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GhNAC18 is a novel NAC gene that was isolated from cotton (Gossypium hirsutum L.). The full-length cDNA was 1511 bp including an open reading frame of 1260 bp in length and encodes a protein of 419 amino acids. With qRT-PCR analysis, GhNAC18 was downregulated during natural and dark-induced senescence, implicating this gene as anti-aging gene in cotton. Analysis of its promoter revealed a group of putative cis-acting elements especially, the light and stress responsive elements, indicating that it may have a potential role in leaf development. Additionally, GhNAC18 was found to have transcriptional activation activities on its C-terminal region and by bioinformatics analysis, GhNAC18 was localized in the nucleus. Tissue specific expression analysis indicated that GhNAC18 is constitutively expressed in roots, stems, earlier stages of senescing leaves, fibers and flower parts with high expression levels registered in the young leaves and cotyledon leaves. GhNAC18 was upregulated by exogenous application of various phytohormones including salicylic acid (SA), methyl Jasmonate (MeJA) and ethylene (ET) but downregulated with abscisic acid (ABA). Moreover, the gene was induced by drought (PEG6000), H 2 O 2 , cold (4°C) and wounding but was inhibited by high salinity. These results indicated that GhNAC18 is a transcriptional activator that is involved in leaf development, especially inhibition of leaf senescence and plant stress responses in cotton. This study provides fundamental information on understanding the function of GhNAC18 gene in cotton leaf senescence and stress tolerance and thereafter its manipulation for breeding of late-aging and stress-tolerant cultivars.

Leaf senescence is defined as a deterioration process that continues to the final developmental stage of leaf. This process is usually regulated by both external and internal factors. There are about 5356 senescence associated genes belonging to 44 plant species. A great number of these genes were identified in Arabidopsis. Leaf senescence can be regulated by many transcription factors. In this study, nine gene families were selected according to their expression levels during leaf senescence from our laboratory database. Phylogenetic tree was constructed by MEGA6. Cultivated cotton CCRI-10 seeds were sown in the experimental field of Institute of Cotton Research of CAAS for profiling and leaf development stages analysis. For abiotic (drought and salt) stress and phytohormone (ABA, SA, ET and JA) treatments, CCRI-10 seeds were sown in potting soil at 25 °C in a chamber room. Total RNA was isolated from various samples and the cDNA prepared for qRT-PCR. The comparative CT method was applied to calculate the relative expression levels of genes. For phylogenetic tree, nine cotton genes were divided into two groups, most of homologous genes in previous studies showed roles in phytohormones and abiotic stress. Expression profiling of the nine genes showed different patterns of tissue specific expression. In leaf development stages, majority of cotton genes showed high expression in early and complete senescence stage. Furthermore, most of cotton genes have positive or negative response to phytohormones and abiotic stress. Based on the results of this study, we found four cotton genes CotAD_07559, CotAD_37422, CotAD_21204 and CotAD_54353 as candidate genes for leaves senescence and abiotic stress.

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