THE AUXIN RESPONSE OF ACTIN IS ALTERED IN THE RICE MUTANT YIN-YANG

The rice mutant Yin-Yang has been selected during a screen for resista nce to cytoskeletal drugs and is characterized by alterations in epide rmal cell length and a precocious onset of gravitropism. The elongatio n response of coleoptile segments to auxin does not reveal changes of auxin sensitivity in Yin-Yang. However, in contrast to the wild type, cell elongation in Yin-Yang is highly sensitive to the actin-polymeris ation blocker cytochalasin D. This increased sensitivity to cytochalas in D requires optimal concentrations of auxin to become manifest. The auxin response of actin microfilaments in epidermal cells differs betw een wild type and mutant. In the wild type, the longitudinal microfila ment bundles become loosened in response to auxin. In the mutant, thes e bundles disintegrate partially and are replaced by a network of shor t filaments surrounding the nucleus. Several aspects of the mutant phe notype can be mimicked in the wild type by treatment with cytochalasin D. The mutant phenotype is discussed in terms of signal-dependent cha nges of actin dynamics and the putative role of actin during cell elon gation.