植物内部磷循环利用提高磷效率的研究进展

  【目的】  磷素作为植物生长发育过程中必需的大量营养元素之一，因其在土壤中的难移动性使得根系对磷的获取有限。植物为满足其生长对磷素的需求，已经进化出一系列相应的机制提高对内部磷的再利用，以减少磷肥投入，保证产量的同时实现环境友好。本文以植物内部磷的高效利用为核心，重点剖析植物有机磷库与无机磷库中磷素的活化再利用的途径，综述释放出的无机磷在不同组织和器官中的转运过程，并对今后深入研究磷再利用的有关方向作出展望。  主要进展  植物体内磷的存在形式主要包括无机磷和有机磷两种。植物吸收的多余无机磷会被暂时储存在液泡中，并在植物缺磷时外流到胞质以满足植物对磷的需求，位于液泡膜的磷酸盐转运蛋白负责无机磷在液泡和胞质之间的分配。存在于核酸和磷脂中的有机磷在磷缺乏时由酶类(核酸酶、磷脂酶和紫色酸性磷酸酶等)水解并释放无机磷以供植物生长需要。植物遭受低磷胁迫，营养器官(老叶等)中活化的无机磷由多种磷酸盐转运蛋白转运到幼叶等新的生长中心被利用，从而显著提高磷的再利用效率。磷转运蛋白(PHTs)通过调控磷向籽粒的运输降低了磷在禾谷类作物籽粒中的积累，提高了磷利用效率，同时降低环境风险。  展望  现阶段的研究较为详细地阐述了植物体内磷素再活化的生理分子机制，但对磷转运功能蛋白参与特定磷转运过程的相关研究仍不够全面，比如液泡磷能调控细胞磷稳态，目前已鉴定得到的与其外排有关的转运蛋白极少，其调控机制也有待深入探索。国内外关于PHT1、PHT2、PHT3和PHT4蛋白如何将磷素从源器官转运到库器官缺乏系统的研究。无机磷库和有机磷库中磷的利用对植物应对缺磷的贡献也鲜有报道。因此，植物体内与磷再活化后转运利用相关的分子生物学调控机理还需进一步研究。

Targeting internal phosphorus re-utilization to improve plant phosphorus use efficiency

  【Objectives】  Phosphorus (P) is an essential macronutrient element in the process of plant growth and development. Due to its low mobility in soil, the root system has limited access to phosphorus. To meet the demand of phosphorus for growth, plants have evolved a series of biological processes to maximize the re-use of internal phosphorus, reduce the P fertilizer input and P effluence in the aquatic ecosystem. This review summarizes recent advances in the understanding of mechanisms by which plant utilize the organic P pools and inorganic P pools. Further, the relevant molecular mechanisms involved in the transport of released inorganic P (Pi) in different tissues and organs are explored and an insight on how to further study the relevant directions of P utilization in the future is provided.  Major advances   P in higher plants mainly includes Pi and organic P. The excess Pi absorbed by plants is temporarily stored in vacuoles and this part of Pi is released to cytoplasm under low-P stress to buffer the demand for Pi via phosphate transporter located in the tonoplast. Enzymes such as nuclease, phospholipase and purple acid phosphatase hydrolyze organophosphate such as nucleic acid, phospholipid and release Pi to facilitate its redistribution and utilization in plants. When plants suffer from low P stress, Pi is exported from vegetative organs (old leaves, etc.) and transported to growth centers such as young leaves for use via phosphate transporters, thereby significantly improving P re-use efficiency. Reducing the accumulation of P in grains and controlling grain P within a reasonable range through phosphate transporters is of great significance for improving grain PUE and alleviating eutrophication.  Prospects  Numerous studies have elaborated the mechanisms of P recycling and utilization in plants, but the involvement of phosphate transporters in specific P transport processes is still not clear. For example, vacuolar Pi can be re-used to meet P demand when plant suffer P deficiency. However, few transporters related to its efflux have been identified, and the relevant regulation factors need further exploration. Holistically exploring the role of PHT1, PHT2, PHT3 and PHT4 proteins on P from source to sink is pertinent. The contribution of Pi mobilized from vacuole and the Pi recycled from organophosphate to cope with phosphate deficiency needs to be quantified. Therefore, analyzing the biological regulatory mechanisms underlying the transportation and utilization of P in plants can provide a scientific basis for reducing P fertilizer input, improving P utilization efficiency and cultivating P-efficient crop varieties.