High responsiveness to nitrogen supply in modern maize cultivars is contributed to gibberellin-dependent leaf elongation

Abstract

Modern maize cultivars have high nitrogen responsiveness of grain yield. However, the underlying physiological mechanism is unclear. In the present study, from 17 cultivars released from 1950 s to 2010 s in China, three maize cultivars released in 1950 s (BMY), 1970 s (ZD2), and 1990 s (ZD958) were selected according to their growth responses to nitrogen supply. Nitrogen responsiveness of shoot biomass was highest in the new cultivar ZD958, and the lowest in the old cultivar BMY, with the medium cultivar ZD2 in between. Shoot biomass was closely related to leaf elongation rate. Leaf elongation was further analyzed using kinematic analysis method. Cell division rate and cell production rate were greater and more responsive to nitrogen supply in ZD958 than in the other cultivars. In agreement, there were more cells in the elongation zone of ZD958 and they were more responsive to increasing supply, resulting a longer leaf elongation zone. The interaction effect between cultivar and nitrogen supply on leaf elongation was in consistence with the change in gibberellin (GA1 and GA3) concentration in leaf growth zone. Accordingly, the genes encoding gibberellin biosynthesis or signal transduction was more up-regulated in ZD958, which was positively correlated to the up-regulation of genes related to cell cycle and DNA replication. On the contrary, the genes related to the inactivation of gibberellin was more down-regulated in ZD958. Exogenous application of bioactive gibberellin or its biosynthesis inhibitor eliminated the interaction effect between cultivar and nitrogen supply on leaf elongation rate. In conclusion, modern maize breeding may have increased nitrogen responsiveness in maize by enhancing the gibberellins-dependent leaf elongation response to high nitrogen supply.