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

Abstract

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). IS IT REALLY NECESSARY TO TAKE ACCOUNT OF CHEMICAL SlCNALlNC OF S o l 1 DRYING?To provide firm evidence for root-to-shoot signaling of the effects of soil drying, it is necessary to artificially manipulate the plant to break the link between soil drying and reduced water uptake. Passioura (1987) has done this successfully using a pressure vessel placed around the roots of a plant growing in drying soil. As the soil dries, pressure is increased to balance the increase in soil suction. Pressurized plants show shoot water relations that are similar to those of wellwatered plants, even though the roots are in contact with drylng soil. Reductions in leaf growth rate and stomatal conductance (Passioura, 1988; Gollan et al., 1992) must, therefore, be attributed to an effect of soil drying that does not require a change in shoot water status.In another experiment, Gowing et al. (1990) divided the roots of small apple trees into two containers. Soil dryrng in one container restricted leaf expansion and leaf initiation with no obvious effect on shoot water relations. When roots in contact with drylng soil were severed from the plant, leaf growth rate recovered to that shown by well-watered plants. It seems unlikely that this treatment could make more water available to the shoots, and a more likely explanation for the restriction in shoot growth is the increased supply of an inhibitor origina...

Stomatal Responses in Rainfed Lowland Rice to Partial Soil Drying ; Evidence for Root Signals

“…This is supported by the stronger negative correlation of soil moisture tension with g s than with Ψ L (R = −0.71 * and −0.30 ns , respectively). These findings are consistent with the previous studies of Bano et al (1993), Davies et al (1994), Dingkuhn et al (1999), and Liu et al (2003), all of them suggested a primary role for root signals in regulating g s and leaf growth in rice and other crops under water defi cit. Our data provide strong evidence for root-to-shoot communication in rice, whereby the portion of roots in drying soil wielded a regulatory function on leaves, but this function was gradually removed upon excision of the droughted roots.…”

“…Accumulated evidence suggests that both chemical and hydraulic signals are operative and integrated in the regulation of leaf growth and stomatal conductance when plants are grown under drought stress (Davies et al, 1994;Comstock, 2002). Their relative importance is likely to depend on plant size and growth conditions (Tardieu and Davies, 1993).…”

“…Root tips have been reported to communicate with shoots to regulate stomatal closure as soil dries, even when plant water status is high; this observation is termed root signals (Sharp and Davies, 1979;Davies and Zhang, 1991;Davies et al, 1994). Root tips appear to respond to soil drying by increasing the biosynthesis of ABA Hartung et al, 2002;Hose et al, 2002).…”

Correlations of stomatal conductance with hydraulic and chemical factors in several deciduous tree species in a natural habitat

“…The traditional view is that stomatal conductance (g s ) is controlled chiefly via leaf water status. It now appears that stomata, at least of plants in drying soil, might also be regulated by root-toshoot movement of chemical factors (Zhang et al, 1987 ;Davies et al, 1994 ;Tardieu et al, 1996 ; to exert a predominant influence, with leaf water status acting to modify stomatal sensitivity to the chemical signal (Tardieu & Davies, 1993 ;. Strong correlations between g s and xylem sap [ABA] have been reported under many conditions for several species, including trees (e.g.…”

Do we know how plants sense a drying soil?

“…Consequently, the traditional view of leaf water potential as a measure of water stress in plants (i.e. hydraulic signals) was proposed to be abandoned (SINCLAIR & LUDLOW, 1985;PASSIOURA, 1988a), and chemical signals produced in the roots dominated the view on how plants sense a drying soil during the late 80's and early 90's (ZHANG & DAVIES, 1989;DAVIES & ZHANG, 1991;DAVIES et al, 1994). KRAMER (1988) criticized this view arguing that, at that time, plant physiologists had spent 50 years of progress in the understanding of plant water relations by shifting emphasis away from the soil to the shoots, and now, plant physiologists would have to return the emphasis to the soil based on experiments that used approaches with little application to the real world.…”