How Do Chemical Signals Work in Plants that Grow in Drying Soil?

We have known for many years that the dehydration of plant cells can lead to accumulation of the plant growth regulator ABA. Application of this compound to well-watered plants mimics many of the effects of soil drymg on gene expression, physiology, growth, and development, making this compound a strong candidate for a role in the droughted plant. Dehydration of leaves can result in massive accumulations of ABA, and roots also synthesize the compound in increased amounts as they are exposed to drier and drier soil. Davies and Zhang (1991) argued that an important component of the drought responses of many plants can be an ABA signal moving from the roots to the shoots to regulate physiology and development as a function of soil water status/availability. Many recent reports show relationships between stomatal conductance and soil water status or xylem ABA concentration, which seem to support this view. Nevertheless, critica1 examination of the ABA-signaling hypothesis must show that enough extra ABA moves in the transpiration stream to the shoots to account for the changes in functioning that are recorded. Many other chemicals moving in the xylem to shoots can also provide shoots with 'information" conceming root functioning, and we must consider the nature of such signals. We should also be concemed with the nature of the information that might be transmitted by a root signal (eg. a measure of soil water status or soil water availability) and the form that such a chemical signal might take (e.g. the concentration of the signal molecule in the transpiration stream or the flux of signal molecules to the site of action in the leaf).