| Drought is one of the most significant abiotic stress that influences the normal growth and crop production of plants. Watermelon [Citrullus lanatus(thunb.) Matsum. & Nakai] is an economically important fruit horticultural crop worldwide. However, it is easily influenced by not only waterlogging but also drought stress. During its life cycle, a large amount of water is needed for irrigation. In recent years, the frequently occurring drought stresses have greatly restricted the development of watermelon industry. Therefore, mining and using the new drought tolerant germplasms, and better understanding the mechanisms underlying drought tolerance of those germplasms will provide great practical significance for improving the resistance of the existing watemelon cultivars. In addition, with the increasing proposal of developing water-saving and organic agriculture, the application of bene?cial microorganisms are suggested to be an important developing direction to improve crop plants’ drought tolerance during cultivation in the future. Previous study reported that arbuscular mycorrhizal(AM) colonization could improve the fruit yield and water use ef?ciency of watermelon grown under deficit water conditions; however, the exact mechanisms remain unknown.In this study, with the continuous drought stress treatment to 12 pot-grown watermelon genotypes originated from different regions, their drought resistances were determined according to the drought injury and membership function evaluation methods. On this basis,two watermelon germplasms with marked difference in drought tolerance were further studied:The changes in plant growth, stomatal characteristics, photosynthesis, autioxidant enzyme activities and drought responsive gene expression profiles of them were compared; and the digital gene expression profiles of their leaves were also analysed by the utilization of RNA-Seq. Furthermore, the drought-sensitive watermelon variety(Y34) was inoculated with G. versiforme, and the AM fungal effects on the drought tolerance of watermelon plants were investigated. The main results of the study are as follows:(1) Under drought treatment, the 12 watermelon genotypes exhibited great difference in drought tolerance based on the occurrence of drought injury and the degree of injury severity.According to drought injury index and membership function values, three wild watermelon genotypes including M20, Y-2, and KY-3 were drought tolerant while Y34, 04-1-2 and Golden Girl were drought sensitive, and the rest genotypes were among the medium.(2) Under drought stress, drought-sensitive genotype Y34 presented the induced symptoms of wilting and leaf chlorosis earlier and more severe than the drought-resistant genotype M20, suggesting that Y34 suffered more leaf damage effects. Drought stress inhibited the growth of both genotypes but increased the root/shoot ratio more pronouncedly in M20 than in Y34. Scanning electron microscopy observation showed that leaves from M20 plants had greater trichome densities than thoes from Y34 plants, this coupled with the fact that M20 could close its stoma earlier to minimize the transpiration rate by regulating the transcript levels of WRKY70-like and MYB96-like genes, allowed M20 maintain a higher leaf relative water content than Y34. Compared with Y34, M20 experienced a lower decrease in photosystem II ef?ciency, initial Rubisco activity and chlorophyll concentration, indicating that M20 could more effectively regulate the energy bifurcation between photochemical and non-photochemical events, thus lessening the injury on leaf photosynthetic apparatus from drought stress. Under drought treatment, SOD, CAT, APX and GR activities were increased in both genotypes, but M20 showed higher increasement than Y34; and the related-gene(except APX) expression levels were also higher in M20 plants. Therefore, the increases in leaf H2O2, O2-, and MDA contents were smaller in M20. To counter the progressively reduced soil moisture, M20 accumulated more soluble sugars and proline contents to promote osmotic potential. These adaptive mechanisms enabled M20 to recover more rapidly after re-watering.(3) The digital gene expression profiles showed that drought induced great changes in transcriptional level of genes in watermelon plants. At the early drought stage,drought-resistant genotype M20 could activate many defense responses by up-regulating corresponding genes related to cell redox homeostasis and homeostatic process et al., while drought-sensitive genotype Y34 started to initiate macroautophagy process. Moreover, at the late drought stage, Y34 largely up-regulated the genes related to protein catabolic process.Compared with Y34, M20 had higher constitutive expression levels of antioxidant enzymes encoding genes such as ascorbic acid oxidase and peroxidase, disease-related genes and as well as defense-responsive genes.(4) Under drought stress, AMF colonization increased the leaf RWC and chlorophyll contents, improving the plant growth of watermelon seedlings, especially the root growth.Drought altered and damaged the chloroplast ultrastructure of watermelon leaves, but AMF inoculation mitigated the negative effects on chloroplast ultrastructure, Pn, initial Rubisco activity, Fv/Fm, ΦPS Ⅱ, ETR and q P, and enhanced i WUE and NPQ. Relative to the non-mycorrhizal plants, the SOD, CAT, APX, GR, and MDHAR activities in the drought stressed mycorrhizal plants were enhanced by 23.47%, 24.58%, 10.28%, 69.49%, and 25.85%,respectively; and the corresponding transcriptional levels of related-genes were also increased.Therefore, the contents of MDA, O2-, and H2O2 were at lower levels while the ASA/DHA and GSH/GSSG ratios were maintained at higher levels in the AMF treated plants. All these cumulative effects of AM symbiosis ultimately increased the drought tolerance of watermelon seedlings.(5) AMF inoculation enhanced the root surface area, root volume, forks number, fine root percentage and the root activity. Under drought stress, the accumulation of MDA, H2O2 and O2- in watermelon roots was significantly elevated. However, AMF inoculation led to significant up-regulation of antioxidant enzyme activities in watermelon roots upon drought imposition, and also made the antioxidants keeping at normal levels to sustain the ratios of their reduced states to oxidation states at high values; so that mycorrhizal plants were able to prohibit the overproduction of ROS. Additionally, AMF mycorrhization also increased the accumulation of soluble sugar and proline contents. In conclusion, our study indicated that AMF colonization enhanced the drought resistance of watermelon,which was attribute to the improved root systems, as well as the promoted root antioxidant and osmoregulation abilities. |
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