Tetraploid Rangpur lime rootstock increases drought tolerance via enhanced constitutive root abscisic acid production

Whole‐genome duplication, or polyploidy, is common in many plant species and often leads to better adaptation to adverse environmental condition. However, little is known about the physiological and molecular determinants underlying adaptation. We examined the drought tolerance in diploid (2x) and autotetraploid (4x) clones of Rangpur lime (Citrus limonia) rootstocks grafted with 2x Valencia Delta sweet orange (Citrus sinensis) scions, named V/2xRL and V/4xRL, respectively. Physiological experiments to study root–shoot communication associated with gene expression studies in roots and leaves were performed. V/4xRL was much more tolerant to water deficit than V/2xRL. Gene expression analysis in leaves and roots showed that more genes related to the response to water stress were differentially expressed in V/2xRL than in V/4xRL. Prior to the stress, when comparing V/4xRL to V/2xRL, V/4xRL leaves had lower stomatal conductance and greater abscisic acid (ABA) content. In roots, ABA content was higher in V/4xRL and was associated to a greater expression of drought responsive genes, including CsNCED1, a pivotal regulatory gene of ABA biosynthesis. We conclude that tetraploidy modifies the expression of genes in Rangpur lime citrus roots to regulate long‐distance ABA signalling and adaptation to stress.     

ABA in bryophytes: how a universal growth regulator in life became a plant hormone?

Abscisic acid (ABA) is not a plant-specific compound but one found in organisms across kingdoms from bacteria to animals, suggesting that it is a ubiquitous and versatile substance that can modulate physiological functions of various organisms. Recent studies have shown that plants developed an elegant system for ABA sensing and early signal transduction mechanisms to modulate responses to environmental stresses for survival in terrestrial conditions. ABA-induced increase in stress tolerance has been reported not only in vascular plants but also in non-vascular bryophytes. Since bryophytes are the key group of organisms in the context of plant evolution, clarification of their ABA-dependent processes is important for understanding evolutionary adaptation of land plants. Molecular approaches using Physcomitrella patens have revealed that ABA plays a role in dehydration stress tolerance in mosses, which comprise a major group of bryophytes. Furthermore, we recently reported that signaling machinery for ABA responses is also conserved in liverworts, representing the most basal members of extant land plant lineage. Conservation of the mechanism for ABA sensing and responses in angiosperms and basal land plants suggests that acquisition of this mechanism for stress tolerance in vegetative tissues was one of the critical evolutionary events for adaptation to the land. This review describes the role of ABA in basal land plants as well as non-land plant organisms and further elaborates on recent progress in molecular studies of model bryophytes by comparative and functional genomic approaches.     

AsHSP17, a creeping bentgrass small heat shock protein modulates plant photosynthesis and ABA‐dependent and independent signalling to attenuate plant response to abiotic stress

Heat shock proteins (HSPs) are molecular chaperones that accumulate in response to heat and other abiotic stressors. Small HSPs (sHSPs) belong to the most ubiquitous HSP subgroup with molecular weights ranging from 12 to 42 kDa. We have cloned a new sHSP gene, AsHSP17 from creeping bentgrass (Agrostis stolonifera) and studied its role in plant response to environmental stress. AsHSP17 encodes a protein of 17 kDa. Its expression was strongly induced by heat in both leaf and root tissues, and by salt and abscisic acid (ABA) in roots. Transgenic Arabidopsis plants constitutively expressing AsHSP17 exhibited enhanced sensitivity to heat and salt stress accompanied by reduced leaf chlorophyll content and decreased photosynthesis under both normal and stressed conditions compared to wild type. Overexpression of AsHSP17 also led to hypersensitivity to exogenous ABA and salinity during germination and post‐germinative growth. Gene expression analysis indicated that AsHSP17 modulates expression of photosynthesis‐related genes and regulates ABA biosynthesis, metabolism and ABA signalling as well as ABA‐independent stress signalling. Our results suggest that AsHSP17 may function as a protein chaperone to negatively regulate plant responses to adverse environmental stresses through modulating photosynthesis and ABA‐dependent and independent signalling pathways.     

The PP2A‐interactor TIP41 modulates ABA responses in Arabidopsis thaliana

Modulation of growth in response to environmental cues is a fundamental aspect of plant adaptation to abiotic stresses. TIP41 (TAP42 INTERACTING PROTEIN OF 41 kDa) is the Arabidopsis thaliana orthologue of proteins isolated in mammals and yeast that participate in the Target‐of‐Rapamycin (TOR) pathway, which modifies cell growth in response to nutrient status and environmental conditions. Here, we characterized the function of TIP41 in Arabidopsis. Expression analyses showed that TIP41 is constitutively expressed in vascular tissues, and is induced following long‐term exposure to NaCl, polyethylene glycol and abscisic acid (ABA), suggesting a role of TIP41 in adaptation to abiotic stress. Visualization of a fusion protein with yellow fluorescent protein indicated that TIP41 is localized in the cytoplasm and the nucleus. Abolished expression of TIP41 results in smaller plants with a lower number of rosette leaves and lateral roots, and an increased sensitivity to treatments with chemical TOR inhibitors, indicating that TOR signalling is affected in these mutants. In addition, tip41 mutants are hypersensitive to ABA at germination and seedling stage, whereas over‐expressing plants show higher tolerance. Several TOR‐ and ABA‐responsive genes are differentially expressed in tip41, including iron homeostasis, senescence and ethylene‐associated genes. In yeast and mammals, TIP41 provides a link between the TOR pathway and the protein phosphatase 2A (PP2A), which in plants participates in several ABA‐mediated mechanisms. Here, we showed an interaction of TIP41 with the catalytic subunit of PP2A. Taken together, these results offer important insights into the function of Arabidopsis TIP41 in the modulation of plant growth and ABA responses.     

Abscisic acid prevents pollen abortion under high‐temperature stress by mediating sugar metabolism in rice spikelets

Heat stress at the pollen mother cell (PMC) meiotic stage leads to pollen sterility in rice, in which the reactive oxygen species (ROS) and sugar homeostasis are always adversely affected. This damage is reversed by abscisic acid (ABA), but the mechanisms underlying the interactions among the ABA, sugar metabolism, ROS and heat shock proteins in rice spikelets under heat stress are unclear. Two rice genotypes, Zhefu802 (a recurrent parent) and fgl (its near‐isogenic line) were subjected to heat stress of 40°C after pre‐foliage sprayed with ABA and its biosynthetic inhibitor fluridone at the meiotic stage of PMC. The results revealed that exogenous application of ABA reduced pollen sterility caused by heat stress. This was achieved through various means, including: increased levels of soluble sugars, starch and non‐structural carbohydrates, markedly higher relative expression levels of heat shock proteins (HSP24.1 and HSP71.1) and genes related to sugar metabolism and transport, such as sucrose transporters (SUT) genes, sucrose synthase (SUS) genes and invertase (INV) genes as well as increased antioxidant activities and increased content of adenosine triphosphate and endogenous ABA in spikelets. In short, exogenous application of ABA prior to heat stress enhanced sucrose transport and accelerated sucrose metabolism to maintain the carbon balance and energy homeostasis, thus ABA contributed to heat tolerance in rice.     

Ectopic expression of wheat expansin gene TaEXPA2 improved the salt tolerance of transgenic tobacco by regulating Na+/K+ and antioxidant competence

High salinity is one of the most serious environmental stresses that limit crop growth. Expansins are cell wall proteins that regulate plant development and abiotic stress tolerance by mediating cell wall expansion. We studied the function of a wheat expansin gene, TaEXPA2, in salt stress tolerance by overexpressing it in tobacco. Overexpression of TaEXPA2 enhanced the salt stress tolerance of transgenic tobacco plants as indicated by the presence of higher germination rates, longer root length, more lateral roots, higher survival rates and more green leaves under salt stress than in the wild type (WT). Further, when leaf disks of WT plants were incubated in cell wall protein extracts from the transgenic tobacco plants, their chlorophyll content was higher under salt stress, and this improvement from TaEXPA2 overexpression in transgenic tobacco was inhibited by TaEXPA2 protein antibody. The water status of transgenic tobacco plants was improved, perhaps by the accumulation of osmolytes such as proline and soluble sugar. TaEXPA2‐overexpressing tobacco lines exhibited lower Na⁺ but higher K⁺ accumulation than WT plants. Antioxidant competence increased in the transgenic plants because of the increased activity of antioxidant enzymes. TaEXPA2 protein abundance in wheat was induced by NaCl, and ABA signaling was involved. Gene expression regulation was involved in the enhanced salt stress tolerance of the TaEXPA2 transgenic plants. Our results suggest that TaEXPA2 overexpression confers salt stress tolerance on the transgenic plants, and this is associated with improved water status, Na⁺/K⁺ homeostasis, and antioxidant competence. ABA signaling participates in TaEXPA2‐regulated salt stress tolerance.     

Celastrol,produced by <Emphasis Type="Italic">Tripterygium wilfordii</Emphasis> Hook F. enhances defense response in cucumber seedlings against diverse environmental stresses

Celastrol is an active triterpenoid compound isolated from Tripterygium wilfordii Hook F.. Many reports have highlighted that celastrol is an effective, safe and desirable approach to the treatment of cancers. However, their biological function during environmental stresses in plants is rarely reported. In the present study, the effects of celastrol on the tolerance against high light (HL), polyethylene glycol (PEG) and cold stress in cucumber (Cucumis sativus L.) were investigated. Celastrol pretreatment could enhance cucumber seedlings stress tolerance at a concentration of 1 μg ml^–1. The results showed that pretreatment with 1 μg ml^–1 celastrol clearly induced the activities of antioxidative enzymes, which subsequently alleviated stress-induced oxidative damage in plant cells. We also provided evidence that celastrol upregulated ABA biosynthetic gene NCED2 expression and ABA accumulation in cucumber seedlings, which resulted to the enhanced tolerance in response to environmental stresses. Furthermore, the celastrol-pretreated seedlings showed less photosystem damaged caused by the stress conditions, when compared with the control. Therefore, our findings provide a novel role of celastrol in plant against environmental stresses and indicate that the celastrol-induced activities of antioxidative enzymes and ABA content might contribute to the stress tolerance.     

Plant Response to Integrated Impact of Natural and Anthropogenic Stress Factors

Responses of cultivated plants (Lepidium sativum L. and Lycopersicon esculentum Mill.), i.e., changes in growth characteristics, pigment and proline content, to an integrated impact of natural (temperature) and anthropogenic (substrate acidity, population density, and heavy metals) stress factors were studied under controlled environmental conditions. The experiments were carried out in a phytotron (tomato) and under laboratory conditions (garden cress). The external stress factors produced a general suppression of growth and pigment synthesis and differentially affected individual biological characteristics of the plants. Each of the stress factors narrowed the interval of plant tolerance to other stress factors. Strong impact of one stress factor affected plant homeostasis mechanisms by weakening plant response to other stress factors.     

Comparative Analysis of Endogenous Hormones in Leaves and Roots of Two Contrasting Malus Species in Response to Hypoxia Stress

Plant hormones play important roles in regulating developmental processes and signaling networks involved in plant responses to biotic and abiotic stresses. We comparatively studied the growth and endogenous hormonal levels in leaves and roots in two Malus species (M. sieversii and M. hupehensis) differing in hypoxia tolerance under normoxic and hypoxia stress. The results showed that hypoxia stress inhibited growth of seedlings of both Malus species, but with significant differences in intensity. Exposure to hypoxia altered the levels of endogenous hormones in leaves and roots in both Malus seedlings. Leaf and root abscisic acid (ABA) contents increased in response to hypoxia stress in both genotypes despite different extents. Compared with M. hupehensis, M. sieversii was more responsive to hypoxia stress, resulting in larger increases in leaf and root ABA contents. The changes in leaf and root ABA contents correlating with the different tolerance levels of the genotypes confirm the involvement of this hormone in plant responses to hypoxia stress. Gibberellins (GAs; GA₁ + GA₄) continuously increased in leaves and roots during the whole period of stress, whereas indole-3-acetic acid (IAA) showed a sharp increase at the early stage in both Malus seedlings. In addition, zeatin riboside (ZR), dihydrozeatin riboside (DHZR), and isopentenyl adenine (IPA) differed in their pattern of changes in both Malus seedlings under hypoxia stress. Based on variations in endogenous hormonal levels in both Malus species that differ in their ability to tolerate hypoxia, we conclude that not a single hormone but multiple hormones and their interplay are responsible for hypoxia tolerance.     

Carbon dioxide enrichment alleviates heat stress by improving cellular redox homeostasis through an ABA‐independent process in tomato plants

Plant responses to elevated CO₂ and high temperature are critically regulated through a complex network of phytohormones and redox homeostasis. However, the involvement of abscisic acid (ABA) in plant adaptation to heat stress under elevated CO₂ conditions has not been thoroughly studied. This study investigated the interactive effects of elevated CO₂ (800 μmol·mol^?1) and heat stress (42 °C for 24 h) on the endogenous level of ABA and the cellular redox state of two genotypes of tomato with different ABA biosynthesis capacities. Heat stress significantly decreased maximum photochemical efficiency of PSII (Fv/Fm) and leaf water potential, but also increased levels of malondialdehyde (MDA) and electrolyte leakage (EL) in both genotypes. Heat‐induced damage was more severe in the ABA‐deficient mutant notabilis (not) than in its parental cultivar Ailsa Craig (Ailsa), suggesting that a certain level of endogenous ABA is required to minimise the heat‐induced oxidative damage to the photosynthetic apparatus. Irrespective of genotype, the enrichment of CO₂ remarkably stimulated Fv/Fm, MDA and EL in heat‐stressed plants towards enhanced tolerance. In addition, elevated CO₂ significantly strengthened the antioxidant capacity of heat‐stressed tomato seedlings towards a reduced cellular redox state for a prolonged period, thereby mitigating oxidative stress. However, elevated CO₂ and heat stress did not alter the endogenous level of ABA or the expression of its biosynthetic gene NCED2 in either genotype, indicating that ABA is not involved in elevated CO₂‐induced heat stress alleviation. The results of this study suggest that elevated CO₂ alleviated heat stress through efficient regulation of the cellular redox poise in an ABA‐independent manner in tomato plants.