Interactions Between Nitrogen （NH₄⁺, NO₃^-） And Plant Hormones In Regulation Of Root System Architecture In Arabidopsis

Plant roots are required for the acquisition of water and nutrients, for responses to abiotic and biotic signals in the soil, and to anchor the plant in the ground. The root system architecture is controlled by many intrinsic and extrinsic factors. Nitrogen is one of the most important macronutrients for plants and its availability is a major factor for plant root growth. Plant hormones also play important roles in root system architecture determination. Researches on interactions between nitrogen and plant hormones in the regulation of root system architecture, will tell us the response of plants to environment signals, which will benefit for the control of root system architecture and the use of fertilizers in agronomy.In these experiments, we used Arabidopsis wild type and the mutants on auxin, abscisic acid (ABA) and ethylene, respectively, as materials to study the interactions between plant hormones and two forms of nitrogens (NO3- and NH4+) in regulation of root system architecture.1. NH4+and NO3- have diverse effects in regulate root system architecture in Arabidopsis.In this experiment the role of uniform supply of 0.1-50 mM ammonium (NH4+) and nitrate (NO3-) on root system development was examined in Arabidopsis, in which the different effects between NH4+ and NO3- in root system architecture were revealed.The length of primary root were increased 14.1%～15.7% when treated with 1～5 mM NH4+ and when the concentration of NH4+ was higher than 10 mM, the length of primary root were decreased 15.7%-62.9%. When the concentration of NII4+ was low as 0.1 mM, the density and average length of lateral root were increased 28.9% and 51.4% respectively. When NH4+ were 10-50 mM, the lateral root density and average length were decreased 20.8%-54.2% and 29.9%-65.7% respectively.0.1 mM NH4+ also could stimulate the formation of root hair and it was increased 11.4%. When the concentration of NH4+ was higher than 1 mM, it could induce the formation of branched root hair and when the NH4+ was 50 mM, about 23.5% root hair branched. All the concentrations of NH4+we used in our experiments inhibited root hair elongation.10～20 mM NO3- stimulated the primary root elongate and the primary root was increased 12.2%-19.8%.0.1～1 mM NO3- could also stimulate the lateral root elongation and the length of the lateral root treated with NO3- was 67.5% longer than the control, but NO3- had no effect on the lateral root emergence. When NO3- were higher than 20 mM, the emergence and elongation of lateral root were inhibited. All the concentrations of NO3- we used in our experiment inhibited the formation and elongation of root hair and did not induce the root hair branching.Taking together, the results indicated that NH4+ and NO3- both could modify the root system architecture and there are similarities and differences between them. Both NH4+ and NO3- could stimulate the primary root elongation, but the function concentration of NO3-was higher 4-10 fold than NH4+. NH4+ could stimulate both the emergence and elongation of lateral root but NO3- only could stimulate the lateral root elongation. NH4+ induced the formation and branching of the root hair but NO3- inhibit the formation of root hair and did not induce the root hair branching. Both NH4+ and NO3- inhibited the elongation of root hair.2. Auxin and ABA are involved in the ammonium regulation of lateral root development in Arabidopsis.NH4+ plays important roles in lateral root development in Arabidopsis. The development of lateral root includes the formation and elongation progress, and they all stimulated by low NH4+ and inhibited by high NH4+. We discussed whether auxin and abscisic acid (ABA) involved in the NH4+ regulated progress. We found that auxin involved in the formation but not the elongation progress of lateral root development which regulated by NH4+: low NH4+could not induce the formation of lateral root in wild type seedlings treated by TIBA and the auxin importer mutant of auxl-7; low NH4+ could cooperate with low auxin to stimulate the formation of lateral root in wild type; auxin could rescue the inhibition effect of high NH4+ on the formation of lateral root. Low NH4+ could stimulate the elongation of lateral root in wild type seedlings treated with auxin and the mutant of auxl-7, but auxin could not rescue the inhibition of high NH4+ on the elongation of lateral root in wild type seedlings. ABA inhibited the stimulation effect of low NH4+ on the elongation of lateral root, and the inhibition of high NH4+ to lateral root formation maybe mediate by ABA.3. Effects of plant hormones on Arabidopsis root hair development are dependent on nitrogen supply.Our results indicated that high rates of NH4+ and NO3- supplies had pronounced effect on root hair development:during the concentration increased, the root hair formation and root hair length were inhibited. Under high rates of nitrogen, the roles of ethylene and auxin were partially inhibited. In addition, high rates of NH4+ induced branched root hair formation, reduced the velocity of cytoplasmic streaming and shorten the period of root hair growing stage. ROS may be responsible for NH4+ -induced branched root hair formation. Ethylene and JAs have distinct effects on NH4+ -induced branched root hair formation: ethylene inhibiting branched root hair formation and JA inducing root hair branching.In conclusion, above results indicated that different form of nitrogen had its own effects in root system architecture, which affected by the plant hormones. On the other hand, the effects of plant hormones in root system architecture also depended on the nitrogen supply to some extent.