Soluble sugars and organic acids are important components of fruit flavor and have a strong impact on the overall organoleptic quality of watermelon (Citrullus lanatus) fruit. Several studies have analyzed the expression levels of the genes related to soluble sugar accumulation and the dynamic changes in their content during watermelon fruit development and ripening. Nevertheless, to date, there have been no reports on the organic acid content in watermelon or the genes regulating their synthesis. In this study, the soluble sugars and organic acids in watermelon were measured and a comparative transcriptome analysis was performed to identify the key genes involved in the accumulation of these substances during fruit development and ripening. The watermelon cultivar ‘203Z’ and its near-isogenic line (NIL) ‘SW’ (in the ‘203Z’ background) were used as experimental materials. The results suggested that soluble sugar consist of fructose, glucose and sucrose while malic-, citric-, and oxalic acids are the primary organic acids in watermelon fruit. Several differentially expressed genes (DEGs) related to soluble sugar- and organic acid accumulation and metabolism were identified. These include the DEGs encoding raffinose synthase, sucrose synthase (SuSy), sucrose-phosphate synthase (SPSs), insoluble acid invertases (IAI), NAD-dependent malate dehydrogenase (NAD-cyt MDH), aluminum-activated malate transporter (ALMT), and citrate synthase (CS). This is the first report addressing comparative transcriptome analysis via NILs materials in watermelon fruit. These findings provide an important basis for understanding the molecular mechanism that leads to soluble sugar and organic acid accumulation and metabolism during watermelon fruit development and ripening.

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Peach gummosis, caused by Lasiodiplodia theobromae, is one of the most prevalent diseases that affects peach production. In this study, we investigated the effect of zinc sulfate on inoculated peach shoots, as well as on the growth, morphology, and pathogenicity of L. theobromae in vitro, in the laboratory. Zinc deficiency was detected in diseased peach shoots by micronutrient analysis (Cu, Mn, and Zn) and confirmed by the measurement of transcript levels of zinc transporters (ZIP4, HAM4, and ZAT). The zinc was transferred from the diseased peach shoots to the peach gum. Applying zinc sulfate to the diseased peach shoots reduced the severity of peach gummosis, showing significantly reduced lesion size and gum weight, as well as downregulation of cell wall degradation-related gene (PG and PME) compared with the control. Zinc sulfate also specifically controlled peach gummosis under L. theobromae phytotoxin stress and induced the expression of defense-related genes (PR4, CHI, PAL, PGIP, and GNS3). In addition, in vitro mycelial growth of L. theobromae was significantly inhibited by zinc sulfate compared with the control. Zinc sulfate caused abnormal hyphae at 25 mM and swelling hyphal tips at 50 mM. Exposure of L. theobromae to zinc sulfate for 20 min inhibited the ability of the pathogen to cause peach gummosis. Our physiological and molecular data demonstrated that zinc sulfate has a dual function by reducing susceptibility in the host and by direct inhibition of the pathogen.

Lasiodiplodia theobromae is a causal agent of peach (Prunus persica L.) tree gummosis, a serious disease affecting peach cultivation and production. However, the molecular mechanism underlying the pathogenesis remains unclear. RNA-Seq was performed to investigate gene expression in peach shoots inoculated or mock-inoculated with L. theobromae. A total of 20772 genes were detected in eight samples; 4231, 3750, 3453, and 3612 differentially expressed genes were identified at 12, 24, 48, and 60 h after inoculation, respectively. Furthermore, 920 differentially co-expressed genes (515 upregulated and 405 downregulated) were found, respectively. Gene ontology annotation revealed that phenylpropanoid biosynthesis and metabolism, uridine diphosphate-glucosyltransferase activity, and photosynthesis were the most differentially regulated processes during gummosis development. Significant differences were also found in the expression of genes involved in glycometabolism and in ethylene and jasmonic acid biosynthesis and signaling. These data illustrate the dynamic changes in gene expression in the inoculated peach shoots at the transcriptome level. Overall, gene expression in defense response and glycometabolism might result in the gummosis of peach trees induced by L. theobromae.

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