A sticky seed is a good seed

RA Burton

ARC Centre of Excellence in Plant Cell Walls, University of Adelaide

Many seeds extrude mucilage in aqueous environments; a property called myxospermy. Mucilage may aid germination and seed dispersal in nature but is also turning out to be very useful in the laboratory as a system to study polysaccharide interactions in gel formation; a proxy for more complicated and inaccessible plant cell walls. Polysaccharide amounts and fine structures vary in mucilages; pectin is dominant in Arabidopsis, pectins and xylans are equal in Linum species (flax) whilst Plantago species (psyllium) predominantly contain heteroxylan. The very small amounts of cellulose in all types appears crucial for mucilage release and subsequent gel integrity. We are making progress in defining this complicated process in Plantago that seems quite different to events for Arabidopsis seed release. We are also particularly interested in xylan, a ubiquitous component of cell walls of most plants. Xylan is a linear backbone decorated with substituent groups varying in identity, spacing and amount from species to species and tissue to tissue. Thus xylans can be variably soluble and fermentable, particularly in a human health context, with their structure influencing downstream uses. Using Plantago we are uncovering gene expression patterns governing xylan structural diversity in two ways. Firstly we have used a set of Plantago species with mucilage heterogeneity to track the glycosyl transferase genes driving addition of substituents to the backbone. Secondly we have made a mutant Plantago ovata population using gamma irradiation and have screened many M2 lines for visible and chemical changes using stains, antibodies and monosaccharide profiles and NIR techniques respectively. We have found some exciting mutant phenotypes and using RNAseq and our own Plantago genome assembly are now identifying the causative lesions. These mutants are also proving invaluable in our efforts to unravel the steps in mucilage release.