Effect of abscisic acid on symbiotic nitrogen fixation activity in the root nodules of Lotus japonicus

摘要

The phytohormone abscisic acid (ABA) is known to be a negative regulator of legume root nodule formation. By screening Lotus japonicus seedlings for survival on an agar medium containing 70 (U.M ABA, we obtained mutants that not only showed increasedroot nodule number, but also enhanced nitrogen fixation. The mutant was designated enfl (enhanced nitrogen fixation 1) and was confirmed to be monogenic and incompletely dominant. In long-term growth experiments with M. loti, although some yield parameters were the same for both enfl and wild-type plants, both the dry weight and N content of 100 seeds and entire enfl plants were significantly larger compared than those traits in wild-type seeds and plants. The augmentation of the weight and N content ofthe enfl plants most likely reflects the increased N supplied by the additional enfl nodules and the concomitant increase in N fixation activity. We determined that the endogenous ABA concentration and the sensitivity to ABA of enfl were lower than thatof wild-type seedlings. When wild-type plants were treated with abamine, a specific inhibitor of 9-cis-epoxycarotenoid dioxygenase (NCED), which results in reduced ABA content, the N fixation activity of abamine-treated plants was elevated to the same levels as enfl. We also determined that production of nitric oxide (NO) in enfl nodules was decreased. We conclude that endogenous ABA concentration not only regulates nodulation, but also nitrogen fixation activity by decreasing NO production in nodules.