Endogenous hormones and expression of senescence-related genes in different senescent types of maize

Levels of cytokinins and abscisic acid (ABA) and the expression of senescence-related genes were investigated in two maize (Zea mays L.) cultivars of different senescence type, cv. P3845 (stay-green) and cv. Hokkou 55 (earlier senescent), in a field study. The delay in leaf senescence in P3845 was correlated with increased levels of chlorophyll and nitrogen and a higher photon-saturated photosynthetic rate (P-sat). Compared with the earlier senescent Hokkou 55, P3845 showed enhanced contents of cytokinins (trans-zeatin riboside, t-ZR; dihydrozeatin riboside, DHZR; isopentenyladenosine, iPA) and reduced levels of ABA in its leaves. In roots, P3845 had increased levels of t-ZR, DHZR, and ABA, but decreased concentrations of iPA. It was concluded that a higher rate of cytokinin transport from roots to leaves contributes to the delay of senescence in P3845. By contrast, the translocation of ABA from roots to shoots may be blocked in the stay-green cultivar, which also results in retarded leaf senescence. P3845 ear leaves contained more malondialdehyde (MDA) and higher catalase (CAT) and superoxide dismutase (SOD) activities than Hokkou 55. Since the accumulation of the mRNAs for Rubisco small subunit (rbcS), phosphoenolpyruvate carboxylase (PEPC), and SOD peaked after Chl content and P-sat had reached their maxima, it is speculated that when leaf senescence is initiated, Chl contents decrease first, followed by the degradation of the photosynthetic apparatus and of photosynthesis-related enzymes. See1 and See2 encode senescence-related cysteine proteases; their mRNAs were most abundant in yellowing leaves, suggesting that these proteins are involved in the process of senescence rather than its initiation. mRNAs of both genes were more abundant in Hokkou 55 than in P3845, which suggests a regulation of leaf senescence at the transcriptional level.