Cytokinins (CKs) may be involved in the regulation of plant adaptation to drought stress. The objectives of the study were to identify proteomic changes in leaves and roots in relation to improved drought tolerance in transgenic creeping bentgrass (Agrostis stolonifera) containing a senescence-activated promoter (SAG12) and the isopentyl transferase (ipt) transgene that increases endogenous CK content. Leaves of SAG12-ipt bentgrass exhibited less severe senescence under water stress, as demonstrated by maintaining lower electrolyte leakage and lipid peroxidation, and higher photochemical efficiency (Fv/Fm), compared with the null transformant (NT) plants. SAG12-ipt plants had higher root/shoot ratios and lower lipid peroxidation in leaves under water stress than the NT plants. The suppression of drought-induced leaf senescence and root dieback in the transgenic plants was associated with the maintenance of greater antioxidant enzyme activities (superoxide dismutase, peroxidase, and catalase). The SAG12-ipt and NT plants exhibited differential protein expression patterns under well-watered and drought conditions in both leaves and roots. Under equivalent leaf water deficit (47% relative water content), SAG12-ipt plants maintained higher abundance of proteins involved in (i) energy production within both photosynthesis and respiration [ribulose 1,5-bisphosphate carboxylase (RuBisCO) and glyceraldehyde phosphate dehydrogenase (GAPDH)]; (ii) amino acid synthesis (methionine and glutamine); (iii) protein synthesis and destination [chloroplastic elongation factor (EF-Tu) and protein disulphide isomerases (PDIs)]; and (iv) antioxidant defence system (catalase and peroxidase) than the NT plants. These results suggest that increased endogenous CKs under drought stress may directly or indirectly regulate protein abundance and enzymatic activities involved in the above-mentioned metabolic processes, thereby enhancing plant drought tolerance.
Increased endogenous plant cytokinin (CK) content through transformation with an adenine isopentyl transferase (ipt) gene has been associated with improved plant drought tolerance. The objective of this study is to determine metabolic changes associated with elevated CK production in ipt transgenic creeping bentgrass (Agrostis stolonifera L.) with improved drought tolerance. Null transformants (NTs) and plants transformed with ipt controlled by a stress- or senescence-activated promoter (SAG12-ipt) were exposed to well-watered conditions or drought stress by withholding irrigation in an environmental growth chamber. Physiological analysis confirmed that the SAG12-ipt line (S41) had improved drought tolerance compared with the NT plants. Specific metabolite changes over the course of drought stress and differential accumulation of metabolites in SAG12-ipt plants compared with NT plants at the same level of leaf relative water content (47% RWC) were identified using gas chromatography–mass spectroscopy. The metabolite profiling analysis detected 45 metabolites differentially accumulated in response to ipt expression or drought stress, which included amino acids, carbohydrates, organic acids, and organic alcohols. The enhanced drought tolerance of SAG12-ipt plants was associated with the maintenance of accumulation of several metabolites, particularly amino acids (proline, γ-aminobutyric acid, alanine, and glycine) carbohydrates (sucrose, fructose, maltose, and ribose), and organic acids that are mainly involved in the citric acid cycle. The accumulation of these metabolites could contribute to improved drought tolerance due to their roles in the stress response pathways such as stress signalling, osmotic adjustment, and respiration for energy production.
Water stress reduces endogenous cytokinin (CK) content and may inhibit CK production. Maintenance of endogenous CK levels by genetic transformation with ipt in leaves and roots undergoing senescence may promote stress tolerance. This study was designed to determine the physiological effects of ipt expression on immature and mature leaves and in roots for plants exposed to different levels of water stress for creeping bentgrass (Agrostis stolonifera). Plants containing the ipt gene, encoding the enzyme adenine isopentenyl phosphotransferase for CK synthesis ligated to a senescence-activated promoter (SAG12), and wild-type ‘Penncross’ (WT) were grown hydroponically in a growth chamber and exposed to water stress by weekly additions of polyethylene glycol 8000 to reduce the growing solution osmotic potential from –0.05 to –0.3, –0.5, –0.7, –1.0, and –1.4 MPa. Immature and mature leaves and roots of SAG12-ipt creeping bentgrass were evaluated for ipt expression, CK content, leaf relative water content (RWC), chlorophyll content (Chl), photochemical efficiency (FvFm), osmotic adjustment (OA), photosynthesis rate (Pn), stomatal conductance (gs), transpiration (E), water use efficiency (WUE), carbon isotope discrimination (Δ), and root viability. Expression of ipt was detected in all plant parts and a higher CK content, primarily in the form of isopentyladenine (iPa), was found in SAG12-ipt plants but not in the WT plants under water stress. Immature leaves exhibited higher iPa and OA at all treatment levels. Mature leaves of SAG12-ipt plants maintained higher OA, Pn, Chl, WUE, and Δ, whereas gs and E were relatively unaffected compared to the WT. Roots of SAG12-ipt plants had higher levels of iPa and greater root viability than the WT. The results demonstrate that expression of ipt enhanced the tolerance of creeping bentgrass to water stress, which could be attributed to the positive effects on osmotic adjustment, efficient water use, and maintaining higher photosynthetic rate primarily for mature leaves, as well as increased root viability.
Increased endogenous chitosan (CTS) could be associated with improved drought resistance in white clover (Trifolium repens). Plants were pretreated with or without 1 mg/mL CTS and then were subjected to optimal or water-limited condition in controlled growth chambers for 6 days. Phenotypic and physiological results indicated that exogenous CTS significantly improved drought resistance of white clover. Metabolome results showed that exogenous CTS induced a significant increase in endogenous CTS content during dehydration accompanied by the maintenance of greater accumulation of sugars, sugar alcohols, amino acids, organic acids, and other metabolites (ascorbate, glutathione, flavonoids, putrescine, and spermidine). These compounds are associated with osmotic adjustment, antioxidant defense, stress signaling, and energy metabolism under stress condition. Similarly, transcriptome revealed that many genes in relation to amino acid and carbohydrate metabolism, energy production and conversion, and ascorbate-glutathione and flavonoid metabolism were significantly up-regulated by CTS in response to dehydration stress. CTS-induced drought resistance was associated with the accumulation of stress protective metabolites, the enhancement of ascorbate-glutathione and tricarboxylic acid cycle, and increases in the γ-aminobutyric acid shunt, polyamine synthesis, and flavonoids metabolism contributing to improved osmotic adjustment, antioxidant capacity, stress signaling, and energy production for stress defense, thereby maintaining metabolic homeostasis under dehydration stress.
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