Dynamics of cell wall structure and related genomic resources for drought tolerance in rice

REVIEW Dynamics of cell wall structure and related genomic resources for drought tolerance in rice Showkat Ahmad Ganie 1 · Golam Jalal Ahammed 2 Received: 17 August 2020 / Accepted: 4 D

REVIEW

Dynamics of cell wall structure and related genomic resources

for drought tolerance in rice

Showkat Ahmad Ganie 1  · Golam Jalal Ahammed 2

Received: 17 August 2020 / Accepted: 4 December 2020 / Published online: 2 January 2021

© The Author(s), under exclusive licence to Springer-Verlag GmbH, DE part of Springer Nature 2021

Abstract

Key message Cell wall plasticity plays a very crucial role in vegetative and reproductive development of rice under drought and is a highly potential trait forimproving rice yield under drought.

Abstract Drought is a major constraint in rice (Oryza sativa L.) cultivation severely affecting all developmental stages,

with the reproductive stage being the most sensitive Rice plants employ multiple strategies to cope with drought, in which modification in cell wall dynamics plays a crucial role Over the years, significant progress has been made in discovering the cell wall-specific genomic resources related to drought tolerance at vegetative and reproductive stages of rice However, questions remain about how the drought-induced changes in cell wall made by these genomic resources potentially influence the vegetative and reproductive development of rice The possibly major candidate genes underlying the function of quantita-tive trait loci directly or indirectly associated with the cell wall plasticization-mediated drought tolerance of rice might have

a huge promise in dissecting the putative genomic regions associated with cell wall plasticity under drought Furthermore, engineering the drought tolerance of rice using cell wall-related genes from resurrection plants may have huge prospects for rice yield improvement Here, we review the comprehensive multidisciplinary analyses to unravel different components and mechanisms involved in drought-induced cell wall plasticity at vegetative and reproductive stages that could be targeted for improving rice yield under drought

Keywords Rice · Cell wall · Genomic resources · Drought · Vegetative growth · Reproductive growth · QTL · Resurrection plants

Introduction

Drought has remained one of the most prominent and

per-sistent environmental issues severely affecting plant growth,

development, and yield Information from the Global

Drought Information System reveals that drought is

becom-ing progressively severe and intense across the globe (Sircar

and Parekh 2019) Rice (Oryza sativa L.) is one of the most

important cereal crops feeding a large fraction of the human population worldwide (Ganie and Mondal 2015; Wang et al

2017; Xu et al 2018) To feed the rapidly expanding human population, the production of rice must be doubled by 2050 (Fischer et al 2009) However, this projected increase in rice production is restrained by several biotic and abiotic stresses (Song et al 2012; Li et al 2015a; Ganie et al 2017a; Kou

et al 2017) Although rice is cultivated in diverse tems ranging from flooded wetland to rainfed dryland, it is highly susceptible to drought due to shallow rooting behav-ior (Gornall et al 2010; Wang et al 2010; Xu et al 2018) The production of rice is highly water-intensive and is, therefore, grown under flooded conditions (Wang et al 2017;

ecosys-Xu et al 2019) Rice cultivation consumes almost 24–30%

of the world’s available freshwater (Bahuguna et al 2018) Therefore, a water deficit in the form of drought can result

in the huge yield losses of rice Almost 18 million tons of rice yield is lost globally per year due to drought (Dhakarey

Communicated by Wusheng Liu.

* Showkat Ahmad Ganie

University of Science and Technology, Luoyang 471023,

China

et al 2017) The yield losses of rice are even more severe

when periods of drought coincide with its sensitive growth

stages (Kumar et al 2014) Different developmental growth

stages of rice are affected by drought, with the flowering and

grain-filling stages being the most drought-sensitive,

result-ing in severe yield penalties (Wang et al 2010; Bahuguna

et al 2018) Under such circumstances, the development

of high-yielding drought-tolerant rice varieties suitable for

drought-prone areas is very crucial The prior sound

knowl-edge of cellular and molecular mechanisms regulating the

rice drought tolerance can accelerate the development of

such varieties

The primary and secondary cell walls differ in the

arrangement, flexibility, and structure of matrix polymers,

organization of microfibrils, rheological and mechanical

properties, and their roles in the plant life (Cosgrove and

Jarvis 2012) Primary walls, established during cell

divi-sion in metabolically active and growing cells, are present

in almost all plant cells, adequately flexible and hydrated to

enable cell wall expansion during growth In contrast, the

secondary cell walls are present only in differentiated

tis-sues, synthesized once cell growth has stopped, laid inner

to the primary wall, and characterized by lesser

extensibil-ity, more thickened walls with higher cellulose-content and

mechanical strength, lesser hydration, and increased rigidity

than primary walls (Novaković et al 2018) Despite these

differences, the basic architecture of primary and secondary

cell walls is the same, consisting of highly tensile cellulose

fibers embedded in a physiologically active water-saturated

matrix of non-cellulosic polysaccharides cross-linked with

structural glycoproteins Whereas the cellulose microfibrils

are ubiquitously present in all plant cell walls, the matrix

composition of primary walls differs between different plant

groups (Novaković et al 2018) In dicots, the matrix is

com-posed mainly of xyloglucans and pectic polysaccharides in

which the network of cellulose microfibrils are cross-linked

with xyloglucans In contrast, the matrix in monocots has

a high proportion of glucuronoarabinoxylans and (1 → 3),

(1 → 4)-β-D-glucans that interact with cellulose microfibrils

(Shivaraj et al 2018)

Cellulose is the major constituent of cell walls which is

synthesized by a large enzyme complex called cellulose

syn-thase at the plasma membrane, and is composed of

β-1,4-linked glucan chains that are β-1,4-linked by hydrogen bonds to

form high load-bearing cellulose microfibrils (Somerville

et al 2004) For load-bearing, the cellulose fibrils

cross-link with hemicelluloses and possibly also with pectins

Hemicelluloses are highly branched polymers consisting of

β-(1 → 4)-linked backbones that interact through extensive

hydrogen bonding with cellulose fibers to reinforce their

ten-sile strength (Cosgrove and Jarvis 2012) Xyloglucans and

arabinoxylans are the most predominant hemicelluloses in

plant cell walls (O’Neill and York 2018) Phenolic acids,

such as ferulic and p-coumaric acids can form cross-links

between hemicellulose fibers to reduce the cell wall ibility, which can affect the accessibility of wall modifying proteins (Sasidharan et al 2011) Pectins represent the most complex cell wall polysaccharides consisting of the basic building block galacturonic acid, which includes homoga-lacturonan (HG), xylogalacturonan (XGA), rhamnogalactu-ronan I (RGI), and rhamnogalacturonan II (RGII), with HG being the most abundant pectin (Shivaraj et al 2018) In the presence of Ca2+, the formation of hydrogel through cross-linking in de-esterified HG helps in load-bearing (Feng et al

flex-2018) Besides Ca2+, the formation of pectin network also requires boron, which facilitates the cross-linking of RGII domains through boron-bridges (Chormova et al 2014) Pec-tins play a crucial role in the porosity, stiffness, mechanical strength, and adhesion of cell walls (Baldwin et al 2014) Besides the cellulose-hemicellulose and pectin networks, another network is constituted by carbohydrate-rich glyco-proteins viz insoluble extensins (EXTs) and soluble arabi-nogalactan proteins (AGPs) in the cell wall The three net-works are interlinked, and thus any alteration in the EXT/AGP network can profoundly affect the cell wall structure leading to the biological consequences related to the cell wall (Nguema-Ona et al 2014)

Cell wall metabolism is one of the crucial aspects of plant response to environmental stresses, which is mediated by various cell wall modifying proteins These proteins play crucial roles in modulating cell wall structure and properties, causing changes in cell enlargement and expansion during acclimation to such stresses (Sasidharan et al 2011) The interactions between cellulose microfibrils and hemicellu-loses are modulated by three groups of enzymes: expansins (EXPs), xyloglucan endo-β-transglucosylase/hydrolases (XETs/XTHs), and endo-1,4-β-glucanases (EGases) which promote the cell wall extensibility (Sasidharan et al 2011) More specifically, the EXPs and XTHs facilitate the cell wall loosening by disrupting the hydrogen bonds between cellulose and xyloglucan polymers, whereas EGases do

so by catalyzing the endohydrolysis of 1,4-β-D-glucosidic bonds of cellulose (Bray 2004) XTHs also modulate cell wall extensibility through breaking and reforming of bonds between xyloglucan chains (Hyodo et al 2003) Pectins are acted upon by a group of cell wall modifying enzymes such

as pectinmethylesterase (PME), pectate lyase (PL), and polygalacturonase (PG) to regulate the pectin stiffness, cell expansion, and growth (Bray 2004) PMEs modify cell walls

by catalyzing demethylesterification of HGs to release acidic pectins (Micheli 2001) PMEs act on pectic polysaccharide domains either linearly, causing cell wall stiffening, or ran-domly promoting cell wall loosening, which is determined

by the cellular pH and availability of Ca2+ (Micheli 2001) The de-esterified pectins released by PME action are then degraded by enzymes PL and PG to facilitate cell expansion

through promoting cell wall loosening (Bray 2004) PL

cata-lyzes eliminative cleavage of de-esterified pectin yielding

oligosaccharides with unsaturated galacturonosyl groups at

their non-reducing ends (Marín-Rodríguez et al 2002) In

contrast, PG catalyzes the hydrolytic cleavage of HG regions

in the middle lamella of the cell wall (Bray 2004) Another

group of cell wall enzymes closely involved in cell wall

loosening and stiffening is peroxidases These proteins

pro-mote either cell wall stiffening by facilitating bond

forma-tion through H2O2-mediated oxidation of aromatic cell wall

compounds (monolignols, cinnamic acids, aromatic amino

acids), or cell wall loosening by generating hydroxyl

radi-cals which disrupt covalent bonding in cell wall polymers

(Francoz et al 2015)

The occurrence of the cell wall is one of the most

impor-tant characteristics of plants allowing them to thrive in the

terrestrial environment The cell wall is very crucial for

pro-viding shape and mechanical strength to withstand

chang-ing turgor pressures The cell wall is the frontline, where

a plant cell comes in contact with various environmental

stresses and, therefore, plays a pivotal role in mounting

plant responses to such stresses (Hoson 2002) Concerted

water uptake and irreversible cell wall expansion-driven cell

enlargement are essential for optimal plant growth (Cosgrove

and Li 1993) Drought reduces the growth and productivity

of plants by causing various physiological changes,

includ-ing loss of turgor (Le Gall et al 2015) Turgor pressure is

a crucial factor in regulating cell growth, which is

signifi-cantly governed by the extensibility of the cell wall (Wolf

and Greiner 2012; Tardieu et al 2014) Drought-induced

reduction in the cell turgor, therefore, decreases growth by

decreasing cell expansion and/or elongation (Tardieu et al

2014) Morphological changes in plants under drought are

profoundly governed by the modifications in the

polysac-charide network of the cell wall Therefore, among the other

mechanisms adopted by plants in response to drought,

main-tenance of tissue turgor via osmotic adjustment is very

cru-cial for plant tolerance to drought

Like other plants, it has been found that drought changes

the cell wall properties in rice too (Dhakarey et al 2017; Li

et al 2017; Hazman and Brown 2018; Bang et al 2019)

Drought has been revealed to affect the tissue turgor leading

to the reduced growth of rice plants (Farooq et al 2009a,

b) Thus, drought may lead to rice growth inhibition by

affecting its cell wall flexibility (Cal et al 2013) Due to the

difficulty in analyzing cell wall dynamics, most of the

stud-ies on abiotic stress-induced modifications in the cell wall

mainly focus on genes involved in the metabolism of the

cell wall, rather than studying the precise nature of changes

in the cell wall structure, composition, and properties

(Ten-haken 2015) This difficulty may be due to the limited

avail-ability of imaging and biomechanical tools for studying the

native cell walls, underuse of existing microscopy and NMR

methods in abiotic-stressed plants, stress-specific changes in the cell wall, and complications due to the crosstalk between stress and cell wall integrity signaling pathways (Rui and Dinneny 2020) Expression of several cell wall-related genes

is regulated by drought in rice (Liu et al 2014; Tamiru et al

2015; Lee et al 2016; Jung et al 2017; Choi 2019) fore, in this article, we have comprehensively reviewed the role of different genomic resources in the metabolism of cell wall components for the vegetative and reproductive growth of rice under drought Furthermore, the potential of cell wall-related genes from resurrection plants in improving drought tolerance of rice is also discussed

There-Drought‑responsive cell wall‑related candidate genes associated with vegetative growth of rice

Plant phenology, leaf area, leaf surface properties, and root extension have been reported to act as the major constitutive traits mainly controlling water status in crop plants under drought (Bocco et al 2012; Wang et al 2016; Ahammed

et al 2020) During the vegetative stage, upland and rainfed lowland rice is subjected to a varying degree of drought for different durations Drought during the vegetative growth of rice leads to its tissue death, leaf tip drying, leaf rolling, and stunted growth (Chang et al 1974; de Datta et al 1988; Li

et al 2017; Islam et al 2018; Menge et al 2019) Besides, drought affects leaf area index, tillering capacity, shoot dry weight, and also the length, thickness, and depth of roots in rice (Bañoc et al 2000; Chu et al 2018; Hazman and Brown

2018) Evaluation of drought response at the vegetative stage

of rice can be very beneficial for the early identification of desirable drought-tolerant genotypes for drought tolerance breeding (de Datta et al 1988)

Remodeling of the cell wall in response to drought often causes changes in different aspects of the vegetative growth

of rice, which are essential for drought tolerance (Fig. 1) Several drought-responsive genomic resources have been identified in rice, which are associated with the remodeling

of the cell wall structure in different vegetative tissues and, therefore, with the dynamics of vegetative growth of rice under drought (Table 1)

Root

Drought avoidance is one of the elegant strategies adopted

by plants for sustained growth and development in limited conditions, and this is principally ascribed to root phenes that play a decisive role in water uptake and trans-port to the shoot system (Clark et al 2002; Lynch et al

water-2014) The root surface is where a plant first gets exposed

to drought; therefore, the plasticity in root phenes under

water-limited conditions plays a key role in coping with

drought in cereal crops, including rice (Henry et al 2012;

Kadam et al 2017) The elongation of the plant root

sys-tem occurs as a result of cell division and cell expansion

facilitated by the dynamic structure of the cell wall, which

shows substantial expansion during the growth of root cells

(Cano-Delgado et al 2000; Bassani et al 2004) Several

genomic resources have been identified that are

associ-ated with the remodeling of cell wall structure for

drought-induced root elongation in rice (Table 1) Study of root

proteome in drought-stressed JA-biosynthesis rice mutant

cpm2 (having disrupted allene oxide cyclase gene) at the

vegetative growth stage has revealed that the mutant had better root development in the form of profuse branching, increased length and more depth than wild-type plants (Dha-karey et al 2017) This root phenotype, for enhanced per-

formance of cpm2 plants under drought, was supported by cell wall metabolite analysis of cpm2, where it was found

that the contents of six major enzymes of phenylpropanoid pathway were increased, potentially modifying the cell wall

Drought alters the expression of drought-responsive cell wall-related

genes to promote the modifications in the cell wall structure This

drought-induced cell wall plasticity modifies the different aspects of

vegetative and reproductive growth of rice to withstand the

harm-ful effects of drought Numerals indicate references for

correspond-ing representative candidate genes involved in the different typic aspects of vegetative and reproductive development: 1 Todaka

pheno-et al (2012), 2 Cal pheno-et al (2013), 3 Liang pheno-et al (2018), 4 Redillas

et al (2012), 5 Jung et al (2017), 6 Ji et al (2005), 7, 8, 11 Vydehi (2007), 9 Li et al (2015b), 10 Guo et al (2013)

composition by altering the levels of cell wall linked coumaric and ferulic acid These results suggest that the deficiency of jasmonic acid (JA) can stimulate cell wall adaptations in the roots of rice during drought, and that JA is

biopolymer-a negbiopolymer-ative regulbiopolymer-ator of root growth biopolymer-and drought response in rice Genes in the AP2/ERF transcription factor (TF) family regulate various developmental and physiological processes related to the responses of rice to various abiotic stresses (Nakano et al 2006; Ganie et al 2019) Functional charac-terization of a drought inducible AP2/ERF family TF gene

OsERF48 has revealed that its overexpression leads to the

longer and dense root phenotype in transgenic plants (Jung

et al 2017) The study also showed that this TF regulated the expression of a series of target genes, including cell wall-

related genes such as OsXTH9, OsAGP24, and OsAGP3,

which might be involved in the drought-induced altered root phenotype These target genes of OsERF48 encode XTH and AGPs, which are reported to be associated with cell expansion and cell wall plasticization-mediated root growth under abiotic stress conditions (Yang et al 2006; Seifert and Roberts 2007) Overexpression of another AP2/ERF type

gene OsERF71 in the rice roots has been found to elevate

the expression of cell wall loosening and lignin biosynthetic

genes (EXP, XTH, OsCCR1, OsCCR10, OsCAD, OsC4H),

which modify the root structure and, therefore, confer drought resistance phenotype at the vegetative stage of rice growth (Lee et al 2016) The root-specific overexpression

of the same TF gene was later found to enhance the drought tolerance of rice shoots by increasing their photochemi-cal efficiency under drought (Lee et al 2017a) Although authors provided no evidence for developmental changes, they assume that this increase in the shoot drought tolerance may be due to the modification in the shoot developmental processes such as leaf growth inhibition, leaf water content, and leaf cell wall hardening The NAC (NAM, ATAF, and CUC) group of TFs are among the most extensively stud-ied candidates for improving drought tolerance in plants

Several NAC genes involved in drought tolerance of rice

have been reported (Hu et al 2006; Nakashima et al 2007; Zheng et al 2009; Jeong et al 2010; Redillas et al 2012

etc.); however, only a few of them are involved in drought response by modifying the rice cell wall structure Rice

plants overexpressing OsNAC9 have been shown to exhibit

the altered root architecture in the form of the profoundly enlarged stele and aerenchyma at the vegetative stage of growth under drought conditions (Redillas et al 2012) This root phenotype was attributed to the significantly upregu-

lated genes, including wall-associated kinases (regulating

cell elongation and morphogenesis) and lignin biosynthetic

gene OsCCR1 The NAC gene OsNAC10 has been reported

to confer drought tolerance to rice by increasing the root diameter at the vegetative stage in almost the same way as

in the case of OsNAC9 (Jeong et al 2010) Enlarged root

cortical aerenchyma improves drought tolerance by

dimin-ishing the metabolic costs of the root, which facilitate root

growth and water-uptake from water-limited soil (Redillas

et al 2012); whereas, the increased root diameter results in

drought resistance by encouraging the root penetration in

the soil (Clark et al 2008) Another rice NAC gene OsNAC6

has been reported to orchestrate root structural adaptations

in the form of increased root number and root diameter for

enhancing drought tolerance (Lee et al 2017b) However,

the authors have not reported whether OsNAC6 does so by

remodeling the cell wall structure Together, these results

indicate that rice ERF and NAC TFs recruit factors

associ-ated with cell wall modification to facilitate root

morpholog-ical adaptations in response to rhizosphere drought

Moreo-ver, a lignin biosynthesis gene OsCCR10 (Oryza sativa

CINNAMOYL-COA REDUCTASE 10) is highly induced by

drought in the roots of rice (Choi 2019) The study showed

that the overexpression of this gene enhances the drought

tolerance of rice by decreasing the extent of leaf rolling as

well as wilting, and also by increasing the survival rate,

chlorophyll-content, and photochemical efficiency

In order to elucidate the possible molecular mechanisms

regulating crop development under environmental stresses,

transcriptomic methods such as microarray, RNA-seq, and

cDNA-amplified fragment length polymorphism (AFLP)

have been widely used to identify the stress-responsive

dif-ferentially expressed genes Besides the functional

char-acterization of some cell wall-specific genes for

drought-induced root plasticity, as mentioned above, several other

cell wall-related genes have been identified to be

differ-entially expressed in different types of root tissues of rice

under drought conditions by using transcriptomic methods

In a transcriptomic study, water deficit has been reported to

repress the expression of an expansin gene OsEXP2 in the

seminal and lateral roots, where growth is generally

stimu-lated under water-limited conditions; whereas, its expression

was found to be upregulated in the adventitious roots (Yang

et al 2004) This distinctive expression of OsEXP2 indicates

that this gene may be linked to primary root growth

How-ever, a separate transcriptomic study found that OsEXP2,

along with two other cell wall loosening genes XET and

EGase, was rapidly upregulated in the seminal root tips

during root elongation of rice in response to water-limited

conditions (Yang et al 2003) The contrast in the

expres-sion of OsEXP2 in the two studies is anomalous, because

both studies employed cDNA-AFLP to profile gene

expres-sion in upland rice variety Azucena High expresexpres-sion of

OsEXP2, XET, and EGase, under water-limited conditions

possibly facilitates the extensibility of cell walls ensuing

in cell expansion in the elongation zone, thus stimulating

root elongation (Wu and Cosgrove 2000; Lee et al 2001)

Besides, identification and differential expression of 17 cell

wall-related genes have been reported in the elongation

zone of water-stressed rice roots (Yang et al 2006) Out

of the 17 genes, five genes were found to be associated

with cell wall loosening (OsEXP1, OsEXP2, EGase, and two XETs), six genes with lignin biosynthesis (PAL, C3H,

4CL, CCoAOMT, CAD, and peroxidase), and the six other

genes with the metabolism of cell wall proteins (GRP and

UDP-GlcNAc pyrophosphorylase) and polysaccharides

(OsCslF2, GMPase, xylose isomerase, and

beta-1,3-glu-canase) The four polysaccharide metabolism-related genes

encode enzymes that catalyze the synthesis of hemicellulose, hydrolysis and synthesis of β-glucan cross-links in cell wall polysaccharides, and activation of mannosyl units for cell wall synthesis A south Indian deep-rooted rice landrace, Nootripathu, is an efficient drought-avoider, maintaining its root growth under severe drought (Babu et al 2001)

To decipher the molecular and biochemical bases of high drought tolerance in Nootripathu, comparative drought-responsive transcriptome profiling has been performed

on the roots of this landrace and shallow-rooted sensitive rice variety IR20 (Muthurajan et al 2018) The results revealed significant upregulation of cell wall-related

drought-genes cellulose synthases, endoglucanases, and expansins

in the roots of Nootripathu as compared to IR20 jan et al 2018) The upregulation of these genes might be associated with cell wall loosening leading to the acceler-ated root growth in Nootripathu under drought conditions

(Muthura-In a separate transcriptome analysis, the cell wall biogenesis and modifying genes such as those encoding ENOD93 pro-teins, expansins, and cell wall invertases, were found to be significantly downregulated in the roots of drought-tolerant rice line DK151 (Wang et al 2011) Although these genes are usually upregulated in drought-tolerant genotypes under water-limited conditions, the authors justify their results by speculating that inhibition of cell wall extension (by down-regulation of the corresponding genes) in root tissue under drought is an economic process saving energy for the sus-tenance of rice plants under drought conditions Cell wall peroxidases are involved in the reduction of cell growth by causing the stiffening of cell walls (Fry 1986) Osmotic stress-induced inhibition of root growth in seedlings of rice has been reported to be due to high activity of cell wall peroxidase, which catalyzes the cell wall stiffening process

by forming the cross-links between cell wall polymers (Lin and Kao 2002)

Functional analysis of these cell wall-related responsive genes identified in the different types of root tis-sues of rice may pave the way towards understanding the molecular mechanisms underlying the regulation of cell wall plasticization-mediated root growth in rice under limited-water conditions

Although root is the frontline organ, where a plant

encoun-ters drought, research on leaves is equally important,

because leaves are the source organs that synthesize

assimi-lates for the sink organs to sustain the growth of the whole

plant even during the stress conditions It is also known that

drought affects foliar plant parts more directly than roots

in rice (Farooq et al 2009a, ) Besides, through some cell

wall plasticization-mediated adaptive mechanisms, leaves

also play a crucial role in reducing water loss during the

periods of water scarcity (Cal et al 2013) One of the

impor-tant symptoms of drought in plants is the rolling of leaves,

and it is deemed as an agronomically important trait in rice

breeding Leaf rolling phenotype is partly regulated by the

formation of specific thin-walled and highly vacuolated cells

called bulliform cells, which are present on the adaxial

sur-face of leaves (Fang et al 2012; Xiang et al 2012; Li et al

2017) These specific cell types lose turgor under water

stress, resulting in leaf rolling phenotype (Price et al 1997;

Fang et al 2012) Moderate leaf rolling in rice and other

field-grown crops helps in attaining the erect architecture

of leaves, facilitating the enhanced light capture and gas

exchange for efficient photosynthesis, which results in the

accumulation of dry matter and increased yield (Zhu et al

2001; Lang et al 2004) However, severe leaf rolling has

detrimental effects on plant growth, development, and grain

yield (Li et al 2017) Since moderate leaf rolling diminishes

the water loss by reducing the transpiration rate, it can

facili-tate the survival of plants under drought (Kadioglu et al

2012; Liang et al 2018) Hence, the manipulation of leaf

rolling can prove as one of the most vital approaches towards

enhancing rice productivity under drought (Zou et al 2011)

Keeping its agronomic importance in view, leaf rolling has

greatly attracted the attention of plant scientists, and

conse-quently, many different types of genes, and molecular

mech-anisms underlying the regulation of leaf rolling in rice have

been studied (Zhang et al 2009; Li et al 2010; Zou et al

2011; Xiang et al 2012; Yang et al 2014) However, only a

few of them have been reported to be involved in

drought-induced leaf rolling in rice by modifying the cell wall

struc-ture Functional characterization of a gene Curled Leaf and

Dwarf 1/Semi-Rolled Leaf 1 (CLD1/SRL1), encoding a

glycophosphatidylinositol (GPI)-anchored membrane

pro-tein, has revealed that it is involved in leaf rolling-mediated

drought tolerance in rice by regulating the cell wall

forma-tion and leaf epidermal integrity (Li et al 2017) The study

found that the leaves of cld1 mutant exhibited a defect in cell

wall formation due to the significantly reduced cellulose-

and lignin-contents, which in turn owes to the abnormal

expression of cell wall-related genes (OsCESA1-3, SND1,

OsDRP2B, etc.) and proteins (OsCAD2, Os4CL3, laccase,

OsCesA8, OsXTH8, etc.) The study also maintains that

the defect in epidermal bulliform cells in the leaves of the

cld1 mutant leads to severe leaf rolling, which causes

reduc-tions in the water-retaining capacity and water potential in leaves, resulting in the reduced drought tolerance Leaf roll-

ing and erectness is also regulated by REL1 (Rolled and

mutation in this gene (rel1-D) has been reported to result

in the high drought tolerance in rice due to the significant upregulation of different types of genes, including drought-responsive, ABA-responsive, superoxide dismutase, and cell wall-related genes (LOC_Os01g64860; LOC_Os01g72510; LOC_Os05g35320; LOC_Os12g36810, etc.), implying that

REL1 is a negative regulator of drought tolerance in rice

(Liang et al 2018) The study proposes that rel1-D-mediated

leaf rolling and drought tolerance in rice is caused by the perturbed dynamics of stress response in specific organelles such as cell wall and vacuole In another study, overexpres-

sion of an E3 ubiquitin ligase gene OsDIS1 (Oryza sativa

drought-induced SINA protein 1) has been shown to decrease

the drought tolerance in rice by increasing the leaf rolling and by affecting the expression of a wide range of genes, including those involved in drought response and in cell wall development (Ning et al 2011), indicating that OsDIS1

has a negative role in regulating drought tolerance in rice

Mutation of SLE1 (Slender Leaf 1) gene, encoding a

cel-lulose synthase D4-like enzyme that is potentially involved

in cell plate formation, in rice leads to a distinct rolled- and narrow-leaf phenotype, besides other pleiotropic develop-mental abnormalities (Yoshikawa et al 2013) Although the authors presume that the impaired root development could

cause the drought stress in sle1 mutants, it is also possible

that the mutants might as well have experienced drought due to the severely rolled- and narrow-leaf phenotype A

2OG-Fe (II) oxygenase-encoding gene RL14 (Rolling Leaf

14) has been shown to regulate the leaf rolling and water

transport by altering the expression of cellulose and lignin

biosynthetic genes (SND1, VND4/5/6, MYB58/63, MYB20,

OSLAC4, OSLAC17, OSCEA4), and hence, the secondary

cell wall composition in rice leaves (Fang et al 2012) The

rl14 mutant rice plants were found to have water

deficiency-triggered shrunk bulliform cells and reduced transpiration rate Although these morpho-physiological features are asso-ciated with drought tolerance, the authors have not specifi-

cally reported the drought-responsive role of RL14 in rice; hence, keeping an avenue open for characterization of RL14

as a drought-responsive gene in rice In a similar study, a

NAC family gene OsSND2 has been identified to regulate

leaf rolling and cell wall biosynthesis by increasing lose-content in rice (Ye et al 2018); however, authors have not mentioned whether it can do so under drought conditions

cellu-as well

MicroRNAs (miRNAs) are non-protein coding RNAs that regulate diverse developmental and stress-response

processes, and thus miRNAs are regarded as critical genomic

resources modulating agronomic traits in crop plants (Ganie

et al 2016, 2017b; Li and Zhang 2016; Shriram et al 2016)

In monocots, whether a leaf is rolled or flat is also

deter-mined by the antagonistic expression patterns of small RNAs

and their target transcription factors on the adaxial and

abaxial surface of the leaf (Moon and Hake 2011; Merelo

et al 2016) In rice, OsHB4 (belonging to

homeodomain-zip III family of TFs) has been identified as a major target

gene of miR166, and Short Tandem Target Mimic

system-mediated knockdown of miR166 has been demonstrated to

confer drought tolerance by causing morpho-physiological

changes such as leaf rolling (due to small bulliform cells),

reduced transpiration rate and decreased diameter of xylem

vessels (Zhang et al 2018) Furthermore, in the knockdown

rice lines, the study has also identified the downstream genes

of the miR166-OsHB4 module which are related to cell wall

formation and polysaccharide synthesis, such as CSLA3,

CESA5, CSLF6 and CLAVATA1; and these genes were found

to be strongly enriched among the differentially expressed

genes Overall, these results imply that miR166 regulates the

expression of OsHB4, which in turn regulates the

expres-sion of some cell wall and polysaccharide synthesis-related

genes to modulate the cell wall structure for drought-induced

morphological and developmental plasticity in rice Besides,

functional analysis of drought-responsive miRNA

regula-tory networks has revealed miRNA-target genes involved

in cell wall metabolism in rice (Chen and Li 2018) The

characterization of these miRNA-target modules can,

there-fore, be useful for getting insights into the drought-induced

metabolism of the cell wall in rice

PG, a cell wall hydrolase, is responsible for the

degrada-tion of pectin in the cell wall (Swain et al 2011)

Overex-pression of a drought-inducible gene OsBURP16

(encod-ing β subunit of PG1) enhanced the sensitivity of rice to

drought by decreasing plant survival rate and increasing

the electrolyte leakage, H2O2 accumulation, and water loss

rate of leaves (Liu et al 2014) The increased expression of

OsBURP16 was, therefore, suggested to decrease the cell

wall pectin-content, which affected the cell wall integrity

and transpiration rate, resulting in increased sensitivity to

different abiotic stresses, including drought (Liu et al 2014)

Lignin is another essential constituent of the cell wall, which

strengthens the plant stiffness Plants modify their cell wall

structure in response to drought by accumulating high levels

of lignin (Hu et al 2009; Bang et al 2019) The

hydropho-bic property of lignin prevents water loss from plant tissues

under water-limiting conditions (Lee et al 2017a) A gene,

encoding homeodomain-leucine zipper transcription factor

OsTF1L, has been reported to be a key regulator of

drought-induced lignin accumulation in rice (Bang et al 2019)

Over-expression of this gene in rice was shown to significantly

increase the drought tolerance at the vegetative stage by

promoting shoot lignin accumulation and effective synthetic rate with a concomitant reduction in water loss (Bang et al 2019) The study also maintains that elevated lignin accumulation in transgenic rice plants occurred due

photo-to the upregulation of a series of lignin biosynthetic genes It has been elucidated that besides hormone action, abnormali-ties in the cell wall structure, deposition, and remodeling can result in plant dwarfism by affecting cell elongation or cell size (Reiter 2002; Zhou et al 2006) Mutation in a drought-

responsive cytochrome P450 gene OsDSS1 (Oryza sativa

Dwarf and Small Seed 1) causes dwarf phenotype in rice

by reducing the early seedling growth, mature plant height and internode length (Tamiru et al 2015) Since the expres-

sion of cell wall metabolism-related genes (OsBgal2,

OsC-ESA1, OsEXP3, OsEXP4, and OsGLU1) was affected in the dss1 mutant, the authors relate the dwarfism to the altered

cell size A similar study provided an excellent model for regulation of rice plant height via cell wall biosynthesis and development under drought (Todaka et al 2012) The study reported that a phytochrome-interacting factor (PIF)-like

gene OsPIL1 acts as a negative internodal growth regulator

in rice under drought, resulting in the dwarf phenotype The

authors ascribe this role of OsPIL1 to the inhibition of cell

elongation via down-regulation of its target genes (expansins and cellulose synthase), which are involved in the dynam-ics of the cell wall structure It can be deduced from these two studies that adopting a dwarf phenotype under drought conditions may be a crucial morphological adaptation to drought in rice Overexpression of two drought-responsive

TF genes OsAP2 and OsWRKY24 has been found to reduce the plant and cell size in transgenic Arabidopsis, which

could be due to the decreased expression of some genes encoding EXPs andXTHs—the genes associated with cell elongation through cell wall loosening (Jang and Li 2018)

On the contrary, these two TF genes were earlier found to

be positive regulators of cell elongation in rice (Jang and Li

2017), which indicates that these TFs regulate a different set

of downstream target genes in monocots and dicots Germins and Germin-like proteins (GLPs) play important roles in the tolerance of plants to biotic and abiotic stresses (Ilyas

et al 2016) Functional characterization of the promoter of

OsRGLP1 revealed that it is highly induced by drought and

that its activity is very high in cell walls besides other lular components (Ilyas et al 2019) The cell wall-specific

cel-activity of the OsRGLP1 promoter suggests its possible role

in strengthening the cell wall—a prominent mechanism for combating environmental stresses Furthermore, an effective response of plants to drought depends largely, besides other processes, on efficient adjustment of carbohydrate metabo-lism and source-sink relationships among different organs (Stitt et al 2007; Figueroa and Lunn 2016) It has been found that under drought, source organs (mature leaves) have lower sucrose and starch concentrations but higher hexose