Levels of beta-glucan and lignin in elongating internodes of deepwater rice

Partial submergence or treatment with either ethylene or gibberellic acid (GA) induces rapid growth in deepwater rice (Oryza sativa L.). We correlated the synthesis of two cell wall components with two phases of internodal elongation, namely (1 leads to 3,1 leads to 4)-beta-glucan formation with cell elongation and lignification with differentiation of the secondary cell wall and cessation of growth. The content of beta-glucan was highest in the zone of cell elongation in internodes of air-grown plants and plants that were induced to grow rapidly by submergence. In the intercalary meristem and in the differentiation zone of the internode, beta-glucan levels were ca. 70% lower than in the zone of cell elongation. The outer cell layers, enriched in epidermis, contained more beta-glucan in submerged, rapidly growing internodes than in air-grown, control internodes. The beta-glucan content of the inner, parenchymal tissue was unaffected or slightly lowered by submergence. The epidermis appears to be the growth-limiting structure of rapidly growing rice internodes. We hypothesize that elevated levels of beta-glucan contribute to elongation growth by increasing the extensibility of the cell wall. Lignification was monitored by measuring the content of lignin and the activities of two enzymes of the lignin biosynthetic pathway, coniferyl alcohol dehydrogenase (CAD) and phenylalanine ammonia-lyase (PAL), in growing and non-growing regions of the internode. Using submerged whole plants and GA3-treated excised stem segments, we showed that lignin content and CAD activity were up to sixfold lower in newly formed internodal tissue of rapidly growing rice than in slowly growing tissue. No differences were observed in parts of the internode that had been formed prior to induction of growth. PAL activity was reduced throughout the internode of submerged plants. We conclude that lignification is one of the processes that is suppressed to permit rapid growth.