Annals of Botany

Glucan goes down as rice reaches up

A common response of plants to submergence is elongation growth. This in turn implies changes in cell wall biochemistry and, indeed, several such changes have been demonstrated. These include hydrolysis of the mixed β-1,3: 1,4 glucan polymers, leading to wall loosening and thus allowing cell elongation, as discussed by Kimpara et al. (Saitama and Osaka, Japan; pp. 221–226). These authors have focused on the reciprocal process, namely the synthesis of β-1,3: 1,4 glucans. Rice seedlings, grown in the dark for 60 h, were submerged, with or without aeration for 12 h. β-1,3: 1,4 glucan synthase decreased to less than 40 % of control values during this time. Aeration of submerged plants prevented most of this decline: enzyme activity was approx. 80 % of control values, indicating that the key factor was hypoxia rather than submergence itself. The reduction in synthase activity was at least partly due to down-regulation of gene expression. The synthase is another member of the cellulose-synthase-like (Csl) protein family, of which a particular group, the CslFs, are restricted to the Poaceae. The authors state that the rice genome contains at least eight CslF genes; in this study, the activity of one of these, CslF6, declined in submerged seedlings to less than 20 % of control values. Aeration led only to a small recovery (36 % of control values) so it seems that factors in addition to hypoxia are involved in regulating gene expression. Do these changes in gene expression and in enzyme activity result in a decline in β-1,3: 1,4 glucan synthesis? Certainly the amount of polymer in seedlings submerged for 12 h was only 75 % of that in control seedlings. It is difficult to ascertain the relative roles of decreased synthesis and increased hydrolysis in this reduction but it seems that the amount of β-1,3: 1,4 glucan is controlled by both synthesis and degradation in relation to the submergence-induced elongation growth.