Grafting is an important agricultural technique widely used to improve plant growth, yield, and adaptation to either biotic or abiotic stresses. However, the molecular mechanisms underlying grafting-induced physiological processes remain unclear. Watermelon (Citrullus lanatus L.) is an important horticultural crop worldwide. Grafting technique is commonly used in watermelon production for improving its tolerance to stresses, especially to the soil-borne fusarium wilt disease. In the present study, we used high-throughput sequencing to perform a genome-wide transcript analysis of scions from watermelon grafted onto bottle gourd and squash rootstocks. Our transcriptome and digital gene expression (DGE) profiling data provided insights into the molecular aspects of gene regulation in grafted watermelon. Compared with self-grafted watermelon, there were 787 and 3485 genes differentially expressed in watermelon grafted onto bottle gourd and squash rootstocks, respectively. These genes were associated with primary and secondary metabolism, hormone signaling, transcription factors, transporters, and response to stimuli. Grafting led to changes in expression of these genes, suggesting that they may play important roles in mediating the physiological processes of grafted seedlings. The potential roles of the grafting-responsive mRNAs in diverse biological and metabolic processes were discussed. Obviously, the data obtained in this study provide an excellent resource for unraveling the mechanisms of candidate genes function in diverse biological processes and in environmental adaptation in a graft system.

BackgroundThe plant-specific TCP transcription factor family, which is involved in the regulation of cell growth and proliferation, performs diverse functions in multiple aspects of plant growth and development. However, no comprehensive analysis of the TCP family in watermelon (Citrullus lanatus) has been undertaken previously.ResultsA total of 27 watermelon TCP encoding genes distributed on nine chromosomes were identified. Phylogenetic analysis clustered the genes into 11 distinct subgroups. Furthermore, phylogenetic and structural analyses distinguished two homology classes within the ClTCP family, designated Class I and Class II. The Class II genes were differentiated into two subclasses, the CIN subclass and the CYC/TB1 subclass. The expression patterns of all members were determined by semi-quantitative PCR. The functions of two ClTCP genes, ClTCP14a and ClTCP15, in regulating plant height were confirmed by ectopic expression in Arabidopsis wild-type and ortholog mutants.ConclusionsThis study represents the first genome-wide analysis of the watermelon TCP gene family, which provides valuable information for understanding the classification and functions of the TCP genes in watermelon.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-016-0765-9) contains supplementary material, which is available to authorized users.

Boron (B) is a microelement required in vascular plants at a high concentration that produces excess boron and toxicity in many crops. B stress occurs widely and limits plant growth and crop productivity worldwide. Salicylic acid (SA) is an essential hormone in plants and is a phenolic compound. The goal of this work is to explore the role of SA in the alleviation of excess B (10 mg L−1) in watermelon plants at a morphological and biochemical level. Excess boron altered the nutrient concentrations and caused a significant reduction in morphological criteria; chlorophyll a, b, and carotenoids; net photosynthetic rate; and the stomatal conductance and transpiration rate of watermelon seedlings, while intercellular carbon dioxide (CO2) was significantly increased compared to the control plants (0.5 mg L−1 B). Furthermore, excess boron accelerated the generation of reactive oxygen species (ROS), such as hydrogen peroxide (H2O2) and induced cellular oxidative injury. The application of exogenous SA significantly increased chlorophyll and carotenoid contents in plants exposed to excess B (10 mg L−1), in line with the role of SA in alleviating chlorosis caused by B stress. Exogenously applied SA promoted photosynthesis and, consequently, biomass production in watermelon seedlings treated with a high level of B (10 mg L−1) by reducing B accumulation, lipid peroxidation, and the generation of H2O2, while significantly increasing levels of the most reactive ROS, OH−. SA also activated antioxidant enzymes, such as superoxide dismutase (SOD), peroxidase (POD), and ascorbate peroxidase (APX) and protected the seedlings from an ROS induced cellular burst. In conclusion, SA can be used to alleviate the adverse effects of excess boron.

Watermelon (Citrullus lanatus) is one xerophyte that has relative higher tolerance to drought and salt stresses as well as more sensitivity to cold stress, compared with most model plants. These characteristics facilitate it a potential model crop for researches on salt, drought or cold tolerance. In this study, a genome-wide comprehensive analysis of the ClNAC transcription factor (TF) family was carried out for the first time, to investigate their transcriptional profiles and potential functions in response to these abiotic stresses. The expression profiling analysis reveals that several NAC TFs are highly responsive to abiotic stresses and development, for instance, subfamily IV NACs may play roles in maintaining water status under drought or salt conditions, as well as water and metabolites conduction and translocation toward fruit. In contrast, rapid and negative responses of most of the ClNACs to low-temperature adversity may be related to the sensitivity to cold stress. Crosstalks among these abiotic stresses and hormone (abscisic acid and jasmonic acid) pathways were also discussed based on the expression of ClNAC genes. Our results will provide useful insights for the functional mining of NAC family in watermelon, as well as into the mechanisms underlying abiotic tolerance in other cash crops.

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