Plant, Cell and Environment (2002) 25, 195–210
ABA-based chemical signalling: the co-ordination of responses to stress in plants
S. WILKINSON & W. J.DAVIES
The Lancaster Environment Centre, Lancaster University, Bailrigg, Lancaster, LA1 4YQ, UK
There is now strong evidence that the plant hormone abscisic acid (ABA) plays an important role in the regulation of stomatal behaviour and gas exchange of droughted plants. This regulation involves both long-distance transport and modulation of ABA concentration at the guard cells, as wellas differential responses of the guard cells to a given dose of the hormone. We will describe how a plant can use the ABA signalling mechanism and other chemical signals to adjust the amount of water that it loses through its stomata in response to changes in both the rhizospheric and the aerial environment. The following components of the signalling process can play an important part inregulation: (a) ABA sequestration in the root; (b) ABA synthesis versus catabolism in the root; (c) the efﬁciency of ABA transfer across the root and into the xylem; (d) the exchange of ABA between the xylem lumen and the xylem parenchyma in the shoot; (e) the amount of ABA in the leaf symplastic reservoir and the efﬁciency of ABA sequestration and release from this compartment as regulated by factors suchas root and leaf-sourced changes in pH; (f) cleavage of ABA from ABA conjugates in the leaf apoplast; (g) transfer of ABA from the leaf into the phloem; (h) the sensitivity of the guard cells to the [ABA] that ﬁnally reaches them; and lastly (i) the possible interaction between nitrate stress and the ABA signal. Key-words: abscisic acid (ABA), apoplast, calcium, drought stress, drying soil, leaf,long-distance signalling, low temperature, nitrate, pH, PPFD, root, stomatal guard cells, symplast, VPD, xylem.
ABA AS A STRESS RESPONSE HORMONE
It has long been known that in order to conserve the water, nutrients and carbohydrates required for survival, plants respond to stresses such as soil drying by reducing leaf expansion and closing stomatal pores. In addition, root growth rates may bemaintained in order that the plant may continue to access water. Traditional explanations for drought-induced regulation of gas exchange and leaf growth have emphasized the importance of the decline in shoot water status, which commonly accompanies severe
Correspondence: Sally Wilkinson. Fax: +44 01524 843854; E-mail: email@example.com © 2002 Blackwell Science Ltd
soil drying(e.g. Kramer 1969). It is now accepted, however, that many of the plant’s responses to soil drying can occur in the absence of changes in shoot water status, via chemical signals. In one very clever series of experiments, pressure has been applied to roots to counteract the increasing soil suction that occurs as soil dries. This generated shoot water relations in droughted plants that werecomparable to those of well-watered plants. Despite this, rates of gas exchange and leaf growth were still restricted compared to rates shown by well-watered plants (e.g. Gollan, Schurr & Schulze 1992). Another apparent demonstration of root-sourced chemical signalling lies in the work of Gowing, Davies & Jones (1990). Here, the roots of young apple trees were split between two containers, the soil in oneof which was allowed to dry, while the other container was irrigated as normal. This treatment restricted the rate of leaf growth, which could be restored to control rates by removing the roots in contact with the drying soil, i.e. by removing the source of the chemical signal. Blackman & Davies (1985) used the same technique to demonstrate that chemical signals sent from drying soil could...