GhHUB2, a ubiquitin ligase, is involved in cotton fiber development via the ubiquitin-26S proteasome pathway.

Hao Feng,Jiangli Dong,Xin Li,Chaojun Zhang,Jie Deng,Tao Wang,Xueyan Zhang,Hong Chen,Ji Liu

doi:10.1093/jxb/ery269

Abstract

Cotton fibers, which are extremely elongated single cells of epidermal seed trichomes and have highly thickened cell walls, constitute the most important natural textile material worldwide. However, the regulation of fiber development is not well understood. Here, we report that GhHUB2, a functional homolog of AtHUB2, controls fiber elongation and secondary cell wall (SCW) deposition. GhHUB2 is ubiquitously expressed, including within fibers. Overexpression of GhHUB2 in cotton increased fiber length and SCW thickness, while RNAi knockdown of GhHUB2 resulted in shortened fibers and thinner cell walls. We found that GhHUB2 interacted with GhKNL1, a transcriptional repressor predominantly expressed in developing fibers, and that GhHUB2 ubiquitinated and degraded GhKNL1 via the ubiquitin-26S proteasome pathway. GhHUB2 negatively regulated GhKNL1 protein levels and lead to the disinhibition of genes such as GhXTH1, Gh1,3-β-G, GhCesA4, GhAGP4, GhCTL1, and GhCOBL4, thus promoting fiber elongation and enhancing SCW biosynthesis. We found that GhREV-08, a transcription factor that participates in SCW deposition and auxin signaling pathway, was a direct target of GhKNL1. In conclusion, our study uncovers a novel function of HUB2 in plants in addition to its monoubiquitination of H2B. Moreover, we provide evidence for control of the fiber development by the ubiquitin-26S proteasome pathway.

Highlights

Protein ubiquitination occurs in nearly every step of plant development (Smalle and Vierstra, 2004; Shu and Yang, 2017)
The expression of GhXTH1 and Gh1,3-β-G correlates with fiber length in GhKNL1 transgenic lines (Gong et al, 2014).We examined the expression of GhCesA4, GhAGP4, GhCTL1, and GhCOBL4 in fibers at 6, 10, 15, and 20 d post anthesis (DPA) and found that these genes were up-regulated in the GhHUB2-OE lines but down-regulated in the GhHUB2-knockdown lines (Fig. 7D)
The developmental process mediated by the ubiquitin–proteasome pathway has been studied in depth in Arabidopsis, but little is known regarding its role in triggering cotton fiber development

Summary

Introduction

Protein ubiquitination occurs in nearly every step of plant development (Smalle and Vierstra, 2004; Shu and Yang, 2017). The process of ubiquitination usually involves three enzymatic steps that are catalysed by ubiquitin-activating enzyme (E1), ubiquitin-conjugating enzyme (E2), and ubiquitin-protein isopeptide ligase (E3) (Deshaies and Joazeiro, 2009). Substrates can be monoubiquitinated or polyubiquitinated (Komander and Rape, 2012). Ubiquitin Lys48-linked polyubiquitination is usually coupled with protein degradation by the 26S proteasome. The ubiquitin–proteasome pathway has been well studied with respect to the regulation of plant developmental processes and responses to abiotic and biotic stresses (Nelson and Millar, 2015; Shu and Yang, 2017). Histone H2B monoubiquitination (H2Bub1) in plants can trigger seed

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Conclusion