The Mechanism Of Strigolactone Regulating Rice Root Formation In Response To Nitrogen And Phosphate Nutration

Plants can respond to the environment by altering their development. For example, the root system is highly responsive to nutrient availability and distribution within the soil. This is particularly true of nitrate (NO3-) and phosphate (P) availability, which are the major growth-limiting nutrients in natural environments. Relative to shoot growth, root growth is generally favoured in nutrient-poor soils. However, the mechanisms mediating these changes and their ecological significance are still poorly understood.Root growth and root branching are tightly regulated processes coordinately controlled by several plant hormones. Strigolactones, or closely related compounds, have been recently known to act as phytohormones involving in root growth. Previous studies have demonstrated SLs exudation under NO3and P deficiency in many plant species. Also, besides stigolactone, auxin is playing a key role to regulate root growth. Shoot-derived auxin is delivered to the root tip through the polar transport stream that is facilitated by proteins of the PIN family. And strigolactones and auxin tightly interact and modulate each other’s levels and distribution through a feedback mechanism. In this study, root iniation and elongation was recoreded in wild type (WT) and stigolactone synthesis mutants (d10and d27) under series of NO3(0-2.5mM) and P (0-300μM) concentrations. And the response of root iniation and elongation in to application strigolactone analog GR24was investigated in WT and two mtants under high and low phosphorus and nitrate concentrations. Auxin concentration and related genes for auxin synthesis and polar transport was observed to illustrate interaction auxin and strigolactone regulating rice root growth in response to NO3and P deficiency. The results were listed as follows.1. Hydroponic cultures were conducted to study root initiation and elongation in WT plant (shiokari) under supplied series of NO3-and P concentrations. Compared with under300μM P concentration, length of seminal root and adventitious roots increased significantly with decreasing P concentration, while the density of lateral roots on seminal and adventitious roots decreased significantly with decreasing P concentration. Compared with under300μM P-supplied nutrition, the length of seminal root and adventitious roots increased by91.9%and53.5%and the density of lateral roots decreased by49.8%and27.4%under10μM P-supplied. The response of rice root architecture to NO3-stress was similar to P stress.Length of seminal root and adventitious roots increased significantly with decreasing NO3-concentration, while the density of lateral roots decreased significantly with decreasing NO3-concentration.2. In order to study whether strigolactones participate NO3-and P stress regulating root architecture, the wild type and strigolactone synthesis mutants (d10and d27) were cultivated to observe the changes of root morphology under high and low NO3-or P concentrations with or without GR24application,a the kind of biologically active synthetic strigolactone. Compared with under normal P and N treatment, root length of WT, D10and d27increased undere low P and low N conditions, while lateral roots density decreased. Application GR24under high P supply mimicked the root genotype under low P nutrition through decreasing lateral root density. This indicated that the synthesis of strigolactones is promoted by NO3-and P stress and it potently participate in the development of rice root growth.3. Root development is closely related with auxin biosynthesis and polar transport. In order to explore the relationship between strigolactones and auxin participating NO3-and P stress regulating rice root growth, auxin concentration were observed in wild type and two mutants (d10and d27) under NO3-and P stress. Compared to under300μM P supply, higher aunxin concentration in root, junction and first leaf of the wild type was recorded under0μMP concentration. And this indicated that P deficiency increased auxin synthesis and polar transport from the shoot to root. Similarly, higher auxin concentration was observed in first leaf of wild type under10μM NO3-than under2.5mM NO3-supply, indicating increased auxin synthesis under nitrate deficiency. However, Compared to under2.5mM nitrate supply, less aunxin concentration in root and junction of the wild type is recorded under10μM NO3-than under2.5mM NO3-supply. And this indicated that NO3-deficiency decreased auxin polar transport from the shoot to root. Auxin concentration in synthesis mutant is higher than wild type regardless of treatments and is less responsive to changing NO3-or P concentration than wild type in our experiment, indicating that mutant in strigolactone synthesis may promote auxin synthesis and transport, therefor changed plant architecture.4. In order to further explore the interaction molecular mechanism of stigolactone and auxin in regulating of rice root morphological under different nitrogen and phosphorus supplies, auxin efflux protein OsPIN genes family and AUX1expression were recoreded. Compared with300μM P-supplied nutrition, low P-supplied nutrition enhanced relative expression of OsPIN5a and OsPIN1c in wild type and two mutants, respectively. Relative expression of OsPIN2and AUX1was decreased in wils type and two mutants with decreasing nitrate concentration. And less relative expression of OsPIN2and AUX1was recorded in wild type than in two mutants under low nitrate nutrition. Furthermore, more auxin concentration in wild type under low P supply than under300M P-supplied nutrition may resulted from increased expression of OsPIN5a. And less auxin concentration in wild type under low NO3-supply than under2.5mM NO3--supplied nutrition may resulted from decreased expression of OsPIN2and AUX1.