Morphology of rsw1, a cellulose-deficient mutant of Arabidopsis thaliana

The rsw1 mutant of Arabidopsis thaliana is mutated in a gene encoding a cel lulose synthase catalytic subunit. Mutant seedlings produce almost as much cellulose as the wild type at 21 degreesC but only about half as much as th e wild type at 31 degreesC. We used this conditional phenotype to investiga te how reduced cellulose production affects growth and morphogenesis in var ious parts of the plant. Roots swell in all tissues at 31 degreesC. and tem perature changes can repeatedly switch them between swollen and slender gro wth patterns. Dark-grown hypocotyls also swell. whereas cotyledons and rose tte leaf blades are smaller, their surfaces are more irregular and their pe tioles shorter. Leaf trichomes swell and branch abnormally. Plants readily initiate inflorescences at 31 degreesC which have shorter but not fatter bo lts and stomata which bulge above the uneven surface of internodes. Bolts c arry the normal number of flowers. but their stigmas protrude beyond the sh ortened sepals and petals. Anthers dehisce normally, but self-fertilisation is reduced because the stigma is well above the anthers. Anther filaments are short and show a crumpled surface. Viable pollen develops, but female r eproductive competence and postpollination development are severely impaire d. We conclude that the RSW1 gene is important for cellulose synthesis in m any parts of the plant and that reduced cellulose synthesis suppresses orga n expansion rather than organ initiation, causes radial swelling only in th e root and hypocotyl. but makes the surfaces of many organs uneven. We disc uss some possible reasons to explain why different organs vary in their res ponses. The morphological changes suggest that RSW1 contributes cellulose t o primary walls but do not yet exclude a role during secondary-wall deposit ion.