Effect of phytohormones on seedling vigor of rice under cold conditions

We analyzed the relationship between seedling vigor and phytohormones at the seedling stage in foreign rice “Dunghan Shali” and “Italica Livorno” with vigorous initial growth under low temperature conditions, and Japanese rice “Oboroduki” and “Nipponbare”. The initial growth of “Dunghan Shali” and “Italica Livorno” was superior to that of “Oboroduki” and “Nipponbare” at both 27°C and 12°C. The phytohormone gibberellin supports vigorous initial growth of “Dunghan Shali”. The vigorous initial growth of “Italica Livorno” at 27°C could be induced by the effects of gibberellin since we noted increased expression of OsGA20ox1, a gene that promotes gibberellin biosynthesis. Both “Dunghan Shali” and “Italica Livorno” harbor common mutations in the promoter region of OsGA20ox1. At 12°C, the vigorous initial growth of “Italica Livorno” was because of the increased expression of OsGA20ox1. In “Dunghan Shali”, although the expression of OsGA20ox1 was not high, the level of abscisic acid, which suppresses plant growth, was lower than that in the three other varieties. We conclude that the effect of gibberellin was important for the vigorous initial growth of rice seedlings at 27°C, but a balance of multiple phytohormones may be important for the seedling vigor in rice at 12°C.