INTERNODAL ELONGATION AND ORIENTATION OF CELLULOSE MICROFIBRILS AND MICROTUBULES IN DEEP-WATER RICE

Excised stem sections of deepwater rice (Oryza sativa L.) containing t he highest internode were used to study the induction of rapid interno dal elongation by gibberellin (GA). It has been shown before that this growth response is based on enhanced cell division in the intercalary meristem and on increased cell elongation. In both GA-treated and con trol stem sections, the basal 5-mm region of the highest internode gro ws at the fastest rate. During 24 h of GA treatment, the internodal el ongation zone expands from 15 to 35 mm. Gibberellin does not promote e longation of internodes from which the intercalary meristem has been e xcised. The orientation of cellulose microfibrils (CMFs) is a determin ing factor in cell growth. Elongation is favored when CMFs are oriente d transversely to the direction of growth while elongation is limited when CMFs are oriented in the oblique or longitudinal direction. The o rientation of CMFs in parenchymal cells of GA-treated and control inte rnodes is transverse throughout the internode, indicating that CMFs do not restrict elongation of these cells. Changes in CMF orientation we re observed in epidermal cells, however. In the basal 5-mm zone of the internode, which includes the intercalary meristem, CMFs of the epide rmal cell walls are transversely oriented in both GA-treated and contr ol stem sections. In slowly growing control internodes, CMF orientatio n changes to the oblique as cells are displaced from this basal 5-mm z one to the region above it. In GA-treated rapidly growing internodes, the reorientation of CMFs from the transverse to the oblique is more g radual and extends over the 35-mm length of the elongation zone. The C MFs of older epidermal cells are obliquely oriented in control and GA- treated internodes. The orientation of the CMFs parallels that of the cortical microtubules. This is consistent with the hypothesis that cor tical microtubules determine the direction of CMF deposition. We concl ude that GA acts on cells that have transversely oriented CMFs but doe s not promote growth of cells whose CMFs are already obliquely oriente d at the start of GA treatment.