Abscisic acid (ABA), the universal plant stress hormone, is accumulated in roots subjected to a range of external stresses, including drought, salinity, and nutrient deficiencies. This accumulation is regulated by ABA-metabolism (biosynthesis and degradation), -recirculation, and -exudation. Stress ABA serves as a long-distance signal regulating the water relations of shoots (stomata, meristems) and roots (hydraulic conductivity, root development, desiccation tolerance). Endogenous ABA, radial water flows (JV), and radial ABA flows (JABA) are closely coupled. Here we described the relations between these processes that are crucial factors for the role of ABA as a stress hormone and a long-distance stress signal. Crop varieties with high ABA concentrations exhibit an intensified long distance ABA signalling that reduces water consumption and, in the case of grapevine, improves the quality of the berries.
Soil inoculation with the ACC deaminase-containing rhizobacterium Variovorax paradoxus 5C-2 increased pea (Pisum sativum) growth and yield in both well watered and drying soil, with an attenuation of systemic ACC signaling likely key in the latter case.1 However, inoculated plants also had increased xylem ABA concentrations (which may also promote growth) in drying soil. Possible mediation of ABA levels by V. paradoxus 5C-2 was investigated in two experiments in which maize (Zea mays) growth was promoted. Xylem ABA concentration of both inoculated and uninoculated plants increased similarly as leaf water potential decreased. Furthermore, hormone flow modeling showed a decreased phloem flow of ABA back to the root. Thus Variovorax paradoxus 5C-2 does not intensify ABA signaling in planta.
• During harvest, fleshy berry tomato fruits (Solanum lycopersicum) were wounded at their stem scar. Within 3 d, this wound was rapidly sealed by a process covering the wound site with a membranous layer which effectively protects the tomato fruit from excessive water loss, nutrient elution and the entry of pathogens. • Chemical analysis of the de novo synthesized stem scar tissue revealed the presence of aromatic and aliphatic components characteristic of the biopolyester suberin. • Gene expression patterns associated with suberization were identified at the stem scar region. Changes in the relative abundance of different transcripts suggested a potential involvement of the plant hormone abscisic acid (ABA) in the wound-healing processes. • The amount of ABA present in the stem scar tissue showed a significantly increased level during wound healing, whereas ABA-deficient mutants notabilis, flacca and sitiens were largely devoid of this rise in ABA levels. The mutant fruits showed a retarded and less efficient suberization response at the stem scar wound, whereas the rate and strength of this response were positively correlated with ABA content. These results clearly indicate in vivo the involvement of ABA in the suberization-based wound-healing processes at the stem scar tissue of tomato fruits.
Electron transport across the plasma membrane of Valerianella locusta mesophyll cells and intact fronds of Lemna gibba, induced by 10−3 M ferricyanide, was inhibited by tetcyclacis, an inhibitor regarded to be specific for cytochrome P-450 mono-oxygenases. The effect was dependent on the concentration of tetcyclacis and the duration of preincubation. The apparent rate of trans-membrane electron transport increased in the presence of catalase, indicating tetcyclacis-induced H2O2-production or additional tetcyclacis-independent H2O2 release. The findings suggest an interaction of cytochrome P-450 with the plasma membrane-located electron transport chain. This redox-chain could be involved in the degradation of abscisic acid, being located at the plasma membrane. This assumption is supported by the finding that ABA inhibits extracellular ferricyanide reduction.
Data on pH in free space, cytoplasm, chloroplasts and vacuoles in leaf tissue are used to calculate the distribution of abscisic acid (ABA) amongst these compartments, assuming that the intervening membranes are permeable to undissociated ABA only. Data on the permeability of membranes to ABA are used to calculate the time constant for equilibration between the free space and the other components. It is concluded that changes in pH in the chloroplast stroma due to irradiance, or other factors, will change the amount of ABA available to the guard cells via the free space, and that the time constant is similar to that for light-induced stomatal movement. The possibility that such changes play a role in modulating stomatal aperture is discussed.
• The effects of drought on European beech (Fagus sylvatica) were assessed in a pot experiment under controlled conditions. • Plants from 11 autochthonous provenances originating from regions in Germany, which differed in annual precipitation, were exposed to a 3-wk drought period in a glasshouse after the first stage of shoot growth had been completed. • Drought reduced the water content to 97% of control in leaves and axes and to 92% in the roots. A strong reduction of predawn water potential in roots and shoots, as well as on transpiration rate, was found. In the roots, the effect on water potential was the same for all provenances, but differences were observed in the shoot water potential. Leaf concentrations of abscisic acid (ABA), proline and sucrose increased in the drought-treated plants compared with the controls. • Two extreme clusters from opposite climatic sites were identified by cluster analysis. A drought-sensitive cluster, originating from regions with high annual precipitation, had low water potential and transpiration rates, as well as high concentrations of fructose, ABA and proline after drought. Water potential and transpiration rates were less affected by drought in the other cluster, which comprised two provenances of relatively dry habitats, and concentrations of hexose, ABA and proline were low.
Chamaegigas intrepidus Dinter (syn. Lindernia intrepidus (Dinter) Oberm.) is a poikilohydric aquatic plant that lives in rock pools on granitic outcrops in Central Namibia. The pools are only filled intermittently during the summer rains, and the plants can pass through 15–20 rehydration/dehydration cycles during a single wet season. Rehydrated plants also have to cope with substantial diurnal fluctuations in the pool pH as a result of photosynthetic CO 2 uptake. We have used in vivo 31 P NMR spectroscopy to investigate the effect of external pH and dehydration (low water potential) on intracellular pH in the roots and submerged leaves of C. intrepidus . Increasing the external pH from 6 to 10 had no effect on the steady state cytoplasmic and vacuolar pH values of submerged leaves, but caused a slight alkalinization of the root cytoplasm. Similarly dehydration with PEG‐600 at either pH 6 or pH 10 had no effect on the cytoplasmic pH of the leaves, but it did cause a small alkalinization of the leaf vacuoles at pH 10. These results imply an unusually effective regulation of intracellular pH, consistent with the adaptation of C. intrepidus to the extreme environmental conditions of its habitat. The NMR analysis also showed that dehydration had no effect on the inorganic phosphate and phosphocholine pools, and this was taken to indicate that the cell membranes were well protected from the effects of the low water potential.
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