It is clear that plants have evolved sophisticated pathways for sensing and responding to changes in different components of the external nitrogen supply as well as their own internal nitrogen status. We speculate on the possibility that the effects elicited by external L-glutamate represent a novel form of foraging response that could potentially enhance a plant's ability to compete with its neighbours and micro-organisms for localized sources of organic nitrogen.

Background and Aims Regulation of water channel aquaporins (AQPs) provides another mechanism by which abscisic acid (ABA) may influence water flow through plants. To the best of our knowledge, no studies have addressed the changes in ABA levels, the abundance of AQPs and root cell hydraulic conductivity (Lp Cell ) in the same tissues. Thus, we followed the mechanisms by which ABA affects root hydraulics in an ABA-deficient barley mutant Az34 and its parental line 'Steptoe'. We compared the abundance of AQPs and ABA in cells to determine spatial correlations between AQP abundance and local ABA concentrations in different root tissues. In addition, abundance of AQPs and ABA in cortex cells was related to Lp Cell.Methods Root hydraulic conductivity (Lp Root ) was measured by means of root exudation analyses and Lp Cell using a cell pressure probe. The abundance of ABA and AQPs in root tissues was assessed through immunohistochemical analyses. Isoform-specific antibodies raised against HvPIP2;1, HvPIP2;2 and HvPIP2;5 were used.Key Results Immunolocalization revealed lower ABA levels in root tissues of Az34 compared with 'Steptoe'. Root hydraulic conductivity (Lp Root ) was lower in Az34, yet the abundance of HvPIPs in root tissues was similar in the two genotypes. Root hair formation occurred closer to the tip, while the length of the root hair zone was shorter in Az34 than in 'Steptoe'. Application of external ABA to the root medium of Az34 and 'Steptoe' increased the immunostaining of root cells for ABA and for HvPIP2;1 and HvPIP2;2 especially in root epidermal cells and the cortical cell layer located beneath, parallel to an increase in Lp Root and Lp Cell . Treatment of roots with Fenton reagent, which inhibits AQP activity, prevented the ABA-induced increase in root hydraulic conductivity.Conclusion Shortly after (<2 h) ABA application to the roots of ABA-deficient barley, increased tissue ABA concentrations and AQP abundance (especially the plasma-membrane localized isoforms HvPIP2;1 and HvPIP2;2) were spatially correlated in root epidermal cells and the cortical cell layer located beneath, in conjunction with increased Lp Cell of the cortical cells. In contrast, long-term ABA deficiency throughout seedling development affects root hydraulics through other mechanisms, in particular the developmental timing of the formation of root hairs closer to the root tip and the length of the root hair zone.

Although salinity inhibits plant growth, application of appropriate rhizosphere bacteria can diminish this negative effect. We studied one possible mechanism that may underlie this beneficial response. Wheat plants were inoculated with Bacillus subtilis IB-22 and Pseudomonas mandelii IB-Ki14 and their consequences for growth, water relations, and concentrations of the hormone abscisic acid (ABA) were followed in the presence of soil salinity. Salinity alone increased ABA concentration in wheat leaves and roots and this was associated with decreased stomatal conductance, but also with chlorophyll loss. Bacterial treatment raised ABA concentrations in roots, suppressed accumulation of leaf ABA, decreased chlorophyll loss, and promoted leaf area and transpiration. However, water balance was maintained due to increased water uptake by inoculated plants, brought about in part by a larger root system. The effect may be the outcome of ABA action since the hormone is known to maintain root extension in stressed plants. Root ABA concentration was highest in salt-stressed plants inoculated with B. subtilis and this contributed to greater root hydraulic conductivity. We conclude that bacteria can raise salt resistance in wheat by increasing root ABA, resulting in larger root systems that can also possess enhanced hydraulic conductivity thereby supporting better-hydrated leaves.

http://www.publish.csiro.au/nid/102.htm Functional Plant BiologyRapid changes in root HvPIP2;2 aquaporins abundance and ABA concentration are 1 required to enhance root hydraulic conductivity and maintain leaf water potential in 2 response to increased evaporative demand 3 4 Abstract. To address the involvement of abscisic acid (ABA) in regulating transpiration and root 14 hydraulic conductivity (Lp Root ) and their relative importance for maintaining leaf hydration, the 15 ABA-deficient barley mutant Az34 and its parental wild-type (WT) genotype (cv. Steptoe) were 16 grown in hydroponics and exposed to changes in atmospheric vapour pressure deficit (VPD) 17 imposed by air warming. WT plants were capable of maintaining leaf water potential (Ψ L ) that 18 was likely due to increased Lp Root enabling higher water flow from the roots, which increased in 19 response to air warming. The increased Lp Root and immunostaining for HvPIP2;2 aquaporins 20 correlated with increased root ABA content of WT plants when exposed to increased air 21 temperature. The failure of Az34 to maintain Ψ L during air warming may be due to lower Lp Root 22 than WT plants, and an inability to respond to changes in air temperature. The correlation 23 between root ABA content and Lp Root was further supported by increased root hydraulic 24 conductivity in both genotypes when treated with exogenous ABA (10 -5 M). Thus the ability of 25 the root system to rapidly regulate ABA levels (and thence aquaporin abundance and hydraulic 26 conductivity) seems important to maintain leaf hydration. 28Additional keywords: Hordeum vulgare L., absicisic acid, tissue hydration, water relations. 29 Page 2 of 24 http://www.publish.csiro.au/nid/102.htm Functional Plant Biology 54 lasting effects on plant hydraulic properties and aquaporin activity in maize (Parent et al. 2009) 55 and tomato (Thompson et al. 2007) plants. However, the role of ABA in regulating plant water 56 relations is likely to be most critical in response to abrupt step-changes in environmental 57 conditions. Thus we compared leaf water relations, AQPs abundance and ABA content and 58 localization in roots of the ABA deficient barley mutant (Az34) and its parental line cv. Steptoe 59in response to air heating (that increased evaporative demand). The goal of the work was to 60 check the ability of the root system to rapidly regulate ABA levels (and thence hydraulic 61 conductivity) and its importance to maintain leaf hydration. 63Material and Methods

The aim of the present report was to demonstrate how a novel approach for immunohistochemical localization of cytokinins in the leaf and particularly in the phloem may complement to the study of their long-distance transport. Different procedures of fixation were used to conjugate either cytokinin bases or their ribosides to proteins of cytoplasm to enable visualization and differential localization of these cytokinins in the leaf cells of wheat plants. In parallel to immunolocalization of cytokinins in the leaf cells, we immunoassayed distribution of free bases of cytokinins, their nucleotides and ribosides between roots and shoots of wheat plants as well as their presence in phloem sap after incubation of leaves in a solution supplemented with either trans-zeatin or isopentenyladenine. The obtained data show ribosylation of the zeatin applied to the leaves and its elevated level in the phloem sap supported by in vivo localization showing the presence of ribosylated forms of zeatin in leaf vessels. This suggests that conversion of zeatin to its riboside is important for the shoot-to-root transport of zeatin-type cytokinins in wheat. Exogenous isopentenyladenine was not modified, but diffused from the leaves as free base. These metabolic differences may not be universal and may depend on the plant species and age. Although the measurements of cytokinins in the phloem sap and root tissue is the most defining for determining cytokinin transport, study of immunolocalization of either free cytokinin bases or their ribosylated forms may be a valuable source of information for predicting their transport in the phloem and to the roots.

Although changes in root architecture in response to the environment can optimize mineral and water nutrient uptake, mechanisms regulating these changes are not well-understood. We investigated whether P deprivation effects on root development are mediated by abscisic acid (ABA) and its interactions with other hormones. The ABA-deficient barley mutant Az34 and its wild-type (WT) were grown in P-deprived and P-replete conditions, and hormones were measured in whole roots and root tips. Although P deprivation decreased growth in shoot mass similarly in both genotypes, only the WT increased primary root length and number of lateral roots. The effect was accompanied by ABA accumulation in root tips, a response not seen in Az34. Increased ABA in P-deprived WT was accompanied by decreased concentrations of cytokinin, an inhibitor of root extension. Furthermore, P-deficiency in the WT increased auxin concentration in whole root systems in association with increased root branching. In the ABA-deficient mutant, P-starvation failed to stimulate root elongation or promote branching, and there was no decline in cytokinin and no increase in auxin. The results demonstrate ABA’s ability to mediate in root growth responses to P starvation in barley, an effect linked to its effects on cytokinin and auxin concentrations.

Lipid transfer proteins (LTPs) participate in many important physiological processes in plants, including adaptation to stressors, e.g., salinity. Here we address the mechanism of this protective action of LTPs by studying the interaction between LTPs and abscisic acid (ABA, a “stress” hormone) and their mutual participation in suberin deposition in root endodermis of salt-stressed pea plants. Using immunohistochemistry we show for the first time NaCl induced accumulation of LTPs and ABA in the cell walls of phloem paralleled by suberin deposition in the endoderm region of pea roots. Unlike LTPs which were found localized around phloem cells, ABA was also present within phloem cells. In addition, ABA treatment resulted in both LTP and ABA accumulation in phloem cells and promoted root suberization. These results suggested the importance of NaCl-induced accumulation of ABA in increasing the abundance of LTPs and of suberin. Using molecular modeling and fluorescence spectroscopy we confirmed the ability of different plant LTPs, including pea Ps-LTP1, to bind ABA. We therefore hypothesize an involvement of plant LTPs in ABA transport (unloading from phloem) as part of the salinity adaptation mechanism.

An attempt was made to look into participation of cytokinins in regulation of stomatal behavior and growth response of plants to the presence of competitors in the neighborhood. The plants of tobacco (Nicotiana tabacum L.) and lettuce (Lactuca sativa L.) were accommodated by one or several plants per pot. In rival plants growing by two or three per pot, the level of transpiration considerably fell, accumulation of shoot biomass decelerated, and leaf area decreased as compared with single plants. Reduction in transpiration and plant biomass was accompanied by a decrease in the content of endogenous cytokinins. When the plants were grown in the presence of competitors for a long time, an increase in the content of hormones in trans genic tobacco plants as a result of heat induction of ipt gene and inoculation of lettuce plants with bacteria Bacillus subtilis, strain IB 22, producing cytokinin, reduced suppression of transpiration and shoot growth in competing plants. This suggests that cytokinins participate in late stages of plant adaptation to competitors.