Physiological Mechanisms Of Nitric Oxide Participating Lateral Root Growth Regulated By Partial Nitrate Nutrition In Rice Genotypes With Different Nitrate Responsivity

Rice roots can excrete O2 to surrounding soil and this can convert ammonium (NH4+) to nitrate (NO3-) in the process of nitrification at root surface or in rhizosphere. And rice roots are actually always exposed to a mixture of NH4+ and NO3-although the predominant species of mineral nitrogen (N) in the soil collected from the lowland rice field is NH4+. Therefore NO3-nutrition on the surface of rice roots is an important part of N nutrition of rice plants. Low concentration NO3-can stimulate the growth of rice roots, especially the lateral roots. As sensitivity of different varieties of rice to NO3-stimulation is different, the growth and development of lateral root is different too. Previous results showed that NO have an important role in regulating on the growth of plant roots. The roles of NO in LR formation in response to partial NO3- availability are not known. In order to explore the mechanism of NO involved in the partial nitrate nutrition on root growth rice genotypes with different nitrate responsivity, Hydroponics and Agar culture were conducted to study the growth of lateral root, NO accumulation and related gene in response to two ratio of NH4+ to NO3-(100/0 and 75/25) in two rice cultivars (Nanguang, sensitive to NO3-; Elio, un sensitive to NO3-) and the its absorbing state of N through Nanguang, one kind of rice which has a strong response to NO3- and the Elio which has a weak response to NO3- in the experiment. The results are listed as follows:1. Hydroponic experiments were conducted to study the effects of different ratios of NH4+ to NO3- (NH4+:NO3- as 100:0,75:25) on the growth of root in rice (Oryza sativa L.) at the seedling stage. The results showed that root hair and lateral root density in Nanguang increased significantly under the mixed NH4+ and NO3- nutrition compared with sole NH4+ nutrition. No difference was recorded in Elio under two N treatments.2. Hydroponic experiments were conducted to study the NO content in the root tip and lateral root zone of two rice cultivars under two ratios of NH4+ to NO3-(100/0 and 75/25). Compared with the sole NH4+ nutrition, the NO content in root tip and lateral root zone of Nanguang increased significantly under the mixed solution of NH4+ and NOs- while no significant was recorded in Elio under two N treatments. According to the source of NO, we found several genes (OsNiR?OsNia2) associated with NO synthesis and gene OsMAPK5 associated with lateral root starting up and compared them under different forms of nitrate nitrogen processing through their expression in Nanguang and Elio. Compared with the condition of total NH4+, the expression of these genes (OsNiR?OsNia2?OsMAPK5) in Nanguang raised significantly under the mixed solution of NH4+ and NO3- while the genes in Elio had also been raised though they were not particularly significant. It shows that there were NO participated in the growth of different nitrate responsivity rice roots.3. Hydroponic and Agar experiments were conducted to study the effect of donor and scavenger of NO (SNP and cPTIO) and the inhibitor of nitrate reductase (sodium tungstate) and the inhibitor of NOS (L-NAME)on the formation of lateral root in two rice cultivars. The results showed that the application of 2.5?M SNP with sole NH4+ nutrition could promote the initiation of lateral root to the same level as under mixture of N nutrition. And the application of the cPTIO and sodium tungstate and L-NAME with mixture of N nutrition could inhibit the initiation of lateral root to the same level as under sole NH4+ nutrition. These results confirmed that NO, resulting from nitrate reductase, participate the lateral root growth.4. Hydroponic experiments were conducted to study the variation of nitrate reductase activity in Nanguang and Elio under two N nutrition. Compared with the sole NH4+ nutrition, the NR activity in Nanguang increased significantly under the mixed N nutrition while no difference was recorded in Elio between two N treatments. Furthermore, Compared with the sole NH4+ nutrition, no differences was recorded in the lateral root density of nia2-2 (the Nitrate reductase mutants) under two N treatments. Ihese indicated that NO participate in controling the growth of rice lateral root through the synthesis of nitrate reductase...