Plant pathology: many roads lead to resistance

Genetic analysis of tolerance to the root lesion nematode Pratylenchus neglectus in the legume Medicago littoralis

BACKGROUND

The nematode Pratylenchus neglectus has a wide host range and is able to feed on the root systems of cereals, oilseeds, grain and pasture legumes. Under the Mediterranean low rainfall environments of Australia, annual Medicago pasture legumes are used in rotation with cereals to fix atmospheric nitrogen and improve soil parameters. Considerable efforts are being made in breeding programs to improve resistance and tolerance to Pratylenchus neglectus in the major crops wheat and barley, which makes it vital to develop appropriate selection tools in medics.

RESULTS

A strong source of tolerance to root damage by the root lesion nematode (RLN) Pratylenchus neglectus had previously been identified in line RH-1 (strand medic, M. littoralis). Using RH-1, we have developed a single seed descent (SSD) population of 138 lines by crossing it to the intolerant cultivar Herald. After inoculation, RLN-associated root damage clearly segregated in the population. Genetic analysis was performed by constructing a genetic map using simple sequence repeat (SSR) and gene-based SNP markers. A highly significant quantitative trait locus (QTL), QPnTolMl.1, was identified explaining 49% of the phenotypic variation in the SSD population. All SSRs and gene-based markers in the QTL region were derived from chromosome 1 of the sequenced genome of the closely related species M. truncatula. Gene-based markers were validated in advanced breeding lines derived from the RH-1 parent and also a second RLN tolerance source, RH-2 (M. truncatula ssp. tricycla). Comparative analysis to sequenced legume genomes showed that the physical QTL interval exists as a synteny block in Lotus japonicus, common bean, soybean and chickpea. Furthermore, using the sequenced genome information of M. truncatula, the QTL interval contains 55 genes out of which five are discussed as potential candidate genes responsible for the mapped tolerance.

CONCLUSION

The closely linked set of SNP-based PCR markers is directly applicable to select for two different sources of RLN tolerance in breeding programs. Moreover, genome sequence information has allowed proposing candidate genes for further functional analysis and nominates QPnTolMl.1 as a target locus for RLN tolerance in economically important grain legumes, e.g. chickpea.

Effect of lead (Pb) on the systemic movement of RNA viruses in tobacco (Nicotiana tabacum var. Turkish)

Effect of various lead (Pb) concentrations on the systemic movement of RNA viruses was examined in tobacco plants. Prior to inoculation, plants were grown hydroponically for 6 days in Hoagland's solution supplemented with five concentrations of lead nitrate [Pb(NO(3))(2)]: 0.0 (control), 10, 15, 50, and 100 μM. Four different RNA viruses with different cell-to-cell movement mechanisms were used. Two weeks after inoculation lower and upper leaves of each treatment were harvested and examined for the presence of viral coat protein. In plants inoculated with Tobacco mosaic virus, Potato virus X, and Tobacco etch virus, TEM images and western blot assays confirmed the presence of viral coat proteins in the upper leaves of all lead treatments. However, in plants inoculated with Turnip vein-clearing virus (TVCV), no signs of viral particles were detected in the upper leaves of plants treated with 10 μM or 15 μM lead nitrate. In contrast, plants treated with high concentrations of lead nitrate (50 μM or 100 μM) showed viral particles in their upper leaves. In plants treated with 10 μM or 15 μM lead nitrate, callose accumulation was the same as in control plants. This suggests that non-toxic concentrations of lead nitrate may trigger the production of putative cellular factors in addition to callose that interfere with the TVCV systemic movement. In contrast, plants treated with 100 μM lead nitrate showed less callose as compared to control plants, facilitating the systemic movement of TVCV.

Molecular mechanisms of self-incompatibility in flowering plants and fungi - different means to the same end

Hermaphrodite flowering plants and fungi face the same sexual dilemma - how to avoid self-fertilization. Both have evolved ingenious recognition systems that reduce or eliminate the possibility of selfing. These self-incompatibility (SI) systems offer unique opportunities to study recognition and signalling in non-animal cells and also represent model systems for studying the evolution of breeding systems at a molecular level. In this review, the authors discuss recent molecular data that predict an astonishing diversity in the cellular mechanisms of SI operating in flowering plants and fungi.

Polymorphism of a complex resistance gene candidate family in wild populations of common bean (Phaseolus vulgaris) in Argentina: comparison with phenotypic resistance polymorphism

Fifteen populations of wild bean (Phaseolus vulgaris), located in three provinces in Argentina, were analysed for their polymorphism for a complex resistance gene candidate (RGC) family clustered on the genome and for resistance phenotypes to strains of Colletotrichum lindemuthianum. Results indicate that RGC polymorphism is high. Population structure obtained for markers related to resistance was compared to population structure obtained for RAPD markers in order to infer the evolutionary forces driving polymorphism for resistance in wild populations at both molecular and phenotypic levels. Hierarchical analysis of differentiation showed that, within provinces, populations were differentiated for RAPD as well as for molecular and phenotypic markers of resistance. In contrast, provinces were differentiated only for RAPD and RGC markers and not for resistance phenotypes. The discrepancies found between diversity structures for molecular markers (RAPD and RGCs) and for resistance phenotypes suggest an effect of selection and indicate that diversity for resistance may not be driven by the same selective forces at the molecular and phenotypic levels. We further discuss whether specific selective forces are exerted on RGC markers and underline the importance of spatial scale of analysis for demonstrating an impact of selection.

The SOD2 gene, encoding a manganese-type superoxide dismutase, is up-regulated during conidiogenesis in the plant-pathogenic fungus Colletotrichum graminicola

The SOD2 gene, encoding a manganese-type superoxide dismutase (MnSOD), was identified from Colletotrichum graminicola among a collection of cDNAs representing genes that are up-regulated during conidiogenesis. The SOD2 gene consists of a 797-bp open reading frame that is interrupted by three introns and is predicted to encode a polypeptide of 208 amino acids. All conserved residues of the MnSOD protein family, including four consensus metal binding domains, are present in the predicted SOD2 protein. However, the predicted protein does not appear to contain a signal peptide that would target it to the mitochondria. Northern hybridizations revealed that expression of the approximately 900-bp SOD2 transcript is closely associated with differentiation of both oval and falcate conidia. Southern analysis indicated that there is only a single copy of the gene. SOD2 disruption strains were morphologically and pathogenically indistinguishable from wild-type strains. The dispensability of the MnSOD enzyme may be due to the activities of two other SOD enzymes, a highly expressed iron-type superoxide dismutase and a much less abundant copper/zinc type, that were also detected in C. graminicola.

Collateral gene expression changes induced by distinct plant viruses during the hypersensitive resistance reaction in Chenopodium amaranticolor

Hypersensitive reactions to plant diseases are typically mediated by R genes. Many R genes that have been cloned only confer resistance to a particular pathogen. However, Chenopodium spp. have multivirus hypersensitive resistance, thus making the understanding of this broad-spectrum resistance mechanism attractive. Using tobacco mosaic virus (TMV) tagged with green fluorescent protein to follow infection over time, cDNA-AFLP to find genes up-regulated during virus infection in C. amaranticolor and quantitative RT-PCR to accurately measure gene expression at different time points, the first dissection of this significant defense response pathway is presented. The detected disease-expressed sequences in C. amaranticolor (DESCA) are similar to those that encode p450 monooxegenases, hypersensitivity-related genes, cellulases, ABC transporters, receptor-like kinases, serine/threonine kinases, phosphoribosylanthranilate transferases and hypothetical R genes, many of which are associated with pathogen defense in other plants. The expressions of these DESCA genes are also induced by infection with the taxonomically distinct tobacco rattle virus (TRV) in C. amaranticolor. In particular, DESCA1, one of the gene fragments from C. amaranticolor that lacks similarity to any other sequence in the GenBank database, is induced at least 200 fold 4 d after infection (dai) by both TMV and TRV. The potential role of DESCA genes in a C. amaranticolor multivirus defense response with regard to their levels and time of gene expression is discussed.

Pathogen-induced programmed cell death in plants, a possible defense mechanism

As much as the definition of life may be controversial, the definition of death also may prove problematic. In recent years it became apparent that the death of a living cell may follow more than one possible scenario: it may result from an externally applied physical injury (an accidental death), or it may be the outcome of activating an internal pathway for cell suicide (a programmed death). That cells can participate in their own execution may indicate that certain types of cell deaths that were previously considered to be caused by foreign agents such as pathogens or drugs may actually result from the activation of a programmed cell death pathway that is normally latent in cells. Here, we describe the activation of such a cell suicide pathway in plant cells upon the recognition of an invading pathogen. We discuss the possible use of this pathway as a defense mechanism against infection and the possibility that in many ways the use of this type of cell death in plants is functionally analogous to that used by mammalian cells in response to infection by pathogens.

A single locus determines sensitivity to bacterial flagellin in Arabidopsis thaliana

Peptides corresponding to the most conserved domain of eubacterial flagellin act as potent elicitors in cells of different plant species. In intact Arabidposis thaliana seedlings these peptides (flg22 and flg15) caused callose deposition, induction of genes coding for pathogenesis-related proteins and a strong inhibition of growth. Half-maximal growth inhibition occurred at peptide concentrations of approximately 100 nM. In contrast, peptides representing the corresponding flagellin domains of the plant-associated bacteria A. tumefaciens and R. meliloti were inactive even at concentrations of 10 microM. With the exception of Ws-0, all ecotypes of A. thaliana tested were sensitive to flg22. Crosses of Ws-0 with the sensitive ecotypes Col-0 and La-er, respectively, resulted in sensitive F1 seedlings. In the F2 generation of both crosses, sensitivity segregated as a single trait with markers of chromosome 5 and a ratio of 3:1. Dominance of the locus sensing flagellin, termed FLS-1, suggests that it encodes an element which is important for the perception of the flagellin signal.

The hypersensitive response. A programmed cell death associated with plant resistance

In plants, the hypersensitive response (HR) is defined as a rapid cell death occurring in response to pathogen attack, and is closely related to active resistance. Initiation of the HR process begins with the recognition of the pathogen by the plant, which is mediated mainly by the pathogen avirulence genes and the plant resistance genes. Then, complex signal transduction pathways intervene, involving changes in protein phosphorylation, production of reactive oxygen species and modification of ion fluxes. Components required for the regulation of the HR cell death are now being identified genetically by the isolation of mutants, in contrast to those involved in the execution of the cell death programme, which are still largely unknown. Further genetic and biochemical analyses will undoubtedly answer the question as to whether this form of programmed cell death (PCD) can be compared with other forms of PCD in plants and with apoptosis in animals.

Expression of Xa1, a bacterial blight-resistance gene in rice, is induced by bacterial inoculation

The Xa1 gene in rice confers resistance to Japanese race 1 of Xanthomonas oryzae pv. oryzae, the causal pathogen of bacterial blight (BB). We isolated the Xa1 gene by a map-based cloning strategy. The deduced amino acid sequence of the Xa1 gene product contains nucleotide binding sites (NBS) and a new type of leucine-rich repeats (LRR); thus, Xa1 is a member of the NBS-LRR class of plant disease-resistance genes, but quite different from Xa21, another BB-resistance gene isolated from rice. Interestingly, Xa1 gene expression was induced on inoculation with a bacterial pathogen and wound, unlike other isolated resistance genes in plants, which show constitutive expression. The induced expression may be involved in enhancement of resistance against the pathogen.

Cell death: fatal induction in plants

The recent molecular characterization of three plant genes in which mutations cause phenotypes that mimic disease responses has provided interesting new insights into cell death in plants, but many questions remain about the nature and physiological importance of the process.