Signaling in Arabidopsis roots in response to beneficial rhizobacteria

Abstract

Root colonization by selected strains of beneficial soil-resident bacteria is known to improve plant growth, influence root system architecture and trigger a systemic immune response that is effective against a broad range of pathogens, known as induced systemic resistance (ISR). In this thesis we explore signaling mechanisms that are activated in the roots in response to ISR-inducing bacteria. We demonstrate that the plant growth-promoting rhizobacterium Pseudomonas fluorescens WCS417 secretes molecules that positively influence plant growth in Arabidopsis, and confer significant alterations on root morphology evidenced by increased number of lateral roots and root hairs. By employing confocal microscopy and genetic analysis, we highlight auxin signaling as the central hormonal pathway to mediate these effects. In Arabidopsis, WCS417-ISR requires the root-specific transcription factor MYB72 that locally operates in the generation or translocation of a thus-far unidentified systemic signal. MYB72 expression is strongly activated by WCS417, but it is also known to be specifically induced under iron limited conditions. We demonstrate that ISR-inducing rhizobacteria upregulate the iron-deficiency response Strategy I, independently of the iron availability status, ultimately improving host’s iron nutrition. We further demonstrate that rhizobacteria-induced MYB72 expression depends on FIT1, the central transcription factor that orchestrates plant responses to iron limitation, and that MYB72 is similarly regulated as the iron uptake genes FRO2 and IRT1. A search for bacterial determinants involved in MYB72 expression revealed that volatile organic compounds emitted by WCS417 are potent elicitors of the iron deficiency response in roots. By employing whole-genome transcript profiling we identified the β-glucosidase BGLU42 as an essential component of ISR. In addition, we demonstrate that overexpression of BGLU42 in the absence of rhizobacteria conferred a significant level of protection against different pathogens. Consistent with the fact that overexpression of MYB72 does not lead to constitutive disease resistance, we found none of the MYB72-target genes to be constitutively upregulated in roots of plants overexpressing MYB72. This suggests that MYB72 may function synergistically with other cellular components for the initiation of ISR. A search for MYB72-interacting proteins revealed that MYB72 interacts with the closely-related MYB10 transcription factor, and co-operatively trigger the expression of the AT5G55620 gene. In addition, we show that MYB72 and MYB10 function redundantly in regulating the expression of genes involved in the shikimate, the general phenylpropanoid, and the lignin biosynthesis pathway. We further show that overexpression of MYB72 or MYB10 suppresses the expression of a large group of defense-related genes upon root colonization by WCS417 bacteria, suggesting that these transcription factors may be targets of bacterial effectors that function during the interaction in order to suppress innate immune responses and enable bacteria to establish long-term associations with host roots. In conclusion, our results demonstrate that volatiles of root-colonizing rhizobacteria manipulate signaling mechanisms of host plants related to root development and mineral nutrition, ultimately resulting in growth promotion and disease resistance. Identification of the chemical identity of these volatiles will provide tools to develop new strategies for improved nutrition and crop protection.