Calcium and Second Messengers in Hormonal Regulation

Abstract

From my point of view, there are three major ambiguities in the present understanding of the plant hormone field: (1) There is a frustrating lack of specificity in the functions regulated by the various hormones; that is, nearly any major developmental function in plant growth and development can be altered by any of the five plant hormones. This confusing overlap in regulation has been recognized for more than a decade [14]; (2) In many instances there is no apparent correlation between the amount of a given hormone present in plants and the activity or function which the hormone is presumed to regulate. Thus, a hormone which is presumed to be an inhibitor of growth, such as ABA, is often found in greatest abundance in tissues which are growing most rapidly. The report of high ABA levels in rapidly growing rice plants is a vivid example (Takahashi, this volume); and (3) After five decades of research on plant hormones, we still are ignorant of the mechanism of action of any one of them.