An oomycete effector manipulates Arabidopsis Auxin and Brassinosteroid signaling pathways.

Effectors are molecules secreted by pathogens that alter host defense responses and metabolic processes. HaRxL106 is an effector protein from Hyaloperonospora arabidopsidis (Hpa), an obligate biotrophic oomycete pathogen of Arabidopsis thaliana. Constitutive expression of HaRxL106 renders plants more susceptible to Hpa and Pseudomonas syringae. Moreover, HaRxL106-expressing transgenic lines phenocopy mutants that display constitutive shade avoidance when grown in normal light conditions. Here we identified two putative target proteins of HaRxL106 using BiFC and CoIP assays. HaRxL106 interacts with Arabidopsis proteins IAA11 and BIM1. The first is a negative regulator of Auxin signaling, while BIM1 activates BR responsive gene expression.
Interestingly, in infection assays bim1 mutants show reduced bacterial and Hpa growth compared to wild type plants, while iaa11 mutants are more susceptible. We also expressed HaRxL106 in iaa11 and bim1 mutant backgrounds. Moreover, only the mutations in BIM1 suppress the constitutive shade avoidance conferred by the YFP-HaRxL106 transgene. We find that Hpa infection enhances expression of BR and Auxin-related genes and that inducible expression of HaRxL106 is sufficient for activation of the Auxin reporter DR5:GUS. The finding of these new targets could explain the developmental phenotypes showed by HaRxL106 transgenics, helping to understand how Hpa manipulates plant hormone signaling.