CsWAKL08, a pathogen-induced wall-associated receptor-like kinase in sweet orange, confers resistance to citrus bacterial canker via ROS control and JA signaling

Qiang Li,Jingjing Qi,Shanchun Chen,Yongrui He,Xiujuan Qin,Lanzhen Xu,Xiuping Zou,Wanfu Dou,Anhua Hu

doi:10.1038/s41438-020-0263-y

Abstract

Citrus bacterial canker (CBC) is a disease resulting from Xanthomonas citri subsp. citri (Xcc) infection and poses a grave threat to citrus production worldwide. Wall-associated receptor-like kinases (WAKLs) are proteins with a central role in resisting a range of fungal and bacterial diseases. The roles of WAKLs in the context of CBC resistance, however, remain unclear. Here, we explored the role of CsWAKL08, which confers resistance to CBC, and we additionally analyzed the molecular mechanisms of CsWAKL08-mediated CBC resistance. Based on systematic annotation and induced expression analysis of the CsWAKL family in Citrus sinensis, CsWAKL08 was identified as a candidate that can be upregulated by Xcc infection in the CBC-resistant variety. CsWAKL08 can also be induced by the phytohormones salicylic acid (SA) and methyl jasmonic acid (MeJA) and spans the plasma membrane. Overexpression of CsWAKL08 resulted in strong CBC resistance in transgenic sweet oranges, whereas silencing of CsWAKL08 resulted in susceptibility to CBC. The peroxidase (POD) and superoxide dismutase (SOD) activities were significantly enhanced in the CsWAKL08-overexpressing plants compared to the control plants, thereby mediating reactive oxygen species (ROS) homeostasis in the transgenic plants. Moreover, the JA levels and the expression of JA biosynthesis and JA responsive genes were substantially elevated in the CsWAKL08 overexpression plants relative to the controls upon Xcc infection. Based on these findings, we conclude that the wall-associated receptor-like kinase CsWAKL08 positively regulates CBC resistance through a mechanism involving ROS control and JA signaling. These results further highlight the importance of this kinase family in plant pathogen resistance.

Highlights

Plants have evolved a plethora of strategies to defend against microbial pathogens, including bacteria, viruses, and fungi[1]
Identification and bioinformatic analysis of the citrus WAKLs (CsWAKLs) in sweet orange Based on exhaustive data mining and a semiautomatic annotation protocol, we identified 21 WAK-like kinases (WAKLs) from the genome of C. sinensis, which were designated CsWAKL01 to CsWAKL21 according to their chromosomal locations (Table S1)
We identified the WAKL family via systemic annotation (Fig. 1a) and investigated the regulation of CsWAKLs early in the Xanthomonas citri subsp. citri (Xcc) infection process, confirming that three CsWAKL genes (CsWAKL01, CsWAKL08, and CsWAKL20) are differentially expressed upon Xcc infection in Citrus bacterial canker (CBC)-resistant and CBCsensitive varieties (Fig. 1d, e)

Summary

Introduction

Plants have evolved a plethora of strategies to defend against microbial pathogens, including bacteria, viruses, and fungi[1]. DAMPs, including leucine-rich repeats, lectin and lysine motifs, and epidermal growth factor-like extracellular domains[5,6]. RLKs are key mediators of plant innate. Li et al Horticulture Research (2020)7:42 immunity, recognizing and inducing specific intracellular signals in response to PAMPs as a part of specific plasma membrane protein complexes[8]. RLKs activate plant immunity through the activation of transcription factors and through the regulation of reactive oxygen species (ROS) homeostasis, pathogenesis-related (PR) proteins, and phytohormones[9,10]. Phytohormones, including jasmonic acid (JA), salicylic acid (SA) and abscisic acid (ABA), are essential mediators of plant immunity and play major roles in the regulation of plant defense responses[11].

Methods

Results

Conclusion