Background Salt stress is a serious abiotic stress that caused crop growth inhibition and yield decline. Previous studies have reported on the the synthesis of gamma-aminobutyric acid (GABA) and its relationship with plant resistance under various abiotic stress. However, the relationship between exogenous GABA alleviating plant salt stress damage and ion flux, amino acid synthesis, and key enzyme expression remains largely unclear. We investigated plant growth, Na+ transportation and accumulation, reactive oxygen species (ROS) metabolism and evaluated the effect of GABA on amino acids, especially SlGADs gene expression and the endogenous GABA content of tomato (Solanum lycopersicum L.) seedlings treated with or without 5 mmol·L− 1 GABA under 175 mmol·L− 1 NaCl stress. Results Exogenous application of GABA significantly reduced the salt damage index and increased plant height, chlorophyll content and the dry and fresh weights of tomato plants exposed to NaCl stress. GABA significantly reduced Na+ accumulation in leaves and roots by preventing Na+ influx in roots and transportation to leaves. The transcriptional expression of SlGAD1–3 genes were induced by NaCl stress especially with GABA application. Among them, SlGAD1 expression was the most sensitive and contributed the most to the increase in glutamate decarboxylase (GAD) activity induced by NaCl and GABA application; Exogenous GABA increased GAD activity and amino acid contents in tomato leaves compared with the levels under NaCl stress alone, especially the levels of endogenous GABA, proline, glutamate and eight other amino acids. These results indicated that SlGADs transcriptional expression played an important role in tomato plant resistance to NaCl stress with GABA application by enhancing GAD activity and amino acid contents. GABA significantly alleviated the active oxygen-related injury of leaves under NaCl stress by increasing the activities of antioxidant enzymes and decreasing the contents of active oxygen species and malondialdehyde. Conclusion Exogenous GABA had a positive effect on the resistance of tomato seedlings to salt stress, which was closely associated with reducing Na+ flux from root to leaves, increasing amino acid content and strengthening antioxidant metabolism. Endogenous GABA content was induced by salt and exogenous GABA at both the transcriptional and metabolic levels.

Transcriptome profiling of watermelon during its incompatible interactions with Fusarium oxysporum f.sp. niveum (FON) was performed using an Agilent custom microarray, which contains 15,000 probes representing approximately 8,200 watermelon genes. A total of 24, 275, 596, 598, and 592 genes showed significant differential expression in FONinfected plant roots, as compared with mockinoculated roots, at 0.5, 1, 3, 5 and 8 days post inoculation (dpi), respectively. Bioinformatics analysis of these differentially expressed genes revealed that during the incompatible interaction between watermelon and FON, the expression of a number of pathogenesis-related (PR) genes, transcription factors, signalling/regulatory genes, and cell wall modification genes, was significantly induced. A number of genes for transporter proteins such as aquaporins were down-regulated, indicating that transporter proteins might contribute to the development of wilt symptoms after FON infection. In the incompatible interaction, most genes involved in biosynthesis of jasmonic acid (JA) were expressed stronger and more sustained than those in a compatible interaction in FON-infected tissues. Similarly, genes associated with shikimate-phenylpropanoid-lignin biosynthesis were also induced during the incompatible interaction, but expression of these genes were not changed or repressed in the compatible interaction. Those results demonstrate that JA biosynthesis and shikimatephenylpropanoid-lignin pathways might play important roles in watermelon against FON infection and thus provides new insights in understanding the molecular basis and signalling network in watermelon plants in response to FON infection. We also performed confocal imaging of watermelon roots infected with the green fluorescent protein (GFP)-tagged FON1 to revealed histological characteristics of the infection.

Interactions between watermelon and a green fluorescent protein (GFP)‐tagged isolate of Fusarium oxysporum f.sp. niveum race 1 (Fon‐1) were studied to determine the differences in infection and colonization of watermelon roots in cultivars resistant to and susceptible to Fusarium wilt. The roots of watermelon seedlings were inoculated with a conidial suspension of the GFP‐tagged isolate, and confocal laser scanning microscopy was used to visualize colonization, infection and disease development. The initial infection stages were similar in both the resistant and susceptible cultivars, but the resistant cultivar responded differentially after the pathogen had penetrated the root. The pathogen penetrated and colonized resistant watermelon roots, but further fungal advance appeared to be halted, and the fungus did not enter the taproot, suggesting that resistance is initiated postpenetration. However, the tertiary and secondary lateral roots of resistant watermelon also were colonized, although not as extensively as susceptible roots, and the hyphae had penetrated into the central cylinder of lateral roots forming a dense hyphal mat, which was followed by a subsequent collapse of the lateral roots. The initial infection zone for both the wilt‐susceptible and wilt‐resistant watermelon roots appeared to be the epidermal cells within the root hair zone, which the fungus penetrated directly after forming appressoria. Areas where secondary roots emerged and wounded root tissue also were penetrated preferentially.