多环芳烃跨作物根系细胞膜的运输机制研究

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous organic pollutants and potentially carcinogenic, mutagenic and toxic to both human and non-human organisms. Over 90% of PAHs in the environment reside in surface soil. Plants grown in PAH-contaminated soils or water can become contaminated with PAHs due to their absorption. They may pose human and animal health hazards. In fact, dietary intake of PAHs is a dominant route of exposure for the general population where food crops are a major source of dietary PAHs. Therefore, it is important to understand the mechanisms of PAH uptake by crops in PAH-contaminated soils in order to control the accumulation and translocation of PAHs in crops, to develop strategies (either agronomic practices, breeding or genetic engineering) for reducing PAH uptake by crops, to phytoremediate soil or water contaminated by PAHs, and to model the potential uptake for risk assessment. However, the mechanism underlying plant root uptake of PAHs remains unclear. This project aims to elucidate the transport process of PAHs across crop root cell membrane. Our hypothesis is that PAH uptake into plant roots proceeds partly by an active PAHs/H+ symport system and partly by passive permeation, i.e., aquaporins and simple diffusion (In general, simple diffusion is especially prevalent for passive uptake in higher plants). To reveal whether active transport is involved in root uptake of PAHs and what the proportion of active transport to the total uptake of PAHs by roots is, time course of uptake, concentration-dependent uptake, effects of temperature and metabolic inhibitors on PAH uptake will be investigated through batches of hydroponic experiment. Transmembrane transport of PAHs in crop root cells will be studied via PAH uptake with and without aquaporin inhibitors, confirmation of proton/PAHs symport, and changes in cytoplasmic pH and micro-morphology of root surface during PAH uptake. Atomic force microscopy, confocal fluorescence microscopy, and microelectrode will be employed for the measurements of root surface micro-morphology, cytoplasmic pH, and membrane potential, respectively. The expected results will provide insight into PAH uptake mechanism in plant roots in the nanometer scale at the cytological and enzymological levels that is relevant to strategies for reducing PAH accumulation in wheat for food safety, improving phytoremediation of PAH-contaminated soils or water by agronomic practices and genetic modification to target remedial plants for higher PAH uptake capacity. They are also beneficial for export of crop products.

作物根系如何吸收多环芳烃（PAHs）是亟待探明的理论课题。本项目拟采用近年来发展的并已在环境学、化学、生物学和土壤学领域成功运用的新方法和新技术（如纳米尺度上的原子力显微、激光共聚焦显微和细胞电化学技术等）通过水培试验研究作物根系吸收PAHs的动力学特征、温度和代谢抑制剂对PAHs吸收的影响，以揭示作物根系吸收PAHs的主/被动问题；然后在此基础上深入探究作物根系吸收PAHs的通道、细胞膜电位和细胞酶活性变化、细胞质pH的变化及调控、根系界面微形态变化和验证H+-PAHs共运门运输的假设，同时进行土培试验佐证。在作物细胞和酶学水平及纳米尺度上，阐明PAHs跨作物根系细胞膜的运输机制，为预防和阻控土壤或水环境中PAHs等持久性有机污染物对食物链的污染、农产品的安全生产、PAHs污染土壤和水体的植物修复及高效能修复植物的筛选提供科学依据和技术支撑，为我国农产品的出口创汇服务。

多环芳烃（PAHs）具有强的“致癌、致畸、致突变”作用，是环境中普遍存在的一类疏水性有机污染物，农产品中时有检出。然而，植物根系如何吸收PAHs是国内外学术界长期以来尚未明晰的一个科学问题。本项目主要采用水培试验和微电极技术研究了植物根系吸收PAHs的方式，验证了与氢质子共运吸收途径的存在，同时探究了根系PAHs吸收过程中细胞质pH的变化及其机制。研究发现并证实了主/被动过程均参与了作物根系PAHs的吸收。被动吸收除了简单扩散外，水通道也参与了；主动吸收是以与氢质子共运方式进行。根系吸收PAHs会导致细胞质酸化，随后通过细胞质的物理化学缓冲作用、质膜质子泵向胞外泵氢、液泡膜质子泵向液泡内泵氢、硝酸根的还原及苹果酸的降解作用消耗氢质子，恢复细胞质的pH。植物根系比表面积和脂肪含量是决定其PAHs吸收能力的2个关键因子，比表面积的贡献是脂肪的2倍。生理酸/碱性肥料可以通过与根系细胞质膜上质子泵的泵氢过程相耦联，进而调控植物根系PAHs的吸收。PAHs积累会导致植物叶片黄化，叶绿素降解加速和叶片含水量的增加是PAHs诱导叶片黄化的根本原因。项目的研究结果在微观上揭示了植物根系吸收PAHs和叶片受PAHs胁迫的细胞学机理，在宏观上为PAHs植物根系吸收的调控进而保障农产品生产的安全和强化PAHs污染环境植物修复的效果提供了理论依据与技术策略，因而具有广阔的应用前景。