RESUMO
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 microM ABA, we obtained mutants that not only showed increased root nodule number but also enhanced nitrogen fixation. The mutant was designated enhanced nitrogen fixation1 (enf1) and was confirmed to be monogenic and incompletely dominant. The low sensitivity to ABA phenotype was thought to result from either a decrease in the concentration of the plant's endogenous ABA or from a disruption in ABA signaling. We determined that the endogenous ABA concentration of enf1 was lower than that of wild-type seedlings, and furthermore, when wild-type plants were treated with abamine, a specific inhibitor of 9-cis-epoxycarotenoid dioxygenase, which results in reduced ABA content, the nitrogen fixation activity of abamine-treated plants was elevated to the same levels as enf1. We also determined that production of nitric oxide in enf1 nodules was decreased. We conclude that endogenous ABA concentration not only regulates nodulation but also nitrogen fixation activity by decreasing nitric oxide production in nodules.
Assuntos
Ácido Abscísico/farmacologia , Genes de Plantas/genética , Lotus/efeitos dos fármacos , Lotus/genética , Mutação/genética , Fixação de Nitrogênio/efeitos dos fármacos , Nodulação/efeitos dos fármacos , Ácido Abscísico/metabolismo , Cruzamentos Genéticos , Flores/efeitos dos fármacos , Flores/fisiologia , Regulação da Expressão Gênica de Plantas/efeitos dos fármacos , Genes Dominantes/genética , Lotus/crescimento & desenvolvimento , Lotus/microbiologia , Proteínas Mutantes/isolamento & purificação , Proteínas Mutantes/metabolismo , Óxido Nítrico/metabolismo , Fixação de Nitrogênio/genética , Fenótipo , Nodulação/genética , RNA Mensageiro/genética , RNA Mensageiro/metabolismo , Rhizobium/efeitos dos fármacos , Rhizobium/fisiologia , Nódulos Radiculares de Plantas/efeitos dos fármacos , Nódulos Radiculares de Plantas/metabolismo , Simbiose/efeitos dos fármacos , Fatores de TempoRESUMO
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 µM ABA, we obtained mutants that not only showed increased root nodule number, but also enhanced nitrogen fixation. The mutant was designated enf1 (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 enf1 and wild-type plants, both the dry weight and N content of 100 seeds and entire enf1 plants were significantly larger compared than those traits in wild-type seeds and plants. The augmentation of the weight and N content of the enf1 plants most likely reflects the increased N supplied by the additional enf1 nodules and the concomitant increase in N fixation activity. We determined that the endogenous ABA concentration and the sensitivity to ABA of enf1 were lower than that of 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 enf1. We also determined that production of nitric oxide (NO) in enf1 nodules was decreased. We conclude that endogenous ABA concentration not only regulates nodulation, but also nitrogen fixation activity by decreasing NO production in nodules.