The role of MYB34, MYB51 and MYB122 in the regulation of camalexin biosynthesis in Arabidopsis thaliana.

Henning Frerigmann,Erich Glawischnig,Tamara Gigolashvili

doi:10.3389/fpls.2015.00654

Abstract

The phytoalexin camalexin and indolic glucosinolates share not only a common evolutionary origin and a tightly interconnected biosynthetic pathway, but regulatory proteins controlling the shared enzymatic steps are also modulated by the same R2R3-MYB transcription factors. The indolic phytoalexin camalexin is a crucial defense metabolite in the model plant Arabidopsis. Indolic phytoalexins and glucosinolates appear to have a common evolutionary origin and are interconnected on the biosynthetic level: a key intermediate in the biosynthesis of camalexin, indole-3-acetaldoxime (IAOx), is also required for the biosynthesis of indolic glucosinolates and is under tight control by the transcription factors MYB34, MYB51, and MYB122. The abundance of camalexin was strongly reduced in myb34/51 and myb51/122 double and in triple myb mutant, suggesting that these transcription factors are important in camalexin biosynthesis. Furthermore, expression of MYB51 and MYB122 was significantly increased by biotic and abiotic camalexin-inducing agents. Feeding of the triple myb34/51/122 mutant with IAOx or indole-3-acetonitrile largely restored camalexin biosynthesis. Conversely, tryptophan could not complement the low camalexin phenotype of this mutant, which supports a role for the three MYB factors in camalexin biosynthesis upstream of IAOx. Consistently expression of the camalexin biosynthesis genes CYP71B15/PAD3 and CYP71A13 was not negatively affected in the triple myb mutant and the MYBs could not activate pCYP71B15::uidA expression in trans-activation assays with cultured Arabidopsis cells. In conclusion, this study reveals the importance of MYB factors regulating the generation of IAOx as precursor of camalexin.

Highlights

Phytoalexins are important defense compounds produced by plants in response to infection by a large variety of microorganisms
MYB51 and MYB122 are Induced by Silver Nitrate and by Pathogen-Associated Molecular Pattern (PAMP) from Pythium aphanidermatum (PaNie) To further validate the importance of R2R3-MYBs in camalexin regulation, we analyzed the induction of MYB34, MYB51, and MYB122 in response to elicitors of camalexin production
WRKY33 binds to the promoters of CYP71B15 and CYP71A13 to activate camalexin biosynthesis, and other regulators have to be involved, because its loss of function leads to low camalexin levels only during early stages of pathogen infection (Birkenbihl et al, 2012)

Summary

Introduction

Phytoalexins are important defense compounds produced by plants in response to infection by a large variety of microorganisms. IAOx is dehydrated to indole-3-acetonitrile (IAN) by CYP71A12 and CYP71A13 (Figure 1; Müller et al, 2015) In accordance with their specific function in phytoalexin biosynthesis, both corresponding genes are expressed at very low levels in the absence of stress and are induced by pathogen infection, application of pathogenassociated molecular patterns (PAMPs), or by AgNO3 (Nafisi et al, 2007; Millet et al, 2010). Camalexin and IGs possess a tightly interconnected biosynthetic pathway, but are at least partially regulated by the same R2R3-MYB transcription factors

Results

Discussion

Experimental Procedures