Structural and functional insights into the modulation of the activity of a flax cytokinin oxidase by flax rust effector AvrL567-A.

Li Wan,Hitoshi Sakakibara,Jeffrey G Ellis,Adrienne R Hardham,Markus Koeck,Bostjan Kobe,Mikiko Kojima,Daniel J Ericsson,Anthony R Ashton,David A Jones,Peter N Dodds,Gregory J Lawrence,Christine Böttcher,Simon J Williams

doi:10.1111/mpp.12749

Abstract

SummaryDuring infection, plant pathogens secrete effector proteins to facilitate colonization. In comparison with our knowledge of bacterial effectors, the current understanding of how fungal effectors function is limited. In this study, we show that the effector AvrL567‐A from the flax rust fungus Melampsora lini interacts with a flax cytosolic cytokinin oxidase, LuCKX1.1, using both yeast two‐hybrid and in planta bimolecular fluorescence assays. Purified LuCKX1.1 protein shows catalytic activity against both N6‐(Δ2‐isopentenyl)‐adenine (2iP) and trans‐zeatin (tZ) substrates. Incubation of LuCKX1.1 with AvrL567‐A results in increased catalytic activity against both substrates. The crystal structure of LuCKX1.1 and docking studies with AvrL567‐A indicate that the AvrL567 binding site involves a flexible surface‐exposed region that surrounds the cytokinin substrate access site, which may explain its effect in modulating LuCKX1.1 activity. Expression of AvrL567‐A in transgenic flax plants gave rise to an epinastic leaf phenotype consistent with hormonal effects, although no difference in overall cytokinin levels was observed. We propose that, during infection, plant pathogens may differentially modify the levels of extracellular and intracellular cytokinins.

Highlights

The outcomes of plant–microbe interactions are largely determined by the effector repertoire secreted by pathogenic microbes during infection (Dou and Zhou, 2012; Toruno et al, 2016)
Intracellular effectors often function to interfere with pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI), which involves the recognition of PAMPs by plant cell surface receptors, known as pattern recognition receptors
Some effectors that function in plant cells, called avirulence (Avr) proteins, are recognized by plant resistance proteins of the NLR class, leading to effector-triggered immunity (ETI) (Dodds and Rathjen, 2010)

Summary

Introduction

The outcomes of plant–microbe interactions are largely determined by the effector repertoire secreted by pathogenic microbes during infection (Dou and Zhou, 2012; Toruno et al, 2016). Intracellular effectors often function to interfere with pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI), which involves the recognition of PAMPs by plant cell surface receptors, known as pattern recognition receptors. Some effectors that function in plant cells, called avirulence (Avr) proteins, are recognized by plant resistance proteins of the NLR (nucleotide-binding leucine-rich repeat receptor) class, leading to effector-triggered immunity (ETI) (Dodds and Rathjen, 2010). Bacterial pathogens utilize type III secretion systems to deliver effectors into plant cells, and many studies have shown that these effectors subvert plant immunity and promote pathogen survival by interfering with PTI, ETI, proteasome-dependent protein degradation, phytohormone signalling, cytoskeleton assembly, vesicle transport or gene expression (Buttner, 2016; Toruno et al, 2016)

Results

Discussion

Conclusion