Whole-seed Immunolabeling of Arabidopsis Mucilage Polysaccharides

Seed hemicelluloses tailor mucilage properties and salt tolerance

While Arabidopsis seed coat epidermal cells have become an excellent genetic system to study the biosynthesis and structural roles of various cell wall polymers, the physiological function of the secreted mucilaginous polysaccharides remains ambiguous. Seed mucilage is shaped by two distinct classes of highly substituted hemicelluloses along with cellulose and structural proteins, but their interplay has not been explored. We deciphered the functions of four distinct classes of cell wall polymers by generating a series of double mutants with defects in heteromannan, xylan, cellulose, or the arabinogalactan protein SALT-OVERLY SENSITIVE 5 (SOS5), and evaluating their impact on mucilage architecture and seed germination during salt stress. We discovered that muci10 seeds, lacking heteromannan branches, had elevated tolerance to salt stress, while heteromannan elongation mutants exhibited reduced germination in calcium chloride (CaCl₂ ). By contrast, xylan made by MUCILAGE-RELATED21 (MUCI21) was found to be required for the adherence of mucilage pectin to microfibrils made by CELLULOSE SYNTHASE5 (CESA5) as well as to a SOS5-mediated network. Our results indicate that the substitution of xylan and glucomannan in seeds can fine-tune mucilage adherence and salt tolerance, respectively. The study of germinating seeds can thus provide insights into the synthesis, modification and function of complex glycans.

Mechanistic insights from plant heteromannan synthesis in yeast

Heteromannan (HM) is one of the most ancient cell wall polymers in the plant kingdom, consisting of β-(1-4)-linked backbones of glucose (Glc) and mannose (Man) units. Despite the widespread distribution of HM polysaccharides, their biosynthesis remains mechanistically unclear. HM is elongated by glycosyltransferases (GTs) from the cellulose synthase-like A (CSLA) family. MANNAN-SYNTHESIS RELATED (MSR) putative GTs have also been implicated in (gluco)mannan synthesis, but their roles have been difficult to decipher in planta and in vitro. To further characterize the products of the HM synthases and accessory proteins, we chose a synthetic biology approach to synthesize plant HM in yeast. The expression of a CSLA protein in Pichia pastoris led to the abundant production of plant HM: up to 30% of glycans in the yeast cell wall. Based on sequential chemical and enzymatic extractions, followed by detailed structural analyses, the newly produced HM polymers were unbranched and could be larger than 270 kDa. Using CSLAs from different species, we programmed yeast cells to produce an HM backbone composed exclusively of Man or also incorporating Glc. We demonstrate that specific MSR cofactors were indispensable for mannan synthase activity of a coffee CSLA or modulated a functional CSLA enzyme to produce glucomannan instead of mannan. Therefore, this powerful platform yields functional insight into the molecular machinery required for HM biosynthesis in plants.

TRM4 is essential for cellulose deposition in Arabidopsis seed mucilage by maintaining cortical microtubule organization and interacting with CESA3

The differentiation of the seed coat epidermal (SCE) cells in Arabidopsis thaliana leads to the production of a large amount of pectin-rich mucilage and a thick cellulosic secondary cell wall. The mechanisms by which cortical microtubules are involved in the formation of these pectinaceous and cellulosic cell walls are still largely unknown. Using a reverse genetic approach, we found that TONNEAU1 (TON1) recruiting motif 4 (TRM4) is implicated in cortical microtubule organization in SCE cells, and functions as a novel player in the establishment of mucilage structure. TRM4 is preferentially accumulated in the SCE cells at the stage of mucilage biosynthesis. The loss of TRM4 results in compact seed mucilage capsules, aberrant mucilage cellulosic structure, short cellulosic rays and disorganized cellulose microfibrils in mucilage. The defects could be rescued by transgene complementation of trm4 alleles. Probably, this is a consequence of a disrupted organization of cortical microtubules, observed using fluorescently tagged tubulin proteins in trm4 SCE cells. Furthermore, TRM4 proteins co-aligned with microtubules and interacted directly with CELLULOSE SYNTHASE 3 in two independent assays. Together, the results indicate that TRM4 is essential for microtubule array organization and therefore correct cellulose orientation in the SCE cells, as well as the establishment of the subsequent mucilage architecture.