Background: Signal peptides are essential for plant growth and development. In plants, biological processes including cell-cell communication, cellular proliferation and differentiation, cellular determination of self-incompatibility, and defensive responses, all depend heavily on peptide-signaling networks such as CLE (CLAVATA3/Embryo surrounding region-related). The CLEs are indispensable in different periods of plant growth and development, especially in maintaining the balance between proliferation and differentiation of stem cells in various meristematic tissues. The working system of CLE genes in cucumber, an important economical vegetable (Cucumis sativus L.), has not been fully studied yet. The distributional patterns of chromosome-level genome assembly in cucumber provide a fundamental basis for a genome-wide comparative analysis of CLE genes in such plants.
Results: A total of 26 individual CLE genes were identified in Chinese long '9930' cucumber, the majority of which belong to unstable short alkaline and hydrophilic peptides. A comparative analysis showed a close relationship in the development of CLE genes among Arabidopsis thaliana, melon, and cucumber. Half of the exon-intron structures of all CsCLEs genes are single-exon genes, and motif 1, a typical CLE domain near the C-terminal functioning in signal pathways, is found in all cucumber CLE proteins but CsCLE9. The analysis of CREs (Cis-Regulatory Elements) in the upstream region of the 26 cucumber CLE genes indicates a possible relationship between CsCLE genes and certain functions of hormone response elements. Cucumber resulted closely related to Arabidopsis and melon, having seven and 15 orthologous CLE genes in Arabidopsis and melon, respectively. Additionally, the calculative analysis of a pair of orthologous genes in cucumber showed that as a part of the evolutionary process, CLE genes are undergoing a positive selection process which leads to functional differentiation. The specific expression of these genes was vigorous at the growth and development period and tissues. Cucumber gene CLV3 was overexpressed in Arabidopsis, more than half of the transformed plants in T1 generation showed the phenomena of obvious weakness of the development of growing point, no bolting, and a decreased ability of plant growth. Only two bolted strains showed that either the pod did not develop or the pod was short, and its development was significantly inferior to that in the wild type.
Conclusions: In this study, 26 CLE genes were identified in Chinese long '9930' cucumber genome. The CLE genes were mainly composed of alkaline hydrophilic unstable proteins. The genes of the CLE family were divided into seven classes, and shared close relationships with their homologs in Arabidopsis and melon. The specific expression of these genes was evaluated in different periods of growth and tissue development, and CLV3, which the representative gene of the family, was overexpressed in Arabidopsis, suggesting that it has a role in bolting and fruit bearing in cucumber.
Keywords: CLE peptides; Cucumber; Gene family; Phylogenetic analysis.