We have suggested above that it may be possible to use deficit irrigation to exploit the plant's long-distance signalling networks to enhance WUE in agriculture and to increase reproductive crop quality, in part by restricting vegetative crop development and the commitment of resources to this end (Yang et al., 2001; Davies et al., 2002). As soil dries, shoot water status can be sustained by signalling-induced restrictions in stomatal aperture (e.g. Mingo et al., 2003; Sobeih et al., 2004) (Figure 5.4). If as an alternative approach for different circumstances where we want to sustain vegetative growth we can develop genotypes that do not produce chemical leaf growth inhibitors as soil dries or have leaf growth processes that are insensitive to these signals, then we can perhaps also sustain biomass accumulation and yield of vegetative plant parts when water supply for agriculture is restricted. This strategy is dependent on identifying the different chemical signals that limit both stomatal conductance and leaf expansion during drought - if indeed there are different regulators of the two processes. While decreased plant water use (caused by the limitation to both stomatal conductance and leaf expansion) can allow the plant to husband immediately available water resources, another strategy might be for the roots to explore deeper parts of the soil profile (Reid & Renquist, 1997). Manipulation of this variable may provide extra water supply to growing shoots and allow maintenance of shoot growth processes at low bulk soil water status.
In many plants, drought increases root and xylem concentrations of the ethylene precursor ACC (Gomez-Cadenas et al., 1996). Although the delivery of ACC from the root system can account for shoot ethylene evolution (Else & Jackson, 1998) and may thus limit leaf growth under drought, the relationship between xylem ACC concentration and leaf growth of plants exposed to drying soil has not been defined. We have recently shown that in tomato both xylem ACC and ABA concentrations increased in response to partial rootzone drying (PRD), prior to any decrease in shoot water status (Sobeih et al., 2004). It is therefore appropriate to assay the interaction between these two hormones on leaf expansion using well-hydrated plants. Feeding ABA and ACC simultaneously via the xylem to detached shoots inhibits leaf growth additively (I.C. Dodd, unpublished results 2005), suggesting an important role for ethylene in the inhibition of leaf growth in drying soil, when shoot water status is maintained. In contrast, in plants at low water potential, ABA accumulation is necessary to minimise high rates of ethylene synthesis and ethylene-mediated root growth inhibition (LeNoble et al., 2004).
Under drought the plant hormone ethylene can be involved in both the suppression of root growth during soil drying (see above) and the suppression of leaf growth via long-distance chemical signalling, again emphasising a key role for this hormone in the regulation of plant production in water scarce environments. Our recent work has shown that ethylene evolution of wild-type (WT) tomato plants increased as soil dried but could be suppressed using transgenic (ACO1as) plants containing an antisense gene for one isoenzyme of ACC oxidase. Most importantly, ACO1AS plants also showed no inhibition of leaf growth when exposed to PRD, even though both ACO1AS and WT plants showed similar changes in other putative chemical inhibitors of leaf expansion (xylem sap pH and ABA concentration). It seems likely that the enhanced ethylene evolution under PRD is responsible for leaf growth inhibition of WT plants. ACO1AS plants showed no leaf growth inhibition over a range of soil water contents, which significantly restricted growth of WT plants (Figure 5.5), but it is important to note that this lack of drought sensitivity was only apparent when leaf turgor was maintained by ABA/pH signalling, reducing stomatal conductance in response to PRD.
Transgenic approaches to enhance drought tolerance may be effective but are not always socially acceptable. It may be important, therefore, that certain bacteria occurring on the root surface contain high levels of the enzyme ACC deaminase that will degrade the ethylene precursor ACC. Since a dynamic equilibrium of ACC concentration exists between root, rhizosphere and bacterium, bacterial uptake of rhizospheric ACC (for use as a carbon and nitrogen source) may decrease root ACC concentration and root ethylene evolution and may potentially increase root growth (Glick et al., 1998). Our recent experiments (A. Belimov, unpublished results 2005) with the plant growth-promoting bacterium Variovorax showed that pea plants grown with the bacterium added to the soil showed a promotion of root biomass, leaf area and total biomass relative to uninoculated plants in drying soil, suggesting that these effects were mediated by modifying plant ethylene status.
Sustaining leaf growth in water scarce environments has proved to be a particularly intractable problem for plant breeders. We suggest here a model system for plant production in an increasingly water scarce world. It involves a combination of genetic (suppression of ethylene signalling) and agronomical manipulations (promotion of ABA signalling by deficit irrigation) to exploit basic plant physiology and plant developmental responses (in this case, soil to root to shoot signalling pathways). The aim is to sustain some turgor as the soil dries via partial stomatal closure and to sustain leaf growth and biomass production via this maintenance
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