PHYTOCHROME DEPENDENT DECREASE OF GIBBERELLIN-SENSITIVITY - A CASE-STUDY OF CELL EXTENSION GROWTH IN THE MESOCOTYL OF JAPONICA AND INDICA TYPE RICE CULTIVARS

Exogenous gibberellin removes the genetical suppression of mesocotyl elongation in dark-grown seedlings of the rice cultivar 'Nihon Masari' (japonica type). This gibberellin effect can be cancelled by light. All light effects can be accounted for by phytochrome. Dose-response and fluence-response studies show that phytochrome induces a reduction of the sensitivity to exogenous gibberellins. A cytological analysis of cell elongation and cortical microtubules led to a model where gibberellin and red light regulate mesocotyl elongation by controlling microtubule orientation in the epidermis of the mesocotyl. This causes corresponding changes of cellular extension growth, which can account for a large part of the observed growth responses. Comparative studies involving antimicrotubular drugs and gibberellin-synthesis inhibitors in the rice cultivar 'Kasarath' (indica type) and a hybrid cultivar suggest that some of the differences between the cultivars are due to differences in gibberellin-sensitivity.