Summary• Decreased soil water availability can stimulate production of the plant hormone ethylene and inhibit plant growth. Strategies aimed at decreasing stress ethylene evolution might attenuate its negative effects.• An environmentally benign (nonchemical) method of modifying crop ethylene relations -soil inoculation with a natural root-associated bacterium Variovorax paradoxus 5C-2 (containing the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase that degrades the ethylene precursor ACC), was assessed with pea (Pisum sativum) plants grown in drying soil.• Inoculation with V. paradoxus 5C-2, but not with a transposome mutant with massively decreased ACC deaminase activity, improved growth, yield and water-use efficiency of droughted peas. Systemic effects of V. paradoxus 5C-2 included an amplified soil drying-induced increase of xylem abscisic acid (ABA) concentration, but an attenuated soil drying-induced increase of xylem ACC concentration. A local bacterial effect was increased nodulation by symbiotic nitrogen-fixing bacteria, which prevented a drought-induced decrease in nodulation and seed nitrogen content.• Successfully deploying a single bacterial gene in the rhizosphere increased yield and nutritive value of plants grown in drying soil, via both local and systemic hormone signalling. Such bacteria may provide an easily realized, economic means of sustaining crop yields and using irrigation water more efficiently in dryland agriculture.

Stomatal pores of higher plants close in response to decreases in atmospheric relative humidity (RH). This is believed to be a mechanism that prevents the plant from losing excess water when exposed to a dry atmosphere and as such is likely to have been of evolutionary significance during the colonization of terrestrial environments by the embryophytes. We have conducted a genetic screen, based on infrared thermal imaging, to identify Arabidopsis genes involved in the stomatal response to reduced RH. Here we report the characterization of two genes, identified during this screen, which are involved in the guard cell reduced RH signaling pathway. Both genes encode proteins known to be involved in guard cell ABA signaling. OST1 encodes a protein kinase involved in ABA-mediated stomatal closure while ABA2 encodes an enzyme involved in ABA biosynthesis. These results suggest, in contrast to previously published work, that ABA plays a role in the signal transduction pathway connecting decreases in RH to reductions in stomatal aperture. The identification of OST1 as a component required in stomatal RH and ABA signal transduction supports the proposition that guard cell signaling is organized as a network in which some intracellular signaling proteins are shared among different stimuli.

To evaluate the role of root-synthesized ABA in regulating growth and stomatal behaviour under well-watered conditions, isogenic wild-type (WT) and ABA-deficient flacca (flc) tomato (Solanum lycopersicum) were reciprocally and self-grafted just below the cotyledonary node. Since flc scions had lower leaf water potentials due to higher transpiration rates, a subset of all graft combinations was grown under a shoot misting treatment to minimize differences in shoot water status. Misting did not alter the relative effects of the different graft combinations on leaf area. WT scions had the greatest leaf area and lowest whole plant transpiration rate irrespective of the rootstock, implying that shoot ABA biosynthesis was sufficient to account for a WT shoot phenotype. In WT scions, the rootstock had no effect on detached leaf ethylene evolution or xylem concentrations of ABA or the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC). In flc scions, although the WT rootstock suppressed stomatal conductance of individual leaves, there was no detectable effect on whole plant transpiration rate. However, leaf area of flc/WT (scion/rootstock) plants increased 1.6-fold compared to flc self-grafts. WT rootstocks increased xylem ABA concentration in flc scions (relative to flc self-grafts) up to 3-fold, and resulted in xylem ACC concentrations and detached leaf ethylene evolution similar to WT scions. Since the WT rootstock normalized shoot ethylene relations but only partially restored the leaf area of flc scions (relative to that of WT scions), shoot ABA biosynthesis can directly promote leaf area via an unknown, ethylene-independent, mechanism.

Tomato (Lycopersicon esculentum Mill.) plants were grown in either a glasshouse (GH) or a controlled environment cabinet (CEC) to assess the effects of partial rootzone drying (PRD) on biomass allocation. Control and PRD plants received the same amounts of water. In control plants, water was equally distributed between two compartments of a split-root system. In PRD plants, only one compartment was watered while the other was allowed to dry. At the end of each drying cycle, wet and dry compartments were alternated. In the GH, total biomass did not differ between PRD and control plants after four cycles of PRD, but PRD increased root biomass by 55% as resources were partitioned away from shoot organs. In the CEC, leaf water potential did not differ between treatments at the end of either of two cycles of PRD, but stomatal conductance of PRD plants was 20% less at the end of the first cycle than at the beginning. After two cycles of PRD in the CEC, biomass did not differ between PRD and control plants, but PRD increased root biomass by 19% over the control plants. The promotion of root biomass in PRD plants was associated with the alternation of wet and dry compartments, with increased root biomass occurring in the re-watered compartment after previous exposure to soil drying. Promotion of root biomass in field-grown PRD plants may allow the root system to access resources (water and nutrients) that would otherwise be unavailable to control plants. This may contribute to the ability of PRD plants to maintain similar leaf water potentials to conventionally irrigated plants, even when smaller irrigation volumes are supplied.

Partial rootzone drying (PRD) is an irrigation technique where water is distributed unevenly to the root system such that part is irrigated while the remainder is allowed to dry the soil. Tomato (Lycopersicon esculentum Mill.) plants were grown with their roots in two soil columns to compare the physiological consequences of alternation of wet and dry columns during PRD irrigation (alternate PRD, PRD-A) with retention of the same wet and dry columns (fixed PRD, PRD-F). When PRD plants received 50% less water than well-watered (WW) plants, xylem ABA concentration ([X-ABA]) increased and stomatal conductance decreased relative to WW plants. Although both sets of PRD plants received the same amount of water, [X-ABA] of PRD-A plants increased up to 2-fold above that of PRD-F plants, which further decreased stomatal conductance. Differences in [X-ABA] were detected within an hour of alternation, but did not persist beyond the photoperiod of alternation. [X-ABA] increased linearly as whole-pot soil water content (θpot) and leaf water potential (Ψleaf) declined, but the difference in [X-ABA] between the two sets of PRD plants was not due to differences in either θpot or Ψleaf. In PRD-F plants, the unwatered part of the root system contributes proportionally less to the transpiration stream as the soil progressively dries (Yao et al. 2001, Plant, Cell & Environment 24, 227–235). In PRD-A plants, we hypothesise that re-watering the dry part of the root system allows these roots to contribute proportionally more to total sap flux, thus liberating a pulse of ABA to the transpiration stream as the root ABA pool accumulated during soil drying is depleted. Since the enhancement of [X-ABA] caused by PRD-A increased as θpot and Ψleaf declined, an optimal frequency of alternation to maximise the cumulative physiological effects of this ABA pulse must consider possible negative impacts of leaf water deficit as soil water status declines.