Tryptophan-Derived Metabolites Are Required for Antifungal Defense in the Arabidopsis mlo2 Mutant

Abstract

Arabidopsis (Arabidopsis thaliana) genes MILDEW RESISTANCE LOCUS O2 (MLO2), MLO6, and MLO12 exhibit unequal genetic redundancy with respect to the modulation of defense responses against powdery mildew fungi and the control of developmental phenotypes such as premature leaf decay. We show that early chlorosis and necrosis of rosette leaves in mlo2 mlo6 mlo12 mutants reflects an authentic but untimely leaf senescence program. Comparative transcriptional profiling revealed that transcripts of several genes encoding tryptophan biosynthetic and metabolic enzymes hyperaccumulate during vegetative development in the mlo2 mlo6 mlo12 mutant. Elevated expression levels of these genes correlate with altered steady-state levels of several indolic metabolites, including the phytoalexin camalexin and indolic glucosinolates, during development in the mlo2 single mutant and the mlo2 mlo6 mlo12 triple mutant. Results of genetic epistasis analysis suggest a decisive role for indolic metabolites in mlo2-conditioned antifungal defense against both biotrophic powdery mildews and a camalexin-sensitive strain of the necrotrophic fungus Botrytis cinerea. The wound-and pathogen-responsive callose synthase POWDERY MILDEW RESISTANCE4/GLUCAN SYNTHASE-LIKE5 was found to be responsible for the spontaneous callose deposits in mlo2 mutant plants but dispensable for mlo2-conditioned penetration resistance. Our data strengthen the notion that powdery mildew resistance of mlo2 genotypes is based on the same defense execution machinery as innate antifungal immune responses that restrict the invasion of nonadapted fungal pathogens.

Elevated Early Callose Deposition Results in Complete Penetration Resistance to Powdery Mildew in Arabidopsis

“…We could not detect differences between the wild-type plant and the pmr4 mutant in the initial penetration success of the virulent powdery mildew Gc (Fig. 2G), thereby confirming data from Consonni et al (2010). Hence, callose deposition and the formation of a calloseencased penetration peg are not required for initial powdery mildew ingress and haustorium formation in a compatible interaction, which confirms previous reports regarding the development of fungal infection structures in the absence of callose formation (Jacobs et al, 2003).…”

“…Whereas higher callose accumulation contributes to resistance against nectrophic fungal pathogens, such as Alternaria brassicicola and Plectosphaerella cucumerina (Ton and Mauch-Mani, 2004), recent results have suggested that callose deposition is irrelevant for plant resistance to adapted biotrophic powdery mildews (Consonni et al, 2010). However, our results proved that this assumption cannot be generalized.…”

“…However, the generated PMR4-overexpressing lines showed complete resistance to both the nonadapted and the virulent powdery mildew. This strong resistance not only occurred earlier during plant infection but was also more stringent than the resistance in the pmr4 (Jacobs et al, 2003;Nishimura et al, 2003) and mlo2 (Consonni et al, 2006(Consonni et al, , 2010 mutants. Here, we identified PMR4 as a defense-related gene, the overexpression and disruption of which results in increased plant resistance, although through different defense mechanisms.…”

“…An active role of callose in resistance to powdery mildew was further questioned in recent studies of mildew resistance locus O (mlo) mutants of barley (Hordeum vulgare). While Consonni et al (2010) showed that PMR4-dependent callose deposition was not required for the observed mlo2-conditioned penetration resistance in Arabidopsis, this appeared not to be the case in barley, where treatment of mlo-resistant coleoptiles with a callose inhibitor resulted in reduced formation of callose-containing papillae and decreased penetration resistance to powdery mildew (Bayles et al, 1990). Callose inhibition studies with several Gramineae, including barley, wheat (Triticum aestivum), and oat (Avena sativa), revealed that penetration resistance to powdery mildew was especially suppressed in incompatible plant-fungus interactions (Zeyen et al, 2002).…”

Arabidopsis ABCG34 contributes to defense against necrotrophic pathogens by mediating the secretion of camalexin

“…S6D). Camalexin, a major phytoalexin in A. thaliana, is known to be important for preinvasion resistance against powdery mildew, and atpad3-1, a camalexin-deficient mutant, is defective in the preinvasion resistance to both powdery mildew (47) and A. brassicicola (33). Second, A. brassicicola conidia germination was significantly reduced in the AtABCG34-overexpressing lines (Fig.…”