Cellulose structures in seed mucilage: New influencing factors discovered.
IPB scientists have recently gained new insights into the complex genetic regulation of mucilage formation in Arabidopsis seeds. Like some other plant species, Arabidopsis seeds contain large amounts of hydrophilic polysaccharides that swell into a transparent mucilage upon contact with water. The main components of the mucilage are pectins and hemicelluloses, which are held in shape by radially arranged cellulose fibrils. The assembly of these cellulose fibrils is catalyzed by cellulose synthase, a membrane-bound enzyme that migrates along the microtubules located beneath the plasma membrane, resulting in parallel alignment of the fibrils. When a mature seed comes into contact with water, the mixture turns into an expanding mucilage and perforates the outer cell wall of the seed coat. It now forms a transparent halo around the seed. After staining, the radially arranged cellulose fibrils are clearly visible in this mucilage envelope. However, the genetic factors influencing such a deposition of highly ordered cellulose structures in the seed mucilage are still largely unknown.
In their study in New Phytologist, the Halle scientists now identified several key players involved in the organization of microtubules and the deposition of the radiating cellulose fibrils in the seed coat cells. Two of the proteins, the IQ67 DOMAIN 9 (IQD9) and the KINESIN LIGHT CHAIN-RELATED 1 (KLCR1), had previously come to the attention of scientists as highly expressed genes during seed development and through their interactions with the cytoskeletal microtubules. Now the scientists have shown that IQD9 and KLCR1 play important roles in cell wall polysaccharide biosynthesis. Mutations in IQD9 and KLCR1 resulted in more compact mucilage envelopes with aberrant cellulose distribution after imbibing the seeds. Both proteins were found to interact closely with each other and with the cortical microtubules of seed coat cells. A third factor identified by the researchers, TONNEAU1 RECRUITING MOTIF 4 (TRM4), contributed together with IQD9 to maintain the cellulose synthase velocity.
IQD proteins have been intensively studied at IPB for many years. As calmodulin-binding factors, they are involved in numerous growth and developmental processes. Some of the 33 IQDs found in Arabidopsis are tightly coupled to the cytoskeleton and control the shape and size of leaf epidermal cells. Now the Halle scientists could show that they also play a role in seed germination.