Molecular cloning and expression of a sweet potato cysteine protease SPCP1 from senescent leaves

In this report a full-length cDNA, SPCP1, was isolated from senescent leaves of sweet potato (Ipomoea batatas (L.) Lam). SPCP1 contained 1020 nucleotides (339 amino acids) in the open reading frame, and exhibited high amino acid sequence homologies (ca. 58% to 74%) with papain-like cysteine proteases of Alnus glutinosa, Arabidopsis thaliana, Astragalus sinicus, Brassica napus, Daucus carota, Gossypium hirsutum, Hordeum vulgare, Iris hollandica, Medicago truncatula, Nicotiana tabacum, Oryza sativa, Ricinus communis, Trifolium repens. Semi-quantitative RT-PCR and Western blot hybridization showed that SPCP1 gene expression was enhanced significantly in natural senescent leaves and in dark-, ethephon-, and ABA-induced senescent leaves, whereas, was almost not detected in mature green leaves, stems, and roots. Initiation of chlorophyll degradation is earlier than the SPCP1 gene expression during leaf senescence. SPCP1 expression was also induced in sweet potato suspension cells treated with 1 mM ethephon. Evan blue staining showed that suspension cells were not significantly affected by ethephon treatment up to 2 mM, however, most of the cells died when treated with 10 mM ethephon. In conclusion, sweet potato SPCP1 is likely a functional, senescence-associated gene and its expression levels were significantly enhanced at mRNA and protein levels in natural and induced senescent leaves and suspension cells. The physiological role and function of SPCP1 were likely not in association with initiation of chlorophyll degradation and cell death during senescence.