Abstract
Arabinogalactan proteins (AGPs) are a superfamily of hydroxyproline-rich glycoproteins that are massively glycosylated, widely implicated in plant growth and development. No comprehensive analysis of the AGP gene family has been performed in Chinese cabbage (Brassica rapa ssp. chinensis). Here, we identified a total of 293 putative AGP-encoding genes in B. rapa, including 25 classical AGPs, three lysine-rich AGPs, 30 AG-peptides, 36 fasciclin-like AGPs (FLAs), 59 phytocyanin-like AGPs, 33 xylogen-like AGPs, 102 other chimeric AGPs, two non-classical AGPs and three AGP/extensin hybrids. Their protein structures, phylogenetic relationships, chromosomal location and gene duplication status were comprehensively analyzed. Based on RNA sequencing data, we found that 73 AGP genes were differentially expressed in the floral buds of the sterile and fertile plants at least at one developmental stage in B. rapa, suggesting a potential role of AGPs in male reproductive development. We further characterized BrFLA2, BrFLA28 and BrFLA32, three FLA members especially expressed in anthers, pollen grains and pollen tubes. BrFLA2, BrFLA28 and BrFLA32 are indispensable for the proper timing of pollen germination under high relative humidity. Our study greatly extends the repertoire of AGPs in B. rapa and reveals a role for three members of the FLA subfamily in pollen germination.
Highlights
Arabinogalactan proteins (AGPs) are a class of hydroxyproline-rich proteoglycan compounds that are widely present in higher plants [1]
We further demonstrated that three fasciclin-like AGPs (FLAs) members, BrFLA2, BrFLA28 and BrFLA32, are required for maintaining the proper timing of pollen germination under high relative humidity
Those chimeric AGPs (CAGPs) that were not defined as an AGP for lacking N-termi(Table S1) [51,55,56]
Summary
Arabinogalactan proteins (AGPs) are a class of hydroxyproline-rich proteoglycan compounds that are widely present in higher plants [1]. AGPs consist of variable core protein backbones covered by carbohydrate side chains rich in arabinose and galactose, with a molecular mass of about 60–300 kDa. For most of the AGPs, the core protein structure occupies less than 10% of the mature molecule, while more than 90% is related to the carbohydrate side chains [1,2]. The complexity arising from the incredible diversity of the glycans decorating the protein backbone makes AGPs a large complex family in higher plants. A total of 85 and 282 putative AGPs have been identified in the model plant. Arabidopsis thaliana (hereafter, Arabidopsis) and rice, respectively [3,4,5,6,7,8,9,10,11].
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