β‐Glucosidase BGLU 42 is a MYB 72‐dependent key regulator of rhizobacteria‐induced systemic resistance and modulates iron deficiency responses in Arabidopsis roots

Abstract

Summary: Selected soil‐borne rhizobacteria can trigger an induced systemic resistance (ISR) that is effective against a broad spectrum of pathogens. In A rabidopsis thaliana, the root‐specific transcription factor MYB 72 is required for the onset of ISR, but is also associated with plant survival under conditions of iron deficiency. Here, we investigated the role of MYB 72 in both processes. To identify MYB 72 target genes, we analyzed the root transcriptomes of wild‐type C ol‐0, mutant myb72 and complemented 35S:FLAG‐MYB 72/myb72 plants in response to ISR‐inducing P seudomonas fluorescens WCS 417. Five WCS 417‐inducible genes were misregulated in myb72 and complemented in 35S:FLAG‐MYB 72/myb72. Amongst these, we uncovered β‐glucosidase BGLU 42 as a novel component of the ISR signaling pathway. Overexpression of BGLU 42 resulted in constitutive disease resistance, whereas the bglu42 mutant was defective in ISR. Furthermore, we found 195 genes to be constitutively upregulated in MYB 72‐overexpressing roots in the absence of WCS 417. Many of these encode enzymes involved in the production of iron‐mobilizing phenolic metabolites under conditions of iron deficiency. We provide evidence that BGLU 42 is required for their release into the rhizosphere. Together, this work highlights a thus far unidentified link between the ability of beneficial rhizobacteria to stimulate systemic immunity and mechanisms induced by iron deficiency in host plants.