The cell wall is particularly important in plant growth and development because it maintains the form of the plant cell, allows intercellular communication, responds to external environmental variables, and interacts with pathogenic microorganisms (Keegstra, 2010; Xin et al., 2010). The plant cell wall has a complicated and dynamic structure, which is mainly composed of polysaccharide polymer, protein and lignin. O-acetylation occurs extensively in the plant cell wall, most notably with hemicelluloses, pectins, and lignins. The replacements of O-acetyl group usually happen on various specific glycosyl residues of cell wall polysaccharides. In addition, cell wall polysaccharides can be either mono- or diacetylated, and the extent of O-acetylation depends on species, tissue type, and growth status of plants.

The biosynthetic pathway and function of O-acetylation of cell wall polysaccharides have not yet been fully understood. Modifications in O-acetylation levels are known to alter plant growth and development, as well as their defense against pathogens, and that this effect is most likely achieved by altering the cell wall structure. The REDUCED WALL ACETYLATION (RWA) protein, as well as the ALTERED XYLOGLUCAN9 protein and TRICHOME BIREFRINGENCE-LIKE (TBL) protein families, has been identified as being involved in the O-acetylation pathway of plant cell walls. RWA proteins could be acetyl donor transporters, transporting acetyl CoA into the Golgi apparatus (Manabe et al., 2011, 2013). AXY9 protein may be required for O-acetylation of cell wall polysaccharides by producing acetylation intermediates (Schultink et al., 2015).

Unlike the RWA and AXY9 proteins, a number of TBLs have been identified as polysaccharide acetyltransferases, catalyzing the O-acetylation of specific cell wall polymers including xylan (Urbanowicz et al., 2015; Zhong et al., 2017a; Zhong et al., 2018a,d), xyloglucan (Zhong et al., 2018c,e; Zhong et al., 2020), mannan (Zhong et al., 2018b), and pectin (Vogel et al., 2004b; Bischoff et al., 2010a; Stranne et al., 2018a; Chiniquy et al., 2019a). TBL proteins have conserved Asp-x-x-His (DxxH) motif and Gly-Asp-Ser (GDS) motif that is required for acetyltransferase activity (Bischoff et al., 2010b), since a mutation in either the GDS or DXXH motif could cause TBL proteins to lose their function completely (Zhong et al., 2017a; Zhong et al., 2018e). Studies of tbl mutants in Arabidopsis have demonstrated that dwarfism, stem weakness, and stunted growth of plants are associated with the lack of the TBL genes (Bischoff et al., 2010a; Xiong et al., 2013; Schultink et al., 2015), implying that TBL is critical for plant development. Besides, TBL proteins are related to abiotic stress in plants. Compared with wild-type Arabidopsis plant, the cold tolerance of esk1 increased significantly (Xin et al., 2010), while tbl10 showed enhanced drought resistance (Stranne et al., 2018a). Furthermore, TBL proteins have also been associated with plant defense against pathogens. The reduction of O-acetyl degree of pmr5 in Arabidopsis may lead to its enhanced resistance to powdery mildew (Vogel et al., 2004b; Chiniquy et al., 2019a). According to a recent research, simultaneous mutation of the OsTBL1 and OsTBL2 genes in rice leads to lower acetylation levels and higher vulnerability to leaf blight disease (Gao et al., 2017).

Rose (Rosa sp.) is the most important cut flower in the world, with 8,500 hectares of cut-flower rose cultivation worldwide, with an annual production of over 15 billion stems (Bendahmane et al., 2013), and sales of more than $11 billion (Zlesak, 2007). Gray mold caused by Botrytis cinerea is the most devastating disease mainly infecting the flower of rose and affects the production of cut rose all over the world (Gleason and Helland, 2003). The cell wall is the initial barrier that pathogens meet when penetrating the plant, and alterations in cell wall structure might affect the plant’s defense against these microbes. O-acetylation is one of the most important modifications of cell wall polymer. TBL proteins, as the main gene family involved in cell wall O-acetylation, may influence the resistance or susceptibility of plants to pathogen by varying the degree of cell wall acetylation. However, no research on the function of the TBL gene at the plant genome-wide level has been conducted thus far. We performed the first genome-wide analysis of the RcTBL family in rose in this study. Furthermore, a virus-induced gene silencing (VIGS) has confirmed that RcTBL16 was involved in B. cinerea susceptibility.

O-acetylation is a common modification on plant cell walls and is essential for the stability of the polysaccharide network, with a small amount of acetylation modification affecting plant growth and susceptibility to pathogens. TBL is the largest protein family involved in O-acetylation in plants. There are 46 TBLs in Arabidopsis (Bischoff et al., 2010a), 18 of which have been identified as O-acetyltransferases, involved in the O-acetylation of xylan xyloglucan mannan and pectin respectively (Table 3). Several TBL proteins from rice and poplar have also been shown to be xylan O-acetyltransferases (Zhong et al., 2018a,d). However, there is still a gap in the comprehensive analysis of the rose TBL gene family, and their function remains to be discovered. Rose genome sequencing project was completed in 2018, enabling genome-wide analysis of the RcTBL gene family. In this study, we have comprehensively analyzed the RcTBL family, including phylogeny, gene structure, chromosomal location, gene duplication events, sequence characterization, and analysis of expression profiles.

We identified 50 rose TBLs, more than Arabidopsis (46) (Bischoff et al., 2010a) but less than poplar (64) (Zhong et al., 2018d), rice (66) (Gao et al., 2017), tomatoes (69) (Zhong et al., 2020), which indicates that TBL protein family has expanded to varying degrees in different plants during the evolution. In our structural analysis, we found that in addition to the PC-esterase domain, RcTBLs have a cys-rich domain, namely PMR5N domain, which may be another vital characteristic of the TBL family. Gene duplication plays a very important role in the expansion of gene families. Checking the phylogenetic relationships of TBL between rose and Arabidopsis showed that most evolved branches contained different amounts of AtTBL and RcTBL proteins, suggesting that the two species displayed conserved evolution.

The composition of the cell wall is closely correlated with fungal disease resistance, and altered levels of cell wall polysaccharide O-acetylation modification can lead to altered plant resistance to fungi. In some cases, a reduction in the level of acetylation enhances plant resistance to pathogens. For instance, Arabidopsis rwa2 showed a 20% reduction in the degree of cell wall polysaccharide acetylation modification and was more resistant to B. cinerea than wild type. In this study, we also found RcTBL16 negatively regulating resistance to gray mold in rose. PMR5, a member of the TBL family, has the same performance in resisting powdery mildew in Arabidopsis (Vogel et al., 2004a), by the O-acetylation modification of homogalacturonan (Chiniquy et al., 2019a). Therefore, we consider that partial rose TBL genes may be involved in the susceptibility of rose to B. cinerea through acetylating cell wall.

It is clear that changes in the level of O-acetylation modification of cell walls can affect plant resistance or susceptibility to pathogens, but the exact mechanism remains a mystery at present. One hypothesis suggests that alterations in the polysaccharide composition of the cell wall will modify the cell wall integrity (CWI) system, thereby triggering plant defenses and activating specific defense responses (Bacete et al., 2017). Alteration of cell wall xylan acetylation caused by ESK1 impairment was accompanied by an enhanced accumulation of abscisic acid, the constitutive expression of genes encoding antibiotic peptides and enzymes involved in the biosynthesis of tryptophan-derived metabolites, and the accumulation of disease resistance-related secondary metabolites and different osmolites, implying that the damage to CWI triggers defense response of plant (Lugan et al., 2010; Xin et al., 2010; Xu et al., 2014; Escudero et al., 2017). However, it needs to be substantiated by stronger evidence. Overall, more research remains to be done on the certain mechanisms of TBL participant plant–pathogen interactions, but there is no doubt that TBLs in rose is most possibly as a susceptibility factor for the resistance to gray mold, and this result will also provide clues for rose breeding application (i.e., enhancing persistent plant resistance by silencing or knocking out susceptibility genes).

The study performed a genome-wide analysis of RcTBLs, including phylogenetic relationships, collinearity, and expression analysis. A total of 50 non-redundant RcTBL members were identified. The expression analysis showed that transcription regulation of several RcTBL was reduced by B. cinerea infection in rose petals. Based on these analyses and VIGS, we further demonstrated that RcTBL16 was engaged in susceptibility of rose to gray mold. The information provided by these results can promote the further functional analysis of RcTBL genes and application in rose disease resistance breeding.

Publicly available datasets were analyzed in this study. These data can be found at: https://lipm-browsers.toulouse.inra.fr/pub/RchiOBHm-V2/.

ZZ and YT conceived and designed the research and wrote the paper. YT performed the experiments. YT, SZ, XL, and ZZ analyzed the data. All the authors have read and approved the final version of the manuscript. All authors contributed to the article and approved the submitted version.

his study was supported by the National Natural Science Foundation of China (grant number 31772344) to ZZ. It is further supported by the Science and Technology Program of Beijing Municipality (grant number Z181100002518002). The funders played no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.