Barley disease susceptibility factor RACB acts in epidermal cell polarity and positioning of the nucleus

Björn Scheler,Carolina Galgenmüller,Ralph Hückelhoven,Stefanie Ranf,Vera Schnepf

doi:10.1093/jxb/erw141

Björn Scheler, Carolina Galgenmüller + Show 3 more

Open Access

https://doi.org/10.1093/jxb/erw141

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Abstract

RHO GTPases are regulators of cell polarity and immunity in eukaryotes. In plants, RHO-like RAC/ROP GTPases are regulators of cell shaping, hormone responses, and responses to microbial pathogens. The barley (Hordeum vulgare L.) RAC/ROP protein RACB is required for full susceptibility to penetration by Blumeria graminis f.sp. hordei (Bgh), the barley powdery mildew fungus. Disease susceptibility factors often control host immune responses. Here we show that RACB does not interfere with early microbe-associated molecular pattern-triggered immune responses such as the oxidative burst or activation of mitogen-activated protein kinases. RACB also supports rather than restricts expression of defence-related genes in barley. Instead, silencing of RACB expression by RNAi leads to defects in cell polarity. In particular, initiation and maintenance of root hair growth and development of stomatal subsidiary cells by asymmetric cell division is affected by silencing expression of RACB. Nucleus migration is a common factor of developmental cell polarity and cell-autonomous interaction with Bgh RACB is required for positioning of the nucleus near the site of attack from Bgh We therefore suggest that Bgh profits from RACB's function in cell polarity rather than from immunity-regulating functions of RACB.

Highlights

Plants possess an innate immunity, which constantly monitors the cell surface and cytoplasm for the presence or activity of pathogenic organisms
We tested barley wild type (WT) Golden Promise and corresponding transgenic barley plants silenced for RACB by RNAi or overexpressing constitutively activated (CA) RACB for their ability to respond to MAMPs by production of reactive oxygen species (ROS)
WT barley plants showed a typical MAMP-triggered oxidative burst when challenged with a chitin elicitor preparation

Summary

Introduction

Plants possess an innate immunity, which constantly monitors the cell surface and cytoplasm for the presence or activity of pathogenic organisms. Plant immune receptors can detect conserved molecular patterns that derive from microbes (MAMPs, microbe-associated molecular patterns) or from host cell damage (damage-associated molecular patterns). Such receptors are called pattern recognition receptors (PRRs). They are localized in the host plasma membrane and function in basal resistance to non-adapted and virulent pathogens (Macho and Zipfel, 2014). A second class of plant immune receptors co-evolved with specific, largely polymorphic virulence effectors (Jones and Dangl, 2006). Most of these so-called resistance (R) proteins are localized in the cytoplasm and nucleoplasm. Microbes adapt to plant hosts by evolution of virulence effectors that suppress or circumvent host immunity

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