Suppression of HopZ Effector-Triggered Plant Immunity in a Natural Pathosystem.

José S Rufián,Adela Zumaquero,Alberto P Macho,Ainhoa Lucía,Javier Ruiz-Albert,Javier Rueda-Blanco,Carlos M Guevara,Inmaculada Ortiz-Martín,Carmen R Beuzón,Gonzalo Ruiz-Aldea

doi:10.3389/fpls.2018.00977

Abstract

Many type III-secreted effectors suppress plant defenses, but can also activate effector-triggered immunity (ETI) in resistant backgrounds. ETI suppression has been shown for a number of type III effectors (T3Es) and ETI-suppressing effectors are considered part of the arms race model for the co-evolution of bacterial virulence and plant defense. However, ETI suppression activities have been shown mostly between effectors not being naturally expressed within the same strain. Furthermore, evolution of effector families is rarely explained taking into account that selective pressure against ETI-triggering effectors may be compensated by ETI-suppressing effector(s) translocated by the same strain. The HopZ effector family is one of the most diverse, displaying a high rate of loss and gain of alleles, which reflects opposing selective pressures. HopZ effectors trigger defense responses in a variety of crops and some have been shown to suppress different plant defenses. Mutational changes in the sequence of ETI-triggering effectors have been proposed to result in the avoidance of detection by their respective hosts, in a process called pathoadaptation. We analyze how deleting or overexpressing HopZ1a and HopZ3 affects virulence of HopZ-encoding and non-encoding strains. We find that both effectors trigger immunity in their plant hosts only when delivered from heterologous strains, while immunity is suppressed when delivered from their native strains. We carried out screens aimed at identifying the determinant(s) suppressing HopZ1a-triggered and HopZ3-triggered immunity within their native strains, and identified several effectors displaying suppression of HopZ3-triggered immunity. We propose effector-mediated cross-suppression of ETI as an additional force driving evolution of the HopZ family.

Highlights

Type III effectors (T3Es) are bacterial proteins translocated by complex and specialized molecular machines, type III secretion systems (T3SS), directly into the cytosol of eukaryotic cells, where they modify a variety of host cellular processes
We previously showed that HopZ1a expressed from P. syringae pv. tomato DC3000 acts as a general suppressor of effectortriggered immunity (ETI) in Arabidopsis (Macho et al, 2010; Rufián et al, 2015)
We found a small albeit statistically significant attenuation (CI = 0.85 ± 0.035) for the hopZ1a mutant strain compared with that of the Psy 7B40 wild type (Figure 1A)

Summary

Introduction

Type III effectors (T3Es) are bacterial proteins translocated by complex and specialized molecular machines, type III secretion systems (T3SS), directly into the cytosol of eukaryotic cells, where they modify a variety of host cellular processes. The first to be characterized, HopZ1a, suppresses several layers of the plant defense response, including pathogenassociated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity (ETI), as well as systemic acquired resistance (SAR) (Macho et al, 2009, 2010; Lewis et al, 2014; Rufián et al, 2015). The characterization of HopZ1a virulence and avirulence activities has been performed either via heterologous expression from P. syringae strains that do not natively carry hopZ genes in Arabidopsis, or via Agrobacterium-mediated transient or stable expression in Arabidopsis and/or Nicotiana benthamiana (Lewis et al, 2010, 2014; Macho et al, 2010; Jiang et al, 2013). The fact that P. syringae strains natively carrying HopZ1a are poorly characterized and/or have been isolated from technically challenging host plants has probably hindered analysis in a native pathosystem (Ma et al, 2006)

Methods

Results

Conclusion