Plants can maintain growth and reproductive success by sensing changes in the environment and reacting through mechanisms at molecular, cellular, physiological, and developmental levels. Each stress condition prompts a unique response although some overlap between the reactions to abiotic stress (drought, heat, cold, salt or high light) and to biotic stress (pathogens) does occur. A common feature in the response to all stresses is the onset of oxidative stress, through the production of reactive oxygen species (ROS). As hydrogen peroxide and superoxide are involved in stress signaling, a tight control in ROS homeostasis requires a delicate balance of systems involved in their generation and degradation. If the plant lacks the capacity to generate scavenging potential, this can ultimately lead to death. In grapevine, antioxidant homeostasis can be considered at whole plant levels and during the development cycle. The most striking example lies in berries and their derivatives, such as wine, with nutraceutical properties associated with their antioxidant capacity. Antioxidant homeostasis is tightly regulated in leaves, assuring a positive balance between photosynthesis and respiration, explaining the tolerance of many grapevine varieties to extreme environments. In this review we will focus on antioxidant metabolites, antioxidant enzymes, transcriptional regulation and cross-talk with hormones prompted by abiotic stress conditions. We will also discuss three situations that require specific homeostasis balance: biotic stress, the oxidative burst in berries at veraison and in vitro systems. The genetic plasticity of the antioxidant homeostasis response put in evidence by the different levels of tolerance to stress presented by grapevine varieties will be addressed. The gathered information is relevant to foster varietal adaptation to impending climate changes, to assist breeders in choosing the more adapted varieties and suitable viticulture practices.

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Heat stress is a major limiting factor of grapevine production and quality. Acclimation and recovery are essential to ensure plant survival, and the recovery mechanisms can be independent of the heat response mechanisms. An experimental set up with and without acclimation to heat followed by recovery [stepwise acclimation and recovery (SAR) and stepwise recovery (SR), respectively] was applied to two grapevine varieties, Touriga Nacional (TN), and Trincadeira (TR), with different tolerance to abiotic stress. Major differences were found between leaves of SAR and SR, especially after recovery; in SAR, almost all parameters returned to basal levels while in SR they remained altered. Acclimation led to a swifter and short-term antioxidative response, affecting the plant to a lesser extent than SR. Significant differences were found among varieties: upon stress, TN significantly increased ascorbate and glutathione reduction levels, boosting the cell's redox-buffering capacity, while TR needed to synthesize both metabolites, its response being insufficient to keep the redox state at working levels. TR was affected by stress for a longer period and the up-regulation pattern of antioxidative stress genes was more obvious. In TN, heat shock proteins were significantly induced, but the canonical heat-stress gene signature was not evident probably because no shutdown of the housekeeping metabolism was needed.

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Worldwide, extensive agricultural losses are attributed to drought, often in combination with heat in Mediterranean climate regions, where grapevine traditionally grows. The available scenarios for climate change suggest increases in aridity in these regions. Under natural conditions plants are affected by a combination of stresses, triggering synergistic or antagonistic physiological, metabolic or transcriptomic responses unique to the combination. However the study of such stresses in a controlled environment can elucidate important mechanisms by allowing the separation of the effects of individual stresses. To gather those effects, cuttings of two grapevine varieties, Touriga Nacional (TN) and Trincadeira (TR), were grown under controlled conditions and subjected to three abiotic stresses (drought - WS, heat - HS and high light - LS) individually and in combination two-by-two (WSHS, WSLS, HSLS) or all three (WSHSLS). Photosynthesis, water status, contents of H2 O2 , abscisic acid and metabolites of the ascorbate-glutathione cycle were measured in the leaves. Common and distinct response features were identified in the different stress combinations. Photosynthesis was not hindered in TN by LS, while even individual stresses severely affect photosynthesis in TR. Abscisic acid may be implicated in grapevine osmotic responses since it is correlated with tolerance parameters, especially in combined stresses involving drought. Overall, the responses to drought-including treatments were clearly distinct to those without drought. From the specific behaviours of the varieties, it can be concluded that TN shows a higher capacity for heat dissipation and for withstanding high light intensities, indicating better adjustment to warm conditions, provided that water supply is plentiful.

Background: Predicted climate changes announce an increase of extreme environmental conditions including drought and excessive heat and light in classical viticultural regions. Thus, understanding how grapevine responds to these conditions and how different genotypes can adapt, is crucial for informed decisions on accurate viticultural actions. Global transcriptome analyses are useful for this purpose as the response to these abiotic stresses involves the interplay of complex and diverse cascades of physiological, cellular and molecular events. The main goal of the present work was to evaluate the response to diverse imposed abiotic stresses at the transcriptome level and to compare the response of two grapevine varieties with contrasting physiological trends, Trincadeira (TR) and Touriga Nacional (TN).

BackgroundQuantitative real time polymerase chain reaction is becoming the primary tool for detecting mRNA and transcription data analysis as it shows to have advantages over other more commonly used techniques. Nevertheless, it also presents a few shortcomings, with the most import being the need for data normalisation, usually with a reference gene. Therefore the choice of the reference gene(s) is of great importance for correct data analysis. Microarray data, when available, can be of great assistance when choosing reference genes. Grapevine was submitted to water stress and heat stress as well as a combination of both to test the stability of the possible reference genes.ResultsUsing the analysis of microarray data available for grapevine, six possible reference genes were selected for RT-qPCR validation: PADCP, ubiq, TIF, TIF-GTP, VH1-IK, aladin-related. Two additional genes that are commonly used as reference genes were included: act and L2. The stability of those genes was tested in leaves of grapevine in both field plants and in greenhouse plants under water or heat stress or a combination of both. Gene stability was analyzed with the softwares GeNorm, NormFinder and the ΔCq method resulting in several combinations of reference genes suitable for data normalisation. In order to assess the best combination, the reference genes were tested in putative stress marker genes (PCO, Galsynt, BKCoAS and HSP17) also chosen from the same microarray, in water stress, heat stress and the combination of both.ConclusionsEach method selected different gene combinations (PADCP + act, TIF + TIF-GTP and ubiq + act). However, as none of the combinations diverged significantly from the others used to normalize the expression of the putative stress marker genes, then any combination is suitable for data normalisation under the conditions tested. Here we prove the accuracy of choosing grapevine reference genes for RT-qPCR through a microarray analysis.

Grapevine (Vitis vinifera L.) is one of the most economically important fruit crops in the world. Deficit in nitrogen, phosphorus and sulfur nutrition impairs essential metabolic pathways. The influence of mineral stress in the composition of the plant cell wall (CW) has received residual attention. Using grapevine callus as a model system, 6 weeks deficiency of those elements caused a significant decrease in growth. Callus CWs were analyzed by Fourier transform infrared spectroscopy (FT-IR), by quantification of CW components and by immunolocalization of CW epitopes with monoclonal antibodies. PCA analysis of FT-IR data suggested changes in the main components of the CW in response to individual mineral stress. Decreased cellulose, modifications in pectin methyl esterification and increase of structural proteins were among the events disclosed by FT-IR analysis. Chemical analyses supported some of the assumptions and further disclosed an increase in lignin content under nitrogen deficiency, suggesting a compensation of cellulose by lignin. Moreover, polysaccharides of callus under mineral deficiency showed to be more tightly bonded to the CW, probably due to a more extensive cross-linking of the cellulose-hemicellulose network. Our work showed that mineral stress impacts the CW at different extents according to the withdrawn mineral element, and that the modifications in a given CW component are compensated by the synthesis and/or alternative linking between polymers. The overall results here described for the first time pinpoint the CW of Vitis callus different strategies to overcome mineral stress, depending on how essential they are to cell growth and plant development.