Hormones are in the air

Phytohormones act to control many aspects of plant growth, development, and responses to the environment. Genetic and biochemical studies have identified many of the factors that mediate hormonal control of cell division, expansion, differentiation, and, ultimately, death. However, despite major advances in our understanding of how individual hormones act, we know relatively little about how hormonal signals are integrated into overall responses. For example, the hormones indole-3-acetic acid (IAA), gibberellin (GA), brassinolide, and, in some circumstances, ethylene all promote stem elongation. But what is the role of each hormone, and how do they interact to mediate growth? An abundance of hormone-related mutants, particularly in Arabidopsis, has provided some insights into the mechanisms of cooperative hormone action. Note that the term “cross-talk” is frequently used in this context. However, classically, this term defines phenomena in which there is an unwanted transfer of signals from one circuit, channel, etc., to another (Oxford English Dictionary). These interactions are most certainly not “unwanted.” In a narrower context, crosstalk is generally limited to common elements within a signal transduction scheme. Such a definition clearly ignores a major source of phytohormone interactions: effects of one input signal on accumulation of other input signals. For example, genetic analyses have defined a complex relationship among sugars, abscisic acid (ABA), and ethylene (1, 2) that controls aspects of growth and development. This interaction is regulated at the level of both hormone synthesis and signal transduction. However, the molecular details of this and other interactions are far from understood.

Some of the best characterized examples of the complex interactions between phytohormones involve responses to biotic stress. For example, wound responses, such as those induced by chewing insects, integrate jasmonic acid (JA), ethylene, and systemin signaling pathways in a manner yet to be fully established (3, 4). Similarly, …