MYB34, MYB51, and MYB 122 Distinctly Regulate Indolic Glucosinolate Biosynthesis in Arabidopsis thaliana

The MYB34, MYB51, and MYB122 transcription factors are known to regulate indolic glucosinolate （IG） biosynthesis in Arabidopsis thaliana. To determine the distinct regulatory potential of MYB34, MYB51, and MYB122, the accumulation of IGs in different parts of plants and upon treatment with plant hormones were analyzed in A. thaliana seedlings. It was shown that MYB34, MYB51, and MYB122 act together to control the biosynthesis of 13M in shoots and roots, with MYB34 controlling biosynthesis of IGs mainly in the roots, MYB51 regulating biosynthesis in shoots, and MYB122 having an accessory role in the biosynthesis of IGs. Analysis of glucosinolate levels in seedlings of myb34, myb51, myb122, myb34 myb51 double, and myb34 myb51 myb122 triple knockout mutants grown in the presence of abscisic acid （ABA）, salicylic acid （SA）, jasmonate （JA）, or ethylene （ET） revealed that： （1） MYB51 is the central regulator of IG synthesis upon SA and ET signaling, （2） MYB34 is the key regulator upon ABA and JA signaling, and （3） MYB122 plays only a minor role in JA/ET-induced glucosinolate biosynthesis. The myb34 myb51 myb122 triple mutant is devoid of IGs, indicating that these three MYB factors are indispensable for IG production under standard growth conditions.

The Arabidopsis P450 protein CYP82C2 modulates jasmonateinduced root growth inhibition, defense gene expression and indole glucosinolate biosynthesis

Jasmonic acid （JA） is a fatty acid-derived signaling molecule that regulates a broad range of plant defense responses against herbivores and some microbial pathogens. Molecular genetic studies have established that JA also performs a critical role in several aspects of plant development. Here, we describe the characterization of the Arabidopsis mutantjasmonic acid-hypersensitivel-1 （jah1-1）, which is defective in several aspects of JA responses. Although the mutant exhibits increased sensitivity to JA in root growth inhibition, it shows decreased expression of JA-inducible defense genes and reduced resistance to the necrotrophic fungus Botrytis cinerea. Gene cloning studies indicate that these defects are caused by a mutation in the cytochrome P450 protein CYP82C2. We provide evidence showing that the compromised resistance of thejah1-1 mutant to B. cinerea is accompanied by decreased expression of JA-induced defense genes and reduced accumulation of JA-induced indole glucosinolates （IGs）. Conversely, the enhanced resistance to B. cinerea in CYP82C2-overexpressing plants is accompanied by increased expression of JA-induced defense genes and elevated levels of JA-induced IGs. We demonstrate that CYP82C2 affects JA-induced accumulation of the IG biosynthetic precursor tryptophan （Trp）, but not the JA-induced IAA or pathogen-induced camalexin. Together, our results support a hypothesis that CYP82C2 may act in the metabolism of Trp-derived secondary metabolites under conditions in which JA levels are elevated. Thejah1-1 mutant should thus be important in future studies toward understanding the mechanisms underlying the complexity of JA-mediated differential responses, which are important for plants to adapt their growth to the ever-changing environments.

Arabidopsis Heterotrimeric G-protein Regulates Cell Wall Defense and Resistance to Necrotrophic Fungi

The Arabidopsis heterotrimeric G-protein controls defense responses to necrotrophic and vascular fungi. The agbl mutant impaired in the Gβ subunit displays enhanced susceptibility to these pathogens. Gβ/AGB1 forms an obligate dimer with either one of the Arabidopsis Gγsubunits （γ1/AGG1 and γ2/AGG2）. Accordingly, we now demonstrate that the aggl agg2 double mutant is as susceptible as agbl plants to the necrotrophic fungus Plectosphaerella cucumerina. To elucidate the molecular basis of heterotrimeric G-protein-mediated resistance, we performed a comparative transcriptomic analysis of agbl-1 mutant and wild-type plants upon inoculation with P cucumerina. This analysis, together with metab- olomic studies, demonstrated that G-protein-mediated resistance was independent of defensive pathways required for resistance to necrotrophic fungi, such as the salicylic acid, jasmonic acid, ethylene, abscisic acid, and tryptophan-derived metabolites signaling, as these pathways were not impaired in agbl and aggl agg2 mutants. Notably, many mis-reguiated genes in agbl plants were related with cell wall functions, which was also the case in aggl agg2 mutant. Biochemical analyses and Fourier Transform InfraRed （FTIR） spectroscopy of cell walls from G-protein mutants revealed that the xylose content was lower in agbl and aggl agg2 mutants than in wild-type plants, and that mutant walls had similar FTIR spec-tratypes, which differed from that of wild-type plants. The data presented here suggest a canonical functionality of the Gβ and Gγ1/γ2 subunits in the control of Arabidopsis immune responses and the regulation of cell wall composition.

Transcriptional Activation and Production of Tryptophan-Derived Secondary Metabolites in Arabidopsis Roots Contributes to the Defense against the Fungal Vascular Pathogen Verticillium Iongisporum

The soil-borne fungal pathogen Verticillium Iongisporum causes vascular disease on Brassicaceae host plants such as oilseed rape. The fungus colonizes the root xylem and moves upwards to the foliage where disease symptoms become visible. Using Arabidopsis as a model for early gene induction, we performed root transcriptome analyses in re- sponse to hyphal growth immediately after spore germination and during penetration of the root cortex, respectively. Infected roots showed a rapid reprogramming of gene expression such as activation of transcription factors, stress-, and defense-related genes. Here, we focused on the highly coordinated gene induction resulting in the production of tryp- tophan-derived secondary metabolites. Previous studies in leaves showed that enzymes encoded by CYP81F2 and PEN2 （PENETRATION2） execute the formation of antifungal indole glucosinolate （IGS） metabolites. In Verticillium-infected roots, we found transcriptional activation of CYP81F2 and the PEN2 homolog PEL 1 （PEN2-LIKE1）, but no increase in antifungal IGS breakdown products. In contrast, indole-3-carboxylic acid （13CA） and the phytoalexin camalexin accumulated in infected roots but only camalexin inhibited Verticillium growth in vitro. Whereas genetic disruption of the individual metabolic pathways leading to either camalexin or CYP81F2-dependent IGS metabolites did not alter Verticillium-induced disease symptoms, a cyp79b2 cyp79b3 mutant impaired in both branches resulted in significantly enhanced susceptibility. Hence, our data provide an insight into root-specific early defenses and suggest tryptophan-derived metabolites as active anti- fungal compounds against a vascular pathogen.