Regulatory Effect Of Ospin2, A Putative Auxin Efflux Transporter, On Rice Architecture, Root Morphology And Phosphorus Nutrition

The plant hormone auxin (indole-3-acetic acid, IAA) is an essential regulator in many plant developmental processes, including apical dominance, phototropism, gravitropism, root patterning and other physiological processes. Polar transport of auxin is essential for the establishment and maintenance of polar growth and morphological patterning. At the cellular level, the directional transport of auxin results from the asymmetric distribution of different auxin membrane carriers, including the influx carrier AUX1protein family and the efflux-facilitating PIN-FORMED (PIN) family. PINs perform a rate-limiting role in catalyzing the efflux of auxin from cells, and their asymmetric cellular localization determines the direction of cell-to-cell flow which is central to auxin-regulated growth processes.Phosphorus (P) is an essential macronutrient for plant growth and development. Due to inorganic fixation and formation of organic complexes, as well as the slow rate of diffusion in soil, P is one of the least available plant nutrients worldwide. Plants have evolved several strategies to improve P acquisition, especially alterations in root morphology. Recent evidence has shown that auxin participate in the control of root responses to low phosphorus availability. Furthermore, auxin also has effects on the nutrients absorption, transport and distribution within plant. But there are few reports on the physiological and molecular mechanisms of the interactions between mineral nutritions and auxin.The rice(Oryza sativa L.) genome contains12putative PIN genes encoding auxin efflux transporters, including four PIN1genes (named OsPIN1α-1d), OsPIN2, three PIN5genes (OsPIN5α-c), OsPIN8and three monocot-specific PIN genes(OsPIN9, OsPIN10α and OsPIN10b). By over-expression of OsPIN2, the only rice homologue of AtPIN2, through the transgenic approach in rice, we analysis its function in regulating rice architecture, root morphology and phosphorus nutrition. The main results were summarized as follows:1.OsPIN2is localized on chromosome6. Sequence alignment between genomic and cDNA sequences showed that OsPIN2has7exons and6introns. The deduced amino acid sequence of OsPIN2suggests that OsPIN2is a typical intergrating membrane protein with two lyophobic domains and a big hydrophilic loop between them. Subcellular localization analysis by OsPIN2::GFP fusion protein in rice protoplast cells conformed it is localized in the cell plasma membrane.2. Over-expression of OsPIN2significantly increased tiller angle and numbers and decreased height of the rice in comparison with WT. At the ripening stage, the0.vPIN2-over-expressing plants showed shorter panicle length, less number of grains per panicle and lower grain weight per panicle in comparison with the control.3. Free IAA in various organs of the transgenic and WT plants was quantified by high-performance liquid chromatography (HPLC) and visualized by DR5::GUS. In the first and second leaves from the top, transgenic plants contained lower free IAA than that in the control plants. In the sheath of the first leaf, the transgenic plants and WT had nearly the same concentration of free IAA, while in the sheath of the second leaf, transgenic plants had lower free IAA concentration than that in WT. In contrast, in the root-shoot junction (shoot base) and roots, the two transgenic lines contained more free IAA than that in WT. In addition, localized external NPA application at the root-shoot junction had little effects on the root growth of the O.sPIN2-over-expression plants, which demonstrated that OsPIN2affected the polar auxin transport directly.4. Transcriptional analyses have demonstrated that OsPIN2regulated OsLazyl, but not OsTAC1and OsPIN1b in rice, suggesting that OsPIN2has a distinct role, along with OsPIN1b and OsTAC1in the auxin-regulated growth of the leaf canopy.5. The changes in root system architecture (RSA) triggered by phosphate (P) deprivation were studied in rice. Low P availability increased the length of root, lateral roots and the number of adventitious roots. Most effects of low P on RSA were dramatically modified in the transgenic plants or NPA treated wild-type plants. This shows that auxin plays a major role in the P starvation-induced changes of root development.6. At seedling stage, the P concentration in the roots of transgenic plants was higher than that in WT. By contrasting the P concentration in different parts of WT and transgenic plants at the grain-filling and ripe stage, we found0sPIN2was also involved in P translocation from vegetative organs to reproductive organs in rice. But the physiological and molecular mechanism underlying the interation between auxin and phosphorus nutrition needs to be further investigated.Taken together,OsPIN2not only has effects on the rice architecture and root growth, but also plays an important role in phosphorus utilization. Our results provide theoretical and material foundation for breeding new rice varieties with ideal-plant architecture and efficient phosphorus utilization.