丛枝菌根真菌与植物共生对植物水分关系的影响及机理

Effects of arbuscular mycorrhizal fungi and plant symbiosis on plant water relation and its mechanism

自1885年Frank首次提到菌根(mykorhiza)概念以来,大量的试验证实了丛枝菌根真菌(AMF)与植物根系之间形成具有一定结构和功能的共生体,促进植物生长并提高干旱耐受能力,在干旱生态系统中发挥重要的作用。该研究多集中在对宿主植物生理生态的影响及其机制方面,然而菌根共生对宿主植物水分吸收和信号产生、传递的影响研究少而分散,缺少系统总结。综述了最近四十多年丛枝菌根真菌与植物共生体对宿主植物干旱适应性影响研究进展,讨论了菌根共生对植物根冠通讯的影响及机理。干旱胁迫下AMF与植物共生,通过影响宿主植物一系列生理生态过程,提高宿主植物横向根压和纵向蒸腾拉力。经典的Ohm吸水模型是该方向最有代表性的研究成果,该模型揭示了菌根共生的根外菌丝具有不同于根细胞的细胞结构和水分运输性能,这为宿主植物提供一种特殊的快速吸水方式,可提高植物对土壤水分的吸收和运输能力。研究表明,AMF会影响宿主植物根冠通讯过程,如诱发信号级联反应,诱导根系尽早感知水分胁迫并产生非水力根源信号,提高宿主对干旱的耐受性。讨论了AMF在根冠通讯分子机制研究方面存在的问题及可能的解决途径,展望了AMF在干旱农业生产中的应用潜力。

Since Frank proposed ＂mykorhiza＂ for the first time in 1885, extensive studies have demonstrated the formation of mycorrhizae between arbuscular mycorrhizal fungi （AMF） and plant roots, and the functioning of mycorrhizae in improving plant gowth and drought adaptability under drought stress particularly in semiarid and arid ecosystems. However, information is limited on the mechanisms how AMF could affect the host plant water uptake, root signal generation and transfer, while most studies have focused on effects of AMF on their physiological and ecological changes in host plants. In this review, progresses in how AMF could balance water relations and affect root to shoot communications are summarized from studies in the last four decades, and possibly related mechanisms are also concluded. These mechanisms include enhanced water uptake, root hydraulic conductance, antioxidant activity, altered hormone relations, osmotic adjustment, aquaporin expression and nutrition absorption. Studies have showed that AMF associated symbioses have usually altered eco- physiological characteristics, e.g. stomatal conductance, plant size and abscisic acid （ABA） content, and thus enhancing the lateral root pressure and vertical transpiration to benefit for host plant＇s water absorption. The Ohm＇s law model, which is the most representatively traditional progress in water uptake mechanisms, could further reveal how AMF is able to improve soil water absorption and transport. This mode reveals that mycorrhizal hyphae, which are different from plant root cells, having aseptate or coenocytic and elastic hyphal wall at the tip, and only infrequent, adventitious septa, can contribute to transport water rapidly in host plants under drought stress. Thus, AMF in plant root may be able to feel drier soil more quickly and produce non-hydraulic root-sourced signals earlier. AMF can also affect root to shoot communications, such as inducing signaling cascades for root-sourced signal generation and the improvement of drought tolerance from cellular to whole plant level. Nevertheless, the composition of root exudates are complex, and the mechanisms of root to shoot communications still need to be solved： 1 ） how AMF help root cells to perceive root water stress; 2） relationships between early drought-gene expression and non-hydraulic root-sourced signal （nHRS） ; and 3） relationships between late drought gene expression and hydraulic root-sourced signal （HRS）. Possible pathways may further reveal the unknown mechanisms in root to shoot communications that are affected by AMF ： 1 ） the differences in their composition among root exudates and root ingredient under moisture gradients, which may have potential in indicating the perception of water stress signal component; 2） the ABA-binding factor （ABF）, which may be as one of the important transcripts to respond to the early drought stress, and Ca2＋ as a second messenger collaborating ABA to regulates the open and close of guard ceils. Therefore, studies on their distribution of ABA and Ca2~ in root, stem and leaf under moisture gradients may provide insight into relationships between early drought-gene expression and nHRS; and 3） relationships between the whole plant drought tolerance （e.g. plant type and biomass allocation） and the cell drought tolerance （e.g. antioxidant enzymes and penetration substances）, which may address mechanisms involving in the late drought-gene expression and HRS. With the further progresses are made on the contribution of AMF symbiosis to plant water uptake and drought tolerance, we believe that AMF will have potential application in semi-arid and arid agricultural production.