Systematic Analysis and Functional Validation of Citrus XTH Genes Reveal the Role of Csxth04 in Citrus Bacterial Canker Resistance and Tolerance.

Qiang Li,Lixiao Yao,Xiuping Zou,Tiangang Lei,Qin Long,Wanfu Dou,Shanchun Chen,Jingjing Qi,Yongrui He,Anhua Hu

doi:10.3389/fpls.2019.01109

Abstract

In this study, we performed a comprehensive survey of xyloglucan endotransglucosylase/hydrolase (XTH) and a functional validation of Citrus sinensis (Cs) XTH genes to provide new insights into the involvement of XTHs in Xanthomonas citri subsp. citri (Xcc) infection. From the genome of sweet orange, 34 CsXTH genes with XTH characteristic domains were identified and clustered into groups I/II, IIIA, and IIIB. Except for chromosome 9, the CsXTH genes were unevenly distributed and duplicated among all chromosomes, identifying a CsXTH duplication hot spot on chromosome 4. With Xcc induction, a group of citrus canker-related CsXTHs were detected. CsXTH04 was identified as a putative candidate gene, which is up-regulated in citrus bacterial canker (CBC)-resistant varieties and induced by exogenous treatment with salicylic acid (SA) and methyl jasmonate (MeJA). CsXTH04 overexpression conferred CBC susceptibility to transgenic citrus, while CsXTH04 silencing conferred CBC resistance. Taken together, the annotation of the CsXTH family provides an initial basis for the functional and evolutionary study of this family as potential CBC-susceptible genes. CsXTH04, validated in this study, can be used in citrus breeding to improve CBC resistance.

Highlights

The cell wall is a dynamic architecture of celluloses, pectic polysaccharides, enzymes, and many structural proteins (Yokoyama and Nishitani, 2001)
Among the xyloglucan endotransglucosylase/ hydrolase (XTH) family, all genes were predicted by C. sinensis annotation project (CAP), while 31 genes were predicted using the Phytozome protocol
Twelve XTH genes that were incorrectly predicted were manually annotated with Fgenesh++ re-annotation (Solovyev et al, 2006) and EST supports (Table 1 and Supplementary Table 3)

Summary

Introduction

The cell wall is a dynamic architecture of celluloses, pectic polysaccharides, enzymes, and many structural proteins (Yokoyama and Nishitani, 2001). An array of enzymes regulate cell wall processes, including expansins (EXPs) (Li et al, 2003), endo-1,4-b-d-endoglucanase (EGases) (del Campillo, 1999; del Campillo et al, 2004), and xyloglucan endotransglucosylase/hydrolases (XTHs) (Rose et al, 2002; Maldonado-Mendoza et al, 2005; Xuan et al, 2016). These enzymes modulate the shape of the cell wall with primary or secondary wall-loosening mechanisms (Li et al, 2003).

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Discussion

Conclusion