Identification of early response genes to salt stress in roots of melon (Cucumis melo L.) seedlings

In order to better understand the mechanisms by which muskmelons (Cucumis melo L.) respond to salt stress, a cDNA library was constructed using suppression subtractive hybridization (SSH) from the root tissue of a salt-tolerant melon cultivar, Bingxuecui. A total of 339 clones were sequenced from the SSH library, leading to 312 high quality expressed sequence tags (ESTs), with an average size of 450 bp; representing 262 uni-ESTs comprising 29 contigs and 233 singletons. Blast analysis of the deduced protein sequences revealed that 283 ESTs had a high similarity to proteins in the non-redundant database, while 29 had low identity or no similarities. Many of the annotated sequences were homologous to genes involved in abiotic or biotic stress in plants. Functional categorization of the proteins revealed that salt tolerance could be largely determined by various proteins involved in metabolism, energy, transcription, signal transduction, protein fate, cell rescue and defense, implying a complex response to salt stress exists in melon plants. Twenty-seven ESTs were selected and analyzed by real-time PCR; the results confirmed that a high proportion of the ESTs were activated by salt stress. The complete sequences and a detailed functional analysis of these ESTs is required, in order to fully understand the broader impact of these genes in plants subjected to a high salinity environment.