Transport of UDP-rhamnose by URGT2, URGT4, and URGT6 modulates rhamnogalacturonan-I length

Mucilage extracted from Chilean papaya seeds is enriched with homogalacturonan domains

Chilean papaya, also known as mountain papaya (Vasconcellea pubescens), is a fruit valued for its nutritional value and pleasant fragrance. The oblong fruit, featuring five ridges and a seed-filled mucilage cavity, is typically consumed cooked due to its high protease content. The mucilage and the seeds are usually discarded as byproducts. This study analyzed the biochemical composition of mountain papaya seed mucilage using methods such as HPAEC and immunolabeling. Results revealed that papaya seeds yield nearly 20% of their weight in mucilage polysaccharides, which can be separated into soluble and adherent layers. The mucilage exhibited a high proportion of acidic sugars, indicating that homogalacturonan (HG) is the predominant domain. It also contained other domains like rhamnogalacturonan-I (RG-I) and hemicelluloses, predominantly xyloglucan. The HG-rich mucilage, currently considered waste, emerges as a promising source of polysaccharides, indicating its multifaceted utility in various industrial applications.

A novel pectin methylesterase inhibitor, PMEI3, in common bean suggests a key role of pectin methylesterification in Pseudomonas resistance

The mechanisms underlying susceptibility to and defense against Pseudomonas syringae (Pph) of the common bean (Phaseolus vulgaris) have not yet been clarified. To investigate these, 15-day-old plants of the variety Riñón were infected with Pph and the transcriptomic changes at 2 h and 9 h post-infection were analysed. RNA-seq analysis showed an up-regulation of genes involved in defense/signaling at 2 h, most of them being down-regulated at 9 h, suggesting that Pph inhibits the transcriptomic reprogramming of the plant. This trend was also observed in the modulation of 101 cell wall-related genes. Cell wall composition changes at early stages of Pph infection were associated with homogalacturonan methylation and the formation of egg boxes. Among the cell wall genes modulated, a pectin methylesterase inhibitor 3 (PvPMEI3) gene, closely related to AtPMEI3, was detected. PvPMEI3 protein was located in the apoplast and its pectin methylesterase inhibitory activity was demonstrated. PvPMEI3 seems to be a good candidate to play a key role in Pph infection, which was supported by analysis of an Arabidopsis pmei3 mutant, which showed susceptibility to Pph, in contrast to resistant Arabidopsis Col-0 plants. These results indicate a key role of the degree of pectin methylesterification in host resistance to Pph during the first steps of the attack.

Biochemical data documenting variations in mucilage polysaccharides in a range of glycosyltransferase mutants

During Arabidopsis seed coat development, copious amounts of mucilage polysaccharides are produced in the epidermal cells. When hydrated on imbibition, these polysaccharides expand and are released to encapsulate the seed as a two-layered hydrogel. Polysaccharides are synthesized from UDP-sugars by glycosyltransferases (GTs) and several GTs, with differing activities, have been identified that contribute to mucilage polysaccharide synthesis. How these GTs orchestrate production of the complex polysaccharides found in mucilage remains to be determined. In this study, we generated a range of multiple GT mutants using either CRISPR/Cas9 targeted mutation or genetic crosses of existing T-DNA insertion mutants. Four traits for mucilage amounts or macromolecular properties were examined for four replicate seed lots from 31 different GT mutant combinations. This data provides a valuable resource for future genetic, biochemical, structural, and functional studies of the roles and properties of polysaccharides present in Arabidopsis mucilage and the relative contributions of different GTs to mucilage production.

Regulation of seed coat mucilage production and modification in Arabidopsis

The Arabidopsis seed coat mucilage is a polysaccharide-rich matrix synthesized by the seed coat epidermal cells. It is a specialized cell wall mainly composed of three types of polysaccharides (i. e. pectin, hemicellulose, and cellulose), and represents as an ideal model system for plant cell wall research. A large number of genes responsible for the synthesis and modification of cell wall polysaccharides have been identified using this model system. Moreover, a subset of regulators controlling mucilage production and modification have been characterized, and the underlying transcriptional regulatory mechanisms have been elucidated. This substantially contributes to the understanding of the molecular mechanisms underlying mucilage synthesis and modification. In this review, we concisely summarize the various genes and regulators involved in seed coat cell differentiation, mucilage biosynthesis and modification, and secondary cell wall formation. In particular, we put emphasis on the latest knowledge gained regarding the transcriptional regulation of mucilage production, which is composed of a hierarchal cascade with three-layer transcriptional regulators. Collectively, we propose an updated schematic framework of the genetic regulatory network controlling mucilage production and modification in the Arabidopsis mucilage secretory cells.

Rhamnogalacturonan-I forms mucilage: behind its simplicity, a cutting-edge organization

Zhang Y, Yin Q, Qin W, Gao H, Du J, Chen J, Li H, Zhou G, Wu H, Wu A-M. 2022. The Class II KNOX family members KNAT3 and KNAT7 redundantly participate in Arabidopsis seed coat mucilage biosynthesis. Journal of Experimental Botany 73, 3477–3495.

The Class II KNOX family members KNAT3 and KNAT7 redundantly participate in Arabidopsis seed coat mucilage biosynthesis

The production of Arabidopsis seed mucilage involves complex polysaccharide biosynthetic pathways and developmental processes in seed epidermal cells. Although the polysaccharide components of Arabidopsis seed mucilage have been identified, their regulatory mechanism requires further investigation. Here, we show that Class II KNOX gene family members KNAT3 and KNAT7 play an essential role in regulating mucilage production in the early developmental stages of Arabidopsis seeds. Double mutant knat3knat7 resulted in defective seed mucilage production and columellae formation, whereas knat3 showed a normal phenotype compared with wild type, and the mucilage thickness in knat7 was slightly disturbed. Rhamnogalacturonan I (RG-I) and its biosynthetic substrates galacturonic acid and rhamnose were reduced in both the adherent and soluble mucilage of knat3knat7. Comparative transcriptome analysis on whole seeds suggested that polysaccharide, glucosinolate and anthocyanin biosynthetic pathways were specifically repressed in knat3knat7. Transient co-expression of KNAT3 and KNAT7 with promoter regions of candidate genes in Arabidopsis protoplasts revealed that both KNAT3 and KNAT7 act as positive regulators of the RG-I biosynthetic gene MUCILAGE-MODIFIED 4 (MUM4, AT1G53500). Collectively, our results demonstrate that KNAT3 and KNAT7 are multifunctional transcription factors in secondary cell wall development and redundantly modulate mucilage biosynthesis in Arabidopsis seeds.

Pre-Anthesis Cytokinin Applications Increase Table Grape Berry Firmness by Modulating Cell Wall Polysaccharides

The use of plant growth regulators (PGRs) is widespread in commercial table grape vineyards. The synthetic cytokinin CPPU is a PGR that is extensively used to obtain higher quality grapes. However, the effect of CPPU on berry firmness is not clear. The current study investigated the effects of pre-anthesis applications (BBCH15 and BBCH55 stages) of CPPU on 'Thompson Seedless' berry firmness at harvest through a combination of cytological, morphological, and biochemical analyses. Ovaries in CPPU-treated plants presented morphological changes related to cell division and cell wall modification at the anthesis stage (BBCH65). Moreover, immunofluorescence analysis with monoclonal antibodies 2F4 and LM15 against pectin and xyloglucan demonstrated that CPPU treatment resulted in cell wall modifications at anthesis. These early changes have major repercussions regarding the hemicellulose and pectin cell wall composition of mature fruits, and are associated with increased calcium content and a higher berry firmness at harvest.

Sterol Glucosyltransferases Tailor Polysaccharide Accumulation in Arabidopsis Seed Coat Epidermal Cells

The conjugation of sterols with a Glc moiety is catalyzed by sterol glucosyltransferases (SGTs). A portion of the resulting steryl glucosides (SG) are then esterified with a long-chain fatty acid to form acyl-SG (ASG). SG and ASG are prevalent components of plant cellular membranes and influence their organization and functional properties. Mutant analysis had previously inferred that two Arabidopsis SGTs, UGT80A2 and UGT80B1/TT15, could have specialized roles in the production of SG in seeds, despite an overlap in their enzymatic activity. Here, we establish new roles for both enzymes in the accumulation of polysaccharides in seed coat epidermal cells (SCEs). The rhamnogalacturonan-I (RG-I) content of the inner layer of seed mucilage was higher in ugt80A2, whereas RG-I accumulation was lower in mutants of UGT80B1, with double mutant phenotypes indicating that UGT80A2 acts independently from UGT80B1. In contrast, an additive phenotype was observed in double mutants for increased galactoglucomannan (GGM) content. Double mutants also exhibited increased polymer density within the inner mucilage layer. In contrast, cell wall defects were only observed in mutants defective for UGT80B1, while more mucilage cellulose was only observed when UGT80A2 was mutated. The generation of a range of phenotypic effects, simultaneously within a single cell type, demonstrates that the adjustment of the SG and ASG composition of cellular membranes by UGT80A2 and UGT80B1 tailors polysaccharide accumulation in Arabidopsis seeds.