Agronomia

262

Review

TRENDS in Plant Science Vol.6 No.6 June 2001

Abiotic stress signalling pathways: specificity and cross-talk
Heather Knight and Marc R. Knight
Plants exhibit a variety of responses to abiotic stresses that enable them to tolerate and survive adverse conditions. As we learn more about the signalling pathways leading to these responses, it is becoming clear that they constitutea network that is interconnected at many levels. In this article, we discuss the ‘cross-talk’ between different signalling pathways and question whether there are any truly specific abiotic stress signalling responses.

Heather Knight* Marc R. Knight Dept Plant Sciences, University of Oxford, Oxford, UK OX1 3RB. *e-mail: heather.knight@plants. ox.ac.uk

Plants encounter a wide range ofenvironmental insults during a typical life cycle and have evolved mechanisms by which to increase their tolerance of these through both physical adaptations and interactive molecular and cellular changes that begin after the onset of stress. The first step in switching on such molecular responses is to perceive the stress as it occurs and to relay information about it through a signal transductionpathway. These pathways eventually lead to physiological changes, such as guard cell closure, or to the expression of genes and resultant modification of molecular and cellular processes. Our knowledge about the signalling pathways leading from stimulus to end response in plants has increased over recent years. It is increasingly apparent that the linear pathways that we have been studying areactually only part of a more complex signalling network and that there is much overlap between its branches, with, for instance, many genes inducible by more than one particular stimulus. In this article, we discuss two aspects of these abiotic stress signalling networks, namely cross-talk and specificity. We define ‘cross-talk’ as any instance of two signalling pathways from different stressors thatconverge. This might take the form of different pathways achieving the same end or of pathways interacting and affecting each other’s outcome, including the flux through one pathway affecting another. These might act in an additive or negatively regulatory way, or might compete for a target (Fig. 1). We define specificity as any part of a signalling pathway that enables distinction between two ormore possible outcomes and that thus might link a particular stimulus to a particular end response and not to any other end responses. Opportunities for both cross-talk and specificity can occur within a particular pathway.
Cross-talk

nature, however, the plant encounters stress combinations concurrently or separated temporally and must present an integrated response to them. In the case ofphytochrome signalling, the two pathways leading to red-light-induced CHS and CAB gene expression negatively regulate flux through one another1,2. Seemingly separate abiotic stress signalling pathways are also likely to interact in a similar manner. In addition, several abiotic stress pathways share common elements that are potential ‘nodes’ for cross-talk. Cross-talk can also occur between pathways indifferent organs of the plant when a systemic signal such as hydrogen peroxide moves from a stimulated cell into another tissue to elicit a response3.
Specificity

When stress signalling pathways are examined in the laboratory, they are usually considered in isolation from other stresses to simplify interpretation. In

In spite of considerable overlap between many abiotic stress signallingpathways, there might, in some instances, be a benefit to producing specific, inducible and appropriate responses that result in a specific change suited to the particular stress conditions encountered. One advantage would be to avoid the high energy cost of producing stress-tolerance proteins, exemplified by the dwarf phenotype of plants constitutively overexpressing the frost tolerance protein...