Topic editor：HU Yanbo.
The arbuscular mycorrhizal（AM） symbiosis is probably the most ancient form of all plant root symbioses， involving specialized fungi belonging to the phyla Glomeromycota and Mucoromycota， and more than 70% of extant plant species including major crops such as rice， wheat， maize， potatoes， cassava， common beans， cotton， as well as many fruit trees. Originating more than 400 million years ago and now found on all continents， it is widely recognized for its role in plant phosphorus（P） nutrition， particularly important under P deficiency， as well as for promoting plant disease and drought resistances and stabilizing plant coexistence in multi-species communities.
The hyphae of AM fungi can extend more than ten centimetres from the roots and collect soil resources such as P or nitrogen（N） for exchange with their host plant（s） for reduced carbon（C） compounds. In doing so， they extend the soil zone from which a plant can extract resources. Especially for poorly diffusible nutrients such as inorganic orthophosphate or organic forms of P or N， the hyphae need to come into close contact with these resources to be able to acquire them effectively. The size of this zone of direct influence of the hyphae on the soil environment， the hyphosphere（where the physical， chemical and biological properties are different from the bulk soil）， depends on the diffusibility of the soil resources and can range from a few micrometres for orthophosphate to several millimetres for water or highly mobile forms of N such as nitrate. This is also the reason why AM fungi often form many metres of hyphae per gram of soil， in contrast to roots， whose density in the soil usually does not exceed a few centimetres or decimetres per gram.
In recent years， particular attention has been paid to processes in the AM fungal hyphosphere that recycle P and N from organic forms. Previous research suggested that AM fungi could promote the mineralization of organic nutrients and plant acquisition from such sources beyond the direct reach of plant roots， but the exact underlying mechanisms remained unclear. As far as we know， AM fungi are not capable of producing any significant amounts of lytic exoenzymes and therefore could not carry out mineralization on their own. Attention has therefore logically turned to the microorganisms in the hyphosphere zone（the so-called hyphosphere microbiome）. The composition of these microbiomes is very different from the rest of the soil， with prokaryotic taxa such as Pseudomonas， Streptomyces， Paenibacillus， Ramlibacter， Arthrobacter， Bacillus， Massilia， Stenotrophomonas and others being particularly enriched in the AM fungal hyphosphere. More recently， hyphosphere-associated protists have been discovered， recruiting mainly from Cercozoa（e.g.， Massisteria）， Lobosa（e.g.， Vermamoeba） and Chlorophyta（e.g.， Bracteacoccus）. Some of these microbes have been experimentally shown to be involved in the mineralization of organic nutrients（e.g.， phytate， chitin） in the AM fungal hyphosphere， while others have been suggested to play similar or other roles in such processes， but direct experimental evidence is still lacking.
One of the most fascinating discoveries of the last decade has been that AM fungi provide some of their C to（at least some of） their associated prokaryotes to stimulate organic nutrient mineralization. This means that there probably is a finely tuned regulation of organic nutrient mineralization in the AM fungal hyphosphere enabled through trading of AM fungal for prokaryotic resources/services， supporting previously postulated theory of hyper-symbiosis in this enigmatic soil zone. We do not know yet with certainty how large such C inputs into the hyphosphere soil could be and how they vary across different environmental and soil conditions. Likewise， we do not know which consequences such processes could have for soil C stocks， sequestration and turnover. We also do not know how the hyphosphere microbiomes couple the two processes of C consumption and organic nutrients（P and N） mineralization and turnover. In addition， although we know that the C present in the AM fungi derive from recent plant photosynthesis， we do not know how the plant controls or influences the C input into the hyphosphere. But it is assumed that the fungal C inputs into the hyphosphere could be substantial and thus worth further investigation. Besides other mechanisms how AM fungi affect soil properties such as hydrophobility， hydraulic connectivity， buildup and maintenance of soil structure and shaping up soil microbiomes， which all deserve a closer look.
In addition to the mutually beneficial interactions described above， other types of biological interactions take place in the AM fungal hyphosphere. In particular， it has recently been observed， both in model experiments and in a number of unsterile field soils， that the abundance of ammonia-oxidizing bacteria（AOB） is often suppressed in the presence of actively growing AM fungal hyphae. However， this was not the case for ammonia oxidizing archaea（AOA）. Interestingly， AOA have been reported to be highly active in the AM fungal hyphosphere， resulting in their high 13C enrichment upon stable isotope probing of the AM fungal hyphosphere microbiome， despite them being chemolithotrophs that are thought to derive all their energy from the oxidation of ammonia and all their C from the fixation of CO2. Thus， although direct competition for free ammonium ions between AM fungi and AOB has previously been postulated to explain the observed patterns of interactions， there may be more to the interactions between AM fungi and the different prokaryotes， including the production of as yet unknown biological inhibitors of nitrification， the consumption of nitrification intermediates such as nitric oxide， or interactions mediated by other members of the AM fungal microbiome. In this respect， the demonstration of existence of native soil microbial communities with suppressive effects on the growth of AM fungal hyphae indicates that there are still many unknowns regarding the biological interactions between AM fungi and other soil microbes.
So what now？ Where should we， and especially the younger generation of researchers， look to better understand this microscopic world full of changes and surprises？ And to use it to create more sustainable agriculture and more resilient ecosystems？ In addition to open eyes and curious minds willing to observe natural phenomena， it will probably take targeted and creatively designed model experiments and more frequent use of isotopes to directly trace the transformations of C and nutrients. Tracking P is particularly challenging in this respect， as there is only one stable isotope of P in our universe（31P） and all the other isotopes that could potentially be used are radioactive. In addition， we should add to the rapidly growing body of evidence on the composition of hyphosphere microbiomes by attempting to reconstruct them from their components， which will most likely depend on the previous cultivation of the various microorganisms and monoxenic cultivation systems. Of course， in addition to these creative experiments， developing novel theories and frameworks is also indispensable. The concept of the plants-AM fungi-bacteria continuum lays the foundation for in-depth study on the biological interactions between AM fungi and soil microbes， and incorporates plants into the entire holobiont. Such progress is in their infancy and may be slow but potentially very rewarding given the importance of soil nutrient recycling and C sequestration now and in the near future.
丛枝菌根（arbuscular mycorrhizal，AM）共生可能是所有植物根部共生中最古老的一种形式。这一共生关系主要包括球囊菌门（Glomeromycota）和毛霉门（Mucoromycota）的专性真菌，可与水稻（Oryza sativa）、小麦（Triticum aestivum）、玉米（Zea mays）、马铃薯（Solanum tuberosum）、木薯（Manihot esculenta）、菜豆（Phaseolus vulgaris）、棉花（Gossypium hirsutum）等主要农作物，以及许多果树在内的超过70%的植物种类共生。这种共生关系起源于4亿多年前，如今已广泛分布于全球各大洲。由于其在植物磷营养吸收，尤其是在缺磷条件下的重要作用，丛枝菌根共生备受关注。同时，AM真菌能够提高植物对病害和干旱的抵抗力，还能促进植物群落中植物物种的共存与稳定。
AM真菌的菌丝可以从植物根系延伸超过10 cm，从土壤中吸收磷和氮等养分，并将其与宿主植物进行交换以获得还原碳化合物，显著扩大了植物对资源的获取范围。特别是对于扩散性较差的养分，如无机磷酸盐或以有机形式存在的磷和氮，而菌丝需要与这些养分直接接触才能实现高效吸收。菌丝对土壤环境的直接影响范围称为“菌丝际”（hyphosphere），其物理、化学和生物特性与周围的土壤不同。菌丝际的范围取决于土壤中养分的扩散能力，对于无机磷酸盐，这个范围可能仅有几微米，而对于水或移动性较高的硝态氮，则可达到几毫米。这也解释了为何AM真菌的菌丝在1 g土壤中通常可形成多达数米的长度，而植物根系的长度通常仅为几厘米或分米。
近年来，学界对AM真菌菌丝际中有机磷和氮循环过程的研究给予了高度关注。已有研究表明，AM真菌可能通过促进有机养分的矿化作用，帮助植物从超出根系直接作用范围的区域获取养分。然而，其具体作用机制尚未明晰。目前的研究表明，AM真菌本身无法分泌大量的裂解性胞外酶，因而无法独立完成有机养分的矿化过程。因此，研究的重点逐渐转向菌丝际微生物群落（即“菌丝际微生物组”）。这些微生物群落的组成与周围土壤显著不同，其中一些原核生物类群在AM真菌菌丝际中表现出显著的富集特征，其中包括假单胞菌属（Pseudomonas）、链霉菌属（Streptomyces）、类芽孢杆菌属（Paenibacillus）、沙壤土杆菌属（Ramlibacter）、节杆菌属（Arthrobacter）、芽孢杆菌属（Bacillus）、马赛菌属（Massilia）及寡氧单孢菌属（Stenotrophomonas）等原核生物类群。此外，近年来还发现一些与菌丝际密切相关的原生生物，主要包括丝足虫门（如Massisteria）、变形虫目（如Vermamoeba）、以及绿藻门（如Bracteacoccus）。部分研究已通过试验验证，这些微生物能够参与有机养分（如植酸和几丁质）的矿化过程，而另一些微生物可能在类似或其他矿化机制中发挥作用，但尚缺乏直接的试验证据加以证明。
过去的10年里，AM真菌研究的一个重要发现是，它们能够将部分碳供给土壤中的某些原核生物（至少是部分原核生物），以促进有机养分的矿化过程。这一发现表明，AM真菌菌丝际区域，可能通过AM真菌与原核生物之间的资源交换，实现对有机养分矿化过程的精细调控，这一发现支持菌丝际土壤区域存在超共生（hyper-symbiosis）机制的理论假设。然而，目前我们尚不清楚这些碳输入到菌丝际土壤中的具体规模及其在不同环境和土壤条件下的变化规律。同样，我们也不确定这一过程对土壤碳储量、封存和周转的具体影响。此外，菌丝际微生物群如何耦合碳消耗与有机养分（如磷和氮）的矿化和周转过程，仍是未解之谜。尽管我们知道AM真菌中的碳源自植物新近发生的光合作用，但是植物如何控制或调节碳向菌丝际的输入，仍缺乏明确的认识。尽管如此，有研究推测，AM真菌对菌丝际的碳输入可能至关重要，值得进一步深入探索。除了这些作用机制外，AM真菌对土壤特性（如土壤疏水性、水力连接性、土壤结构的形成与维持及土壤微生物群落的塑造）等方面的影响，也同样需要引起更多的关注与研究。
除了上述互利关系外，AM真菌菌丝际还发生着其他类型的生物相互作用。近期研究表明，在模型试验和若干未经灭菌处理的田间土壤中，当AM真菌菌丝处于活跃生长状态时，氨氧化细菌（ammonia-oxidizing bacteria，AOB）的丰度往往受到抑制，而氨氧化古菌（ammonia oxidizing archaea， AOA）则不受影响。值得注意的是，尽管AOA为化能自养生物，其能量来源于氨的氧化，碳来源于CO₂的固定，但研究发现它们在AM真菌菌丝际中表现出较高的活性，并在稳定同位素探测中表现出明显的13C富集。这一现象表明，尽管此前研究推测，AM真菌与AOB之间可能因竞争自由铵离子形成互作模式，但AM真菌与不同原核生物之间的相互作用可能远比我们目前理解的更为复杂。这些相互作用可能涉及尚未被发现的生物硝化抑制因子、硝化中间体（如一氧化氮）的消耗，或由AM真菌微生物群落中的其他成员所介导的相互作用。在此背景下，土壤中存在能够抑制AM真菌菌丝生长的土著微生物群落的现象，进一步表明AM真菌与其他土壤微生物之间的生物相互作用仍有许多未知的方面，亟待深入研究。
那么，接下来我们该如何展开研究？尤其是年轻一代的研究人员，应该从哪些方向着手，以更深入地理解这一充满变数和惊奇的微观世界？并将这一理解应用于创造更可持续的农业模式和更具韧性的生态系统建设？除了保持敏锐的观察力和好奇心，愿意主动探索自然现象外，可能还需要通过精心设计的模型试验，以及更多的运用同位素技术，直接追踪碳和营养元素的转化过程。在这一领域，追踪磷的转化尤其具有挑战性，因为自然界中仅有一种稳定的磷同位素（31P），而其他可能的同位素都具有放射性。此外，我们还需进一步丰富关于菌丝际微生物群组成的证据库，尝试从其组成成分中重建微生物群落——这一过程可能依赖于对各类微生物的先行培养及单菌培养体系的应用。当然，除了这些创新性试验外，发展新的理论框架也是至关重要的。植物-AM真菌-细菌连续体（plants-AM fungi-bacteria continuum）的概念为深入研究AM真菌与土壤微生物之间的生物相互作用提供了理论基础，并将植物整合到生物连续共生体的研究范畴。尽管这一领域仍处于起步阶段，进展也较为缓慢，但鉴于土壤营养循环和碳固存的关键性作用，尤其是在当前及未来的生态环境中，其研究潜力巨大，回报也尤为可期。
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