MYB67, a novel regulator of camalexin biosynthesis in Arabidopsis thaliana

Camalexin, the primary phytoalexin of Arabidopsis thaliana, is an indole-derived secondary metabolite involved in defense mechanisms to counteract invading microorganisms. Camalexin can be synthesised in both shoots and roots and can also be exuded by the roots. Camalexin has emerged more recently as a key component allowing plants to gain biomass after colonisation of roots with plant growth promoting microbes. Although the signalling pathways leading to the production of camalexin have been largely explored, the regulatory networks that control the induction of its biosynthetic steps by pathogens and plant growth promoting microbes remain generally unknown, particularly in the roots. MYB67, a member of the R2R3-MYB family, was found to be associated with the genes CYP71A27 and CYP71A28, encoding cytochrome P-450 enzymes involved in camalexin biosynthesis in the roots. The aim of this work was to characterise MYB67’s physiological function and explore its contribution to the transcriptional response in the roots upon inoculation with the pathogen Burkholderia glumae PG1 and the plant growth promoting rhizobacteria Pseudomonas fluorescens sp. CH267. Spatial expression of ProCYP71A27:GUS after biotic elicitation was found to be influenced by MYB67. Loss of MYB67 led to increased resistance to B. glumae and a more efficient usage of the plant growth promoting effect stimulated by P. fluorescens. Furthermore, in response to both bacteria as well as the chitin oligosaccharide; chitohexaose, MYB67 seems to act as a negative regulator of camalexin biosynthesis at early stages. Even under mock conditions, myb67 was found to have induction of camalexin associated genes. RNA-seq analyses revealed for the first time the transcriptional differences in WT roots between the pathogen and PGPB and provided substantial new knowledge of MYB67’s contribution to the stress responses. Highlighting MYB67’s definite role in plant innate immunity associated functions. Overall, MYB67 was also discovered to have a greater transcriptional regulatory impact in response to the PGPB than the pathogen. The insights in this study support MYB67’s involvement in the plant immune response, particularly the regulation of the camalexin signalling pathway to ensure its optimal homeostasis