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Rust fungi of the order Pucciniales are destructive pathogens of wheat worldwide. Leaf rust caused by the obligate, biotrophic basidiomycete fungus Puccinia triticina ( Pt) is an economically important disease capable of causing up to 50 % yield losses. Historically, resistant wheat cultivars have been used to control leaf rust, but genetic resistance is ephemeral and breaks down with the emergence of new virulent Pt races. There is a need to develop alternative measures for control of leaf rust in wheat. Development of transgenic wheat expressing an antifungal defensin offers a promising approach to complement the endogenous resistance genes within the wheat germplasm for durable resistance to Pt. To that end, two different wheat genotypes, Bobwhite and Xin Chun 9 were transformed with a chimeric gene encoding an apoplast-targeted antifungal plant defensin MtDEF4.2 from Medicago truncatula. Transgenic lines from four independent events were further characterized. Homozygous transgenic wheat lines expressing MtDEF4.2 displayed resistance to Pt race MCPSS relative to the non-transgenic controls in growth chamber bioassays. Histopathological analysis suggested the presence of both pre- and posthaustorial resistance to leaf rust in these transgenic lines. MtDEF4.2 did not, however, affect the root colonization of a beneficial arbuscular mycorrhizal fungus Rhizophagus irregularis. This study demonstrates that the expression of apoplast-targeted plant defensin MtDEF4.2 can provide substantial resistance to an economically important leaf rust disease in transgenic wheat without negatively impacting its symbiotic relationship with the beneficial mycorrhizal fungus. [ABSTRACT FROM AUTHOR]

While the biology of nitrogen-fixing root nodules has been extensively studied, little is known about the evolutionary events that predisposed legume plants to form symbiosis with rhizobia. We have studied the presence and the expression of two pectic gene families in Medicago, polygalacturonases (PGs) and pectin methyl esterases (PMEs) during the early steps of the Sinorhizobium meliloti– Medicago interaction and compared them with related pollen-specific genes. First, we have compared the expression of MsPG3, a PG gene specifically expressed during the symbiotic interaction, with the expression of MsPG11, a highly homologous pollen-specific gene, using promoter- gus fusions in transgenic M. truncatula and tobacco plants. These results demonstrated that the symbiotic promoter functions as a pollen-specific promoter in the non-legume host. Second, we have identified the presence of a gene family of at least eight differentially expressed PMEs in Medicago. One subfamily is represented by one symbiotic gene ( MtPER) and two pollen-expressed genes ( MtPEF1 and MtPEF2) that are clustered in the M. truncatula genome. The promoter- gus studies presented in this work and the homology between plant PGs, together with the analysis of the PME locus structure and MtPER expression studies, suggest that the symbiotic MsPG3 and MtPER could have as ancestors pollen-expressed genes involved in polar tip growth processes during pollen tube elongation. Moreover, they could have been recruited after gene duplication in the symbiotic interaction to facilitate polar tip growth during infection thread formation. [ABSTRACT FROM AUTHOR]