Making mucilage: A key transcriptional regulatory module regulating rhamnogalacturonan I biosynthesis

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Xu et al. identify a key transcriptional regulatory module involved in biosynthesis of the pectic polysaccharide rhamnogalacturonan I in the mucilage surrounding the Arabidopsis seed. https://doi.org/10.1093/plcell/koac011

Background: Pectin is a complex cell wall polysaccharide that plays important roles in modulating cell adhesion, cell morphogenesis, and growth. The Arabidopsis seed coat mucilage is a specialized cell wall mainly composed of rhamnogalacturonan-I (RG-I) (approximately 90%), a type of pectic polysaccharide, making seed mucilage an ideal model system for studying the synthesis and regulation of pectin polysaccharides. Several genes involved in pectin synthesis have been identified, but the upstream transcription factors governing pectin synthesis and their regulatory mechanisms remain largely unknown.

Question: How is the biosynthesis of pectin transcriptionally regulated?

Findings: RG-I biosynthesis is significantly reduced in loss-of-function mutants of the trihelix transcription factor DE1 BINDING FACTOR 1 (DF1). DF1 physically interacts with GLABRA2 (GL2). The DF1–GL2 module cooperatively regulates the expression of the RG-I biosynthesis genes MUCILAGE MODIFIED 4 (MUM4) and GALACTURONOSYLTRANSFERASE-LIKE5 (GATL5). DF1 binds to the promoters of MUM4 and GATL5 through interacting with GL2 and facilitates the transcriptional activity of GL2. In addition, the expression of DF1 and GL2 is directly regulated by TRANSPARENT TESTA GLABRA2 (TTG2) and, in turn, DF1 directly represses the expression of TTG2. Altogether, our study uncovers the transcriptional regulation mechanism of mucilage RG-I biosynthesis dominated by the module comprising DF1, GL2, and TTG2.

Next steps: We are currently working to discover additional transcriptional regulators involved in pectic polysaccharides biosynthesis in the Arabidopsis seed coat mucilage. Further study will be focused on dissecting the transcriptional regulation networks and underlying mechanisms governing pectin biosynthesis.

Reference:

Yan Xu, Yiping Wang, Jinge Du, Shengqiang Pei, Shuaiqiang Guo, Ruili Hao, Dian Wang, Gongke Zhou, Shengjun, Malcolm O’Neill, Ruibo Hu, and Yingzhen Kong (2022) A DE1 BINDING FACTOR 1–GLABRA2 module regulates rhamnogalacturonan I biosynthesis in Arabidopsis seed coat mucilage. https://doi.org/10.1093/plcell/koac011