The Mechanism Of OsPIN1b Involved In Rice Root Elongation Regulated By Low-nitrogen And-phosphate

Nitrogen and phosphorus are two nutrient elements which are necessary for plant growth and development,and the insufficient supply of these two elements will seriously affect crop growth and yield.Under the condition of nitrogen and phosphate stress,plants could improve the nutrient absorption ability by regulating the morphology and structure of roots.Therefore,ideal root morphology is the basis for efficient use of nutrients in plants.In addition to the external environment,plant root morphology is also controlled by internal factors such as plant hormones.Auxin is the imporant plant hormone to be involved in plant root growth regulated by environmental factors including nutrient deficiency.Studies have shown that auxin synthesis and polar transport could significantly affect the growth and development of plant roots.In addition to auxin,nitric oxide,a signaling molecule,has been reported to regulate the growth and development of plant roots,but the physiological and molecular mechanisms are still not clear.In this study,we investigated the morphological changes of rice root under low-nitrogen and-phosphate conditions,analyzed auxin distribution in different parts of rice,[3H]IAA transport from the junction to roots and relative expression of auxin efflux carriers.And we further observed NO fluorecence in root tips and effcet of NO donor and/or scavenger application on rice root growth and auxin polar transport.The results obtained are as follows:1.Compared with control treatment,low-nitrogen and-phosphate condition(LN and LP)significantly affected rice root growth.With concentrations of nitrogen and phosphate decreasing,length of seminal and adventitious roots increased while the aboveground dry weight decreased,ultimatly resulting in the root:shoot ratio increased.Furthermore,the ratio of root to shoot was higher under LP condition than LN condition.2.Compared with control treatment,LN and LP remarkedly inhibited auxin polar transport from the shoot to root.Higher auxin concentration in topmost fully-expanded leaf was recorded under LN and LP conditions in comparison with control treatment;conversely,lower auxin concentration injunction and roots was observed under LN and LP.These suggested th\at LN and LP probably inhibited auxin polar transport form the shoot to root.To visually observe the change of auxin concentration in rice plants,we constructed transgenic rice plants with DR5::GUS.Similar change tendency of DR5::GUS expression was also recorded in LN-and LP-treated rice plats,confirming the auxin concentration in response to LN and LP treatments.To quantitatively analyze auxin polar transport from shoot to root,[3H]IAA transport from junction to roots was conducted in the following experiments.The results showed that LN and LP remarkedly inhibited[3H]IAA transport.At the same time,the expression of auxin efflux protein OsPIN family genes were detected in rice roots.The results of qRT-PCR showed that relative expression of most OsPIN family genes was decreased by LN and LP condition.Interestingly,the most abundant expression of PIN family genes in root tips was OsPIN1b,which was several times higher than the others,and its expression was inhibited by LN and LP treatments.Moreover,compared with wild type,the length of seminal root in two pinlb mutants was less sensitive to LN and LP stress.These results suggested that LN and LP regualted the growth of rice roots probably through lowering auxin polar transport from the shoot to root,which OsPIN1b might paly a important role.3.Compared with control treatment,nitric oxide fluorecence in rice root tips increased significantly under LN and LP conditions.Under normal nutrition conditions,the exogenous application of nitric oxide donor SNP(10 M)could promote the root elongation of rice seeds,and reached the same level as those under LN and LP conditions.Under LN and LP conditions,the application of nitric oxide scavenger(cPTIO)significantly inhibited the elongation of seminal roots and reached the same level as under normal nutrition.These results suggested that nitric oxide was involved in LN-and LP-regulated seminal root elongation in rice plant.Further experiments showede that NO affected rice root elongation probably through lowering auxin polar transpot;because application of NO donor and scanveger could inhibited or increased the[3H]IAA transport,DR5::GUS expression and relative expressin of PIN family genes.Furthermore,the length of seminal root in pinlb mutants did not respond to exogenous NO donor and scavenger,which suggested that OsPIN1b might play the inportant role in NO-regulated seminal root elongation when rice plants facing LN and LP conditions.In summary,OsPIN1b maybe involved in mediating auxin transport to root tip and act as downstream mediator of NO to induce seminal root elongation when rice plants facing N and P deficiencies.