AND A BIODUN O. C LAUDIUS -C OLE 3
14.3. Inheritance and Mechanisms of Resistance
Resistance to nematodes is expressed in many forms and is conditioned by a wide variety of genetic systems (Williamson and Kumar, 2006). In some cases resistance is conditioned by a single dominant gene with a major effect, typically a classic hyper- sensitive necrotic reaction by affected plant cells. The Mi gene for resistance to Meloidogyne spp. in tomato is a well-characterized example of this type of resistance.
Other forms of resistance may be conditioned by multiple genes and may also involve either a hypersensitive plant response or less dramatic responses. In some cases the resistant plant fails to support well-developed giant cells (for root-knot nematodes) or syncytium complex (for cyst and reniform nematodes) and rates of nematode develop- ment and fecundity are reduced. In some soybean and groundnut accessions, resist- ance to root-knot nematodes is expressed as a reduction in the number of invading juveniles that establish feeding sites. Initial penetration of the roots is nearly equal between the susceptible and resistant plants but then there is a high rate of emigration from the roots of a resistant plant. Juveniles that do establish a feeding site in such
resistant plants typically have a slower rate of development than do juveniles on susceptible plants. Roots of immune or resistant cassava cultivars form a callous tissue complex in response to nematode infection, thus making giant cell formation difficult.
In some plants resistance is conditioned by recessive genes. This has been observed in several cotton accessions that express moderate levels of resistance.
Further, resistance in cotton is often transgressive, where progeny of crosses between two moderately resistant parents express a level of resistance that has greater than expected additive effects. In both groundnut and cotton, there are some resistant accessions where the best evidence suggests that resistance is conditioned by two genes. Further, it appears that one gene in each system is primarily responsible for suppression of nematode reproduction, whereas the other gene is primarily responsi- ble for suppression of root galling.
The observed phenotypic resistance to Ditylenchus africanus, the groundnut pod nematode, in groundnut breeding lines point in a different direction (Steenkamp, 2008). AFLP analyses of reciprocal two-way crosses between a susceptible (cultivar) and resistant (line) parent suggested that the resistance is quantitatively inherited (thus, is likely to be polygenic), although the phenotypic expression of resistance is relatively very strong. The linkage analysis indicates that three quantitative trait loci (QTLs) on two separate linkage groups are associated with the observed groundnut resistance to this nematode. Heritability of the trait and maintaining it in progeny is a challenge that would benefit from more in-depth knowledge of the genes or loci that are associated with the trait. Another nematode associated with groundnut, the testa nematode, Aphelenchoides arachidis, was originally discovered in Nigeria in 1970 (Dickson and De Waele, 2005) and was regarded as of only local importance until it was found more recently in Egypt and South Africa. Unfortunately, there is no record of resistance to this nematode in groundnut.
Plant resistance is conditioned by a variety of genetic mechanisms, which may be mono-, oligo- or polygenic. The genes involved may be further classified by their effect on phenotypic expression, ranging from major genes (large effects) to minor genes (small effects). The apparent inheritance of resistance genes can be influenced by the genetic background in which they exist. Resistance to root-knot nematodes in the two accessions of upland cotton, Clevewilt-6 (PI65358) and Wild Mexico Jack Jones (PI593649), is conditioned by a single recessive gene. Mapping studies have confirmed that they are two distinct genes and, further, progeny from a cross of these parents exhibit transgressive segregation. There is evidence that this transgressive segregation is due to recessive genes present in some susceptible genotypes (Wang et al., 2008). A further issue with these sources of resistance is that after multiple backcrosses of progeny from this cross to susceptible parents the resistance appears to be inherited as a dominant trait governed by two genes. Thus, that which initially appeared to be a recessive trait is expressed as a dominant trait after introgression into different genetic backgrounds.
The physiological and molecular basis of resistance to most nematodes is still poorly understood. Phytoalexins and other similar plant-produced compounds with antimicrobial activity have been implicated in some plant resistance responses. In lima bean (Phaseolus lunatus) and soybean (Glycine max) the accumulation of such compounds in necrotic lesions in response to Pratylenchus scribneri or M. incognita has been observed. These compounds appear to act as repellents to the nematodes.
TheMi-1.2 gene from tomato is one of several genes for resistance to root-knot nematodes in the Solanum peruvianum germplasm and has been studied extensively (see Chapter 15). This gene was introgressed into cultivated tomato, S. lycopersi- cum, in the 1940s and is now widely used in commercial tomato production. The Mi-1.2 resistance gene has several unique characteristics. It conditions resistance to three nematode species (M. arenaria, M. incognita and M. javanica, which may be due more to how closely related these species are to each other than to any unique attribute of the resistance gene), it is temperature sensitive and non-functional at temperatures exceeding 28°C. This gene also conditions resistance to potato aphids and whiteflies. Mi-1.2 belongs to the NB-LRR class of plant resistance genes (con- tain a nucleotide binding site (NB) and a leucine-rich repeat (LRR)). The resistance is similar to resistance to other pathogens that is conditioned by this class of resist- ance genes. The interaction of the resistance gene with an effector molecule (aviru- lence gene product), either directly or indirectly, initiates a signal transduction pathway in the host cell. The end result of this pathway is cell death due to a com- plex of biochemical reactions. An important component of this signal transduction pathway is the protein Rme 1. This protein may act at the same step as the Mi-1.2 protein or upstream (Fig. 14.2). A key characteristic of such resistance responses is an oxidative burst with production of oxygen-free radicals, which then react with a host of other compounds. Affected host cells also respond with the production of various pathogenesis-related (PR) proteins and phytoalexins. Membrane integrity is compromised. The resulting host cell death inhibits further giant cell development and results in death of the invading nematode. In the case of the Mi-1.2 gene this host response is rapid, occurring within the first day or two after root penetration.
In other resistant plants a similar necrotic response may not be initiated for several
Inactive Mi-1 protein
Activated Mi-1 protein
Hypersensitive response Reactive oxygen species
Resistance
Salicylic acid
Ethylene Cytokinin
High temperature
Nitric oxide E
E Rme-1
Cytoplasm
Fig. 14.2. Schematic representation of the interaction of the Mi-1.2 resistance gene protein in tomato with effector molecules from the nematode and other host components to initiate the resistant host response. The Rme-1 protein from the host may be involved in the activation of the resistance response. (From Williamson and Roberts, 2009.)
days after invasion by the nematode. In such cases the initial development of the syncytia or giant cells appears normal before initiation of the necrotic response.
The genes Mi-2 through to Mi-9, which are also from accessions of S. peruvianum, have additional attributes (Williamson, 1998). Some of these genes are not tempera- ture sensitive and others are active against nematode populations that are virulent on Mi-1.2. Unfortunately, none of these genes has yet been introgressed into cultivated tomato. The effector molecule from the nematode that initiates the signal transduc- tion pathway has not been definitively identified but one candidate has been reported:
the silencing of the Cg-1 gene in a nematode renders them virulent on Mi-1.2-mediated resistance.
Not all major effect dominant resistance genes condition a necrotic hypersen- sitive host response. The resistance in groundnut is conditioned by a single domi- nant gene that confers near immunity but no hypersensitive necrotic response is observed.
The identification of nematode resistance genes has been made difficult by the fact that they often reside in regions of the plant genome that have limited recombina- tion. Further, these regions of the plant genome are often rich in resistance genes and resistance gene analogues. In several cases, the location of the resistance gene has been mapped to a particular region of a chromosome but it has been difficult to iden- tify the precise gene sequence responsible for nematode resistance. In the case of Mi, there are actually three closely linked genes with a high degree of homology desig- nated 1.1, 1.2 and 1.3, with only Mi-1.2 being the active resistance gene. The genes Gpa 2 and Gro 1 for resistance to the potato cyst nematodes Globodera pallida and G. rostochiensis, respectively, have been sequenced and are also NB-LLR type resist- ance genes. However, the H1 gene for resistance to G. rostochiensis remains elusive.
H1 resides in a large cluster of genes of the CC-NB-LRR type and precise identifica- tion has not yet been achieved.
Similarly, resistance to the soybean cyst nematode, Heterodera glycines, is complex with several QTLs contributing to the resistant phenotype, some of which are dominantly inherited (Rhg4) and some as recessive genes (rhg1).
Although there are many markers linked to these loci the identification of actual genes has been difficult (Concibido et al., 2004). Initial reports that they were receptor kinases have not been confirmed. One report (Cook et al., 2012) provides evidence of two genes, an a-SNAP protein and a WI12 wound-inducible protein being involved in the resistance response (see Chapter 15). Interestingly, suscepti- ble soybean has a single copy of these two linked genes, whereas resistant geno- types have ten tandem repeats of these genes. Another recent report provides strong evidence for Rhg4 being a serine hydroxymethyltransferase involved in serineô glycine interconversions and essential for one-carbon metabolism in the plant (Lui et al., 2012). Although the mechanism by which this protein confers resistance is yet to be characterized, the authors offered three alternative hypoth- eses for the resistance mechanism. Alterations in folate homeostasis may trigger a necrotic hypersensitive host response. The alteration of one-carbon metabolism may lead to production by the plant of a nematicidal compound. Lastly, alteration in the plant’s folate metabolism may result in a severe nutrient deficiency to the nematode parasite. Thus current evidence for resistance to cyst nematodes in soy- bean points to very novel mechanisms based on genes not previously associated with resistance.