Leaf micromorphology and sugar may contribute to differences in drought tolerance for two apple cultivars

Fruit crops combating drought: Physiological responses and regulatory pathways

Drought is a common stress in agricultural production. Thus, it is imperative to understand how fruit crops respond to drought and to develop drought-tolerant varieties. This paper provides an overview of the effects of drought on the vegetative and reproductive growth of fruits. We summarize the empirical studies that have assessed the physiological and molecular mechanisms of the drought response in fruit crops. This review focuses on the roles of calcium (Ca2+) signaling, abscisic acid (ABA), reactive oxygen species signaling, and protein phosphorylation underlying the early drought response in plants. We review the resulting downstream ABA-dependent and ABA-independent transcriptional regulation in fruit crops under drought stress. Moreover, we highlight the positive and negative regulatory mechanisms of microRNAs in the drought response of fruit crops. Lastly, strategies (including breeding and agricultural practices) to improve the drought resistance of fruit crops are outlined.

Engineering drought-tolerant apple by knocking down six GH3 genes and potential application of transgenic apple as a rootstock

Drought poses a major threat to apple fruit production and quality. Because of the apple's long juvenile phase, developing varieties with improved drought tolerance using biotechnology approaches is needed. Here, we used the RNAi approach to knock down six GH3 genes in the apple. Under prolonged drought stress, the MdGH3 RNAi plants performed better than wild-type plants and had stronger root systems, higher root-to-shoot ratio, greater hydraulic conductivity, increased photosynthetic capacity, and increased water use efficiency. Moreover, MdGH3 RNAi plants promoted the drought tolerance of the scion when they were used as rootstock, compared with wild-type and M9-T337 rootstocks. Scions grafted onto MdGH3 RNAi plants showed increased plant height, stem diameter, photosynthetic capacity, specific leaf weight, and water use efficiency. The use of MdGH3 RNAi plants as rootstocks can also increase the C/N ratio of the scion and achieve the same effect as the M9-T337 rootstock in promoting the flowering and fruiting of the scion. Notably, using MdGH3 RNAi plants as rootstocks did not reduce fruit weight and scion quality compared with using M9-T337 rootstock. Our research provides candidate genes and demonstrates a general approach that could be used to improve the drought tolerance of fruit trees without sacrificing the yield and quality of scion fruits.

Fine-tuning of SUMOylation modulates drought tolerance of apple

SUMOylation is involved in various aspects of plant biology, including drought stress. However, the relationship between SUMOylation and drought stress tolerance is complex; whether SUMOylation has a crosstalk with ubiquitination in response to drought stress remains largely unclear. In this study, we found that both increased and decreased SUMOylation led to increased survival of apple (Malus × domestica) under drought stress: both transgenic MdSUMO2A overexpressing (OE) plants and MdSUMO2 RNAi plants exhibited enhanced drought tolerance. We further confirmed that MdDREB2A is one of the MdSUMO2 targets. Both transgenic MdDREB2A OE and MdDREB2A^K192R OE plants (which lacked the key site of SUMOylation by MdSUMO2A) were more drought tolerant than wild-type plants. However, MdDREB2A^K192R OE plants had a much higher survival rate than MdDREB2A OE plants. We further showed SUMOylated MdDREB2A was conjugated with ubiquitin by MdRNF4 under drought stress, thereby triggering its protein degradation. In addition, MdRNF4 RNAi plants were more tolerant to drought stress. These results revealed the molecular mechanisms that underlie the relationship of SUMOylation with drought tolerance and provided evidence for the tight control of MdDREB2A accumulation under drought stress mediated by SUMOylation and ubiquitination.

Use of physiological and biochemical indicators to identify apple varieties resistant to drought

As a result of global climate change in the arid regions of the world, a decrease in the adaptability of fruit crops to drought has been noted. This explains the interest of researchers in the issues of increasing the drought resistance of fruit crops in various ways, one of which is the creation of resistant varieties as a result of selection. The use of physiological and biochemical parameters in the breeding process as diagnostic criteria for drought resistance and the identification of highly adaptive varieties of fruit crops is relevant all over the world. The objects of study are apple varieties of various ecological and geographical origins: Idared, Erli Mac, Dayton (USA), Prikubanskoe, Rassvet, Fortuna (Russia). Physiological parameters: the relative water content, the total content of chlorophylls (a + b), carotenoids, ascorbic acid in the leaves of the studied apple varieties were determined during periods before drought and during drought. Differences in the response of varieties to the impact of drought were revealed. It was found that apple varieties of local selection Prikubanskoye, Fortuna and American variety Idared have a greater ability to adapt to changing environmental conditions in comparison with other studied varieties. The revealed adaptive features during drought (retention of a high relative water content, stability of the pigment system, a low ratio of the total chlorophyll to carotenoids, an increased content of ascorbic acid) make it possible to use them in breeding as donors of drought resistance traits. The data of physiological and biochemical studies, obtained in the unique natural and climatic conditions of the North Caucasus region of Russia, complement fundamental research on the physiological foundations of apple tree adaptation to drought.

Response of sugar metabolism in apple leaves subjected to short-term drought stress

For a comprehensive understanding of gene expression, enzyme activity and sugar concentrations in response to short-term water deficit in apple (Greensleeves), sugar-modulated gene expression and enzyme activities were analyzed. Water stress resulted in the accumulation of sorbitol, glucose, fructose, galactose and starch, accompanied by a significant reduction in photosynthesis and sucrose concentration. In response to short-term water deficits, the activities of aldose-6-phosphate reductase (A6PR; EC 1.1.1.200), sorbitol dehydrogenase (SDH; EC 1.1.1.14), neutral invertase (NINV; EC 3.2.1.26), sucrose synthase (SUSY; EC 2.4.1.13), and fructokinase (FK; EC 2.7.1.4) were higher, whereas cell wall invertase (CWINV; EC 3.2.1.26) and hexokinase (HK; EC 2.7.1.1) activities were lower. In addition, sucrose phosphate synthase (SPS; EC 2.4.1.14) activity increased during the initial stages of dehydration and then decreased as the drought strengthened. Transcript levels of MdA6PR, MdSDH1/2, MdNINV1/2, MdSUSY3, MdFK1/2/4, MdSOT1/2, MdSUC1-3, MdTMT2/3, MdvGT1, MdpGlcT1-4 were upregulated, whereas transcript levels of MdCWINV1/2, MdHK1/2/3/5, and MdTMT1 were downregulated after 6 days of water stress. These findings suggest that the sorbitol metabolism pathway is induced and high levels of hexose derived from photosynthetic products are transported into vacuoles for adjustment to the water deficit. Our results provide insights into the relationships between sugar levels and sugar-modulated gene and enzyme activity in response to the imposition of short-term water stress.

Transcriptomic and physiological analyses reveal drought adaptation strategies in drought-tolerant and -susceptible watermelon genotypes

Drought stress has become one of the most urgent environmental hazards for horticultural crops. In this research, we analyzed watermelon adaptation strategies to drought stress in drought-tolerant (M20) and -susceptible (Y34) genotypes via transcriptomic and physiological analyses. After drought stress, a total of 6228 and 4311 differentially expressed genes (DEGs) were observed in Y34 and M20, respectively. Numerous DEGs were involved in various defense responses such as antioxidation, protein protection, osmotic adjustment, wax accumulation, hormone signaling, and melatonin biosynthesis. Accordingly, the contents of ABA, melatonin, wax, and some osmoprotectants were increased by drought stress in both Y34 and especially M20. Exogenous application of melatonin or ABA induced wax accumulation and drought tolerance and melatonin may function upstream of ABA. In comparison to Y34, M20 was more able to activate ABA signaling, melatonin biosynthesis, osmotic adjustment, antioxidation, and wax accumulation under drought stress. These stronger responses confer M20 tolerance to drought. Photosynthesis and most DEGs involved in photosynthesis and cell growth were decreased by drought stress in both M20 and especially Y34. For drought-susceptible genotypes, growth retardation may be an important mechanism for saving and redistributing resources in order to reprogram stress signaling networks.

The sweet side of global change-dynamic responses of non-structural carbohydrates to drought, elevated CO2 and nitrogen fertilization in tree species

Non-structural carbohydrates (NSC) play a central role in plant functioning as energy carriers and building blocks for primary and secondary metabolism. Many studies have investigated how environmental and anthropogenic changes, like increasingly frequent and severe drought episodes, elevated CO2 and atmospheric nitrogen (N) deposition, influence NSC concentrations in individual trees. However, this wealth of data has not been analyzed yet to identify general trends using a common statistical framework. A thorough understanding of tree responses to global change is required for making realistic predictions of vegetation dynamics. Here we compiled data from 57 experimental studies on 71 tree species and conducted a meta-analysis to evaluate general responses of stored soluble sugars, starch and total NSC (soluble sugars + starch) concentrations in different tree organs (foliage, above-ground wood and roots) to drought, elevated CO2 and N deposition. We found that drought significantly decreased total NSC in roots (-17.3%), but not in foliage and above-ground woody tissues (bole, branch, stem and/or twig). Elevated CO2 significantly increased total NSC in foliage (+26.2%) and roots (+12.8%), but not in above-ground wood. By contrast, total NSC significantly decreased in roots (-17.9%), increased in above-ground wood (+6.1%), but was unaffected in foliage from N fertilization. In addition, the response of NSC to three global change drivers was strongly affected by tree taxonomic type, leaf habit, tree age and treatment intensity. Our results pave the way for a better understanding of general tree function responses to drought, elevated CO2 and N fertilization. The existing data also reveal that more long-term studies on mature trees that allow testing interactions between these factors are urgently needed to provide a basis for forecasting tree responses to environmental change at the global scale.

Leaf physico-chemical and physiological properties of maize (Zea mays L.) populations from different origins

In this study we evaluated the leaf surface properties of maize populations native to different water availability environments. Leaf surface topography, wettability and gas exchange performance of five maize populations from the Sahara desert, dry (south) and humid (north-western) areas of Spain were analysed. Differences in wettability, stomatal and trichome densities, surface free energy and solubility parameter values were recorded between populations and leaf sides. Leaves from the humid Spanish population with special regard to the abaxial side, were less wettable and less susceptible to polar interactions. The higher wettability and hydrophilicity of Sahara populations with emphasis on the abaxial leaf surfaces, may favour dew deposition and foliar water absorption, hence improving water use efficiency under extremely dry conditions. Compared to the other Saharan populations, the dwarf one had a higher photosynthesis rate suggesting that dwarfism may be a strategy for improving plant tolerance to arid conditions. The results obtained for different maize populations suggest that leaf surfaces may vary in response to drought, but further studies will be required to examine the potential relationship between leaf surface properties and plant stress tolerance.

Selenium alleviates chromium toxicity by preventing oxidative stress in cabbage (Brassica campestris L. ssp. Pekinensis) leaves

The beneficial role of selenium (Se) in alleviation of chromium (Cr)-induced oxidative stress is well established. However, little is known about the underlying mechanism. The impacts of exogenous Se (0.1mg/L) on Cr(1mg/L)-induced oxidative stress and antioxidant systems in leaves of cabbage (Brassica campestris L. ssp. Pekinensis) were investigated by using cellular and biochemical approaches. The results showed that supplementation of the medium with Se was effective in reducing Cr-induced increased levels of lipid peroxides and superoxide free radicals (O(-)2(·)), as well as increasing activities of superoxide dismutase (SOD) and peroxidase (POD). Meanwhile, 1mg/L Cr induced loss of plasma membrane integrity, growth inhibition, as well as ultrastructural changes of leaves were significantly reversed due to Se supplementation in the medium. In addition, Se application significantly altered the subcellular distribution of Cr which transported from mitochondria, nucleus and the cell-wall material to the soluble fraction and chloroplasts. However, Se application did no significant alteration of Cr effects on osmotic adjustment accumulating products. The study suggested that Se is able to protect leaves of cabbage against Cr toxicity by alleviation of Cr induced oxidative stress, and re-distribution of Cr in the subcellular of the leaf. Furthermore, free radicals, lipid peroxides, activity of SOD and POD, and subcellular distribution of Cr can be considered the efficient biomarkers to indicate the efficiency of Se to detoxification Cr.