Signals in abscission

Summary Abscission is the term used to describe the process of natural separation of organs from the parent plant. This may be part of the highly programmed development of a plant, or in response to environmental stress. It enables temperate plants to overwinter and hence survive, but in agricultural or horticultural environments premature abscission can lead to significant crop losses. Abscission is the culmination of changes in gene expression, which result in the loosening of adjacent cell walls within the zone and subsequent cell separation. For many years it has been recognized that the balance between the plant hormones ethylene and auxin determine where, and when, separation takes place. As we begin to understand the mechanisms by which plant growth regulator signals are perceived and transduced, we can begin to understand how the process of abscission itself may be induced and regulated. This review details what we know of the signals that lead to the differentiation of zone cells; the environmental signals that promote cell separation, and the possible intracellular signalling events that culminate in organ shedding. Contents Summary 323 I. Introduction 323 II. Differentiation of the abscission zone 324 III. Environmental and developmental signals that induce abscission 327 1. Senescence 327 2. Photoperiod 327 3. Water stress 328 4. Wounding and pathogen attack 328 5. Ozone 329 IV. Inter‐ and intracellular signals 329 1. Role of ethylene and auxin 330 2. Mechanisms of plant hormone signal perception 330 3. Roles of other hormones and components of signal transduction 333 V. Conclusions 335 Acknowledgements 336 References 336