Effect of Exogenous 6-Benzylaminopurine on Retarding Soybean Leaf Senescence and its Relationship with Receptor-Like Protein Kinase RLPK2 Gene

Abstract

A novel receptor-like protein kinase gene RLPK2 (GenBank Accession No. AY687391) is associated with leaf senescence, and 6-benzylaminopurine (6-BA) could delay leaf senescence. However, whether 6-BA could delay leaf senescence through RLPK2 gene remains unclear. The effects of a cytokinin analogue 6-BA on RLPK2 knockout mutant and wild-type soybean leaves were investigated in this study. The results indicated that the contents of chlorophyll and Rubisco in leaves of RLPK2 knockout (rlpk2) treatment were significantly higher than those of the control (wild-type without 6-BA treatment). Similarly, the content of soluble protein in rlpk2 treatment were also significantly increased, whereas the contents of malondialdehyde (MDA), hydrogen peroxide (H2O2) and superoxide ion (O2•–) were significantly decreased. The rlpk2 treatment significantly enhanced the activities of antioxidant enzymes [superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase. (APX), etc.]. The rlpk2 treatment significantly increased the ZR and GA3 contents but decreased the ABA and IAA contents. Meanwhile, the rlpk2 treatment obviously decreased the expression levels of senescence-associated genes GmWRKY53b, GmCIB1, GmCRY2a and GmPAO1. The expression change pattern of the above senescence-associated genes in both 6-BA treatment and 6-BA+rlpk2 treatment was similar to that of in rlpk2 treatment, and even the 6-BA+rlpk2 combined treatment exhibited larger variations in amplitude. Real-time PCR analysis revealed that 6-BA treatment and 6-BA+rlpk2 combined treatment significantly reduced the expression of RLPK2 gene, and the latter showed the most obvious expression. Therefore, we proposed that 6-BA is involved in the regulation of soybean leaf senescence by directly affecting the expression of RLPK2 gene, and RLPK2 may be a signaling molecule involved in the cytokinin-mediated delay of soybean leaf senescence.

Keywords: 6-benzylaminopurine, Soybean RLPK2 Gene, Leaf Senescence, Hormone, Oxidative stress