Rhizosphere manipulations to maximize 'crop per drop' during deficit irrigation.

Although much of global agriculture is rain-fed, production is frequently (and sometimes catastrophically) constrained by rainfall. Supplementary irrigation can stabilize yield from year-to-year and conventionally has aimed to meet full crop evapotranspiration (ET), since the relationship between ET and crop yield is near-linear at suboptimal water supply (Fereres and Soriano, 2007). Changes in climate (rainfall patterns) and/or resource management (irrigation quotas) will mean that future crops, either unintentionally or deliberately, will receive deficit irrigation (DI, less water than crop ET), necessarily drying the soil, limiting leaf expansion and gas exchange and, consequently, yield. Although decreased cellular turgor can limit leaf growth and gas exchange, under many circumstances plant roots can sense drying soil, and transmit chemical signals to the shoots to regulate their physiology (Davies and Zhang, 1991; Dodd et al., 1996). Much work has aimed to substantiate this ‘chemical signalling hypothesis’ by determining the production and distribution of various signals (e.g. the plant hormones ABA, cytokinins, ethylene), their role in regulating plant responses to soil drying (e.g. using mutants and/or transgenics altered in signal synthesis or sensitivity), and the importance of the root system as a signal source (e.g. using reciprocal grafts of wild-type plants and such mutants and/or transgenics; Dodd, 2005; Hartung and Wilkinson, 2009). This research area is largely ‘ABA-centric’, in part, due to its undoubted importance in regulating plant water use (Hartung and Wilkinson, 2009) and relative ease of measurement (Dodd et al., 1996). Other signals have been relatively ignored, even though mild soil drying can significantly change both plant cytokinin (Kudoyarova et al., 2007) and ethylene (Sobeih et al., 2004) status. High-throughput, multianalyte physico-chemical techniques to quantify plant hormones in multiple plant organs (especially reproductive structures that directly influence crop yield) following rhizospheric stress (Albacete et al., 2008) need to be applied to real plants growing in the field under realistic soil drying scenarios. Such information will provide a sound physiological basis to underpin efforts aimed at manipulating long-distance hormonal signalling in planta. Several genetic manipulations have altered plant hormone signalling in crop plants, although the role of the root system (and its contribution to long-distance signalling) has not specifically been elucidated. A maize ACC synthase mutant with decreased leaf ethylene synthesis showed delayed leaf senescence and greater photosynthesis under drought compared to wild-type plants (Young et al., 2004). Brassica napus plants with genetically enhanced stomatal sensitivity to ABA showed increased seed yield under drought (Y Wang et al., 2005). Tomato plants with constitutive ABA overproduction showed increased leaf and whole plant water use efficiency (Thompson et al., 2007), but their delayed leaf area development (hence soil coverage to minimize evaporation) may diminish the expected gains in crop-level water use efficiency. Notwithstanding any physiological limitations of such technologies, socioeconomic factors may mitigate against their widespread adoption, such as consumer acceptance of GM crops and the significant biotechnological effort (and associated cost) required to make such genetic manipulations available in all crops/varieties. For these reasons, there remains a need for management options to minimize the yield penalties of deficit-irrigated crops. This article emphasises progress in understanding how rhizosphere manipulations, specifically partial rootzone drying and the introduction of plant growth-promoting rhizobacteria, alter plant root-to-shoot signalling to regulate leaf expansion and gas exchange and hence yield.