Interactions of beneficial and detrimental root-colonizing filamentous microbes with plant hosts

Understanding commonalities and differences of how symbiotic and parasitic microbes interact with plants will improve advantageous interactions and allow pathogen control strategies in crops. Recently established systems enable studies of root pathogenic and symbiotic interactions in the same plant species.

impacts, plant pathology research has focused on disease resistance, and has therefore been dominated by the study of pathogen modulation of plant immunity [7].
In contrast, research into benefi cial eff ects of plant microbes is mainly guided by nutritional aspects [5] with much less focus on immunity and compatibility aspects. A well-studied example for benefi cial symbiosis is the asso ciation of plant roots with fungi [8]. Th is mycorrhiza can be found in 80% of all land plants. Arbuscular myco rrhiza (AM) relies on an evolutionarily ancient program dating back to early land plants and was key when plants conquered the land. It is conceivable that pathogens take advantage of this symbiosis program to gain access to the host plant's resources.
We have extensive evidence for commonalities between pathogenic and symbiotic lifestyles. Both interaction types follow similar developmental processes of identi fication, plant cell penetration and re-diff erentiation of the host cells to establish intracellular interfaces for nutrient and information exchange ( Figure 1) [9]. Undecorated chitin oligomers of microbe origin, known to be potent inducers of plant immunity [10], were found recently to also activate symbiosis-related signaling [11]. Furthermore, eff ector proteins, hallmarks of animal and plant pathogens and which suppress defense and reprogram the host, were also described recently in mycorrhizal fungi [12,13]. Considering these similarities, it is surprising that very few parallels have been made between modes of pathogen and symbiotic colonization. Since symbiotic mycorrhiza occur only below ground, we are bound to study both types of interactions in roots. Th is will enab le us to generate pathogen-resistant crop plants without aff ecting benefi cial symbiosis. To do so, we need dual research systems that enable these comparative studies.
Our ability to compare principles of colonization is hampered by the traditional separation of plant pathology systems and symbiosis systems on diff erent plant species.
Arabidopsis thaliana, the plant system of choice for numerous plant-pathogen interactions, does not support feeding structure formation by endomycorrhizal fungi, and thus is limited to studies of non-host interactions [14]. Notably, separate research of Phytophthora pathogens in its host plants potato and tomato, and benefi cial AM fungi in legumes and rice, has shown that both follow analogous steps to establish an interaction ( Figure 1). Moreover, both form specialized accommodation structures within plant cells ( Figure 2). Th us, it would be good to have a single plant species that allows direct comparison between pathogenic and symbiotic interactions.

Dual systems enable the study of root colonization by fi lamentous pathogens and symbionts
Dual systems are crop plants whose roots are colonized by fi lamentous symbiotic microbes (for example, the widely used AM fungus Glomus irregularis) but in addition can also be infected by other biotrophic pathogens. Th e legume species Medicago truncatula and Lotus japonicus have served as genetic model plants for symbiosis research [4,8] and a huge genetic resource has been established by the community, rendering these plants prime candidates for systems to study similarities and diff erences between symbiosis and pathogenicity. Another established monocot system for symbiosis research is rice [15]. It is surprising, however, that not many root pathogen infection systems with clearly distinguishable biotrophic stages exist for these plant species.
In AM fungi colonized roots, the ideal microbial pathogen partner to compare with is a naturally root colonizing fi lamentous organism with a broad host range. Other than G. irregularis it should be cultivatable, transformable and effi ciently traceable in living tissuesfor example, by fl uorescent proteins. Several fi lamentous microbes have been employed to unravel the mechanisms involved in root colonization (Table 1). Historically, most research has been carried out using Aphanomyces euteiches [16], Colletotrichum trifolii [17] and Verticillium species [18], and to a major extent using Magnaporthe oryzae [19]. Piriformospora indica colonization of roots and its growth-promoting eff ects have also been studied in the economically relevant barley [20], a monocot plant that also establishes interactions with AM fungi [21].
Notably, C. trifolii and M. oryzae are major leaf colonizers in nature; however, they can be employed for root infection under laboratory conditions [17,22]. C. trifolii experiments have helped to extend the role of the DMI3 (DOESN'T MAKE INFECTIONS 3) calcium/calmodulin kinase, a classical symbiosis signaling element, from symbiotic to pathogenic interactions [17]. Infections with C. trifolii showed diff erential responses between plants that carried either a DMI3 wild-type or a mutated allele.
While C. trifolii and M. oryzae were reported to establish biotrophic stages inside the root, others such as apoplast-colonizing A. euteiches [16] and Verticillium albo-atrum [18] lack the potential to form intracellular structures such as the arbuscular feeding structures of AM fungi, thereby complicating the delimitation of their biotrophic stages. Nevertheless, A. euteiches has been successfully employed to identify signaling elements that are shared between symbiotic and pathogen perception mechanisms. An example is NFP (NOD FACTOR PERCEPTION), a lysin motif receptor-like kinase (LysM-RLK) that is integral to perception of lipochitooligosaccharidic symbiosis factors from AM fungi by the plant. Recently, NFP was shown to also aff ect colonization by the pathogen A. euteiches [16]. Larger sets of LysM-RLK receptor variants can be found in root nodule and AM-forming legumes [23,24] and non-nodulating but AM-forming rice [25] compared with non-mycorrhized and non-nodulated Arabidopsis. Th is enlarged receptor repertoire could correlate with a requirement for higher resolution signal discrimination between pathogens, mycorrhizal fungi and symbiotic bacteria. Further research is required to clarify how specifi city in signal perception is achieved.

Phytophthora palmivora provides new opportunities for comparative studies in roots
A key element of benefi cial AM fungi is nutrient exchange through tree-like arbuscules in the inner cortical cells of the host root ( Figure 2) [9]. Th us, it is of particular interest to compare arbuscules with other morphologically distinguishable intracellular interfaces formed by pathogens in roots. M. oryzae and C. trifolii do form specialized intracellular hyphae and P. indica Following chemical cross-talk, the microbe germinates and forms attachment and penetration structures, termed appressoria and hyphopodia, respectively. Penetration occurs through or between cells, and in the case of AM fungi intracellular hyphae are supported by a plant-derived pre-penetration apparatus [17]. Specialized intracellular interfaces, termed haustoria and arbuscules, form within plant root cells. Phytophthora infections eventually result in cell death of the infected tissue, while obligate biotrophic AM fungi continuously reside in living plant roots. infrequently forms coils in root cells [26]. However, abundant haustoria of pathogenic fungi and oomycetes such as Phytophthora most strikingly resemble dedicated interfaces as observed in arbuscules [9]. Similar to the arbuscules of AM fungi, which are surrounded by a periarbuscular membrane spiked with an exclusively positioned phosphate transporter [27,28], Phytophthora haustoria are enveloped by an extrahaustorial membrane with distinct features to normal plant plasma membranes [29]. We still do not fully understand why Phytophthora forms these intricate struc tures in plant cells. So far, haustoria are known to facilitate the deployment of pathogen-encoded eff ector proteins [30]. Currently, cellular research on Phytophthora haustoria is mainly focused on its interaction with above-ground plant parts [31]. However, the majority of Phytophthora species are prominent root pathogens [32], such as P. palmivora, which forms haustoria in roots and thus allows us to comparatively study arbuscules and haustoria in the same tissue ( Figure 2). Indeed, the P. palmivora-M. truncatula system has recently helped to identify an overlap in the chemical language spoken between microbe and plant. A cutin-derived signal produced by M. truncatula roots was found to be required for the interaction of both benefi cial AM fungi and pathogenic oomycetes [33].
P. palmivora has a broad host range infecting numerous monocot and dicot cash crops such as oil palms, rubber trees, cocoa and coconut. As with other pathogens, its colonization potential resembles that of widespread AM fungi (Table 1). Th e broad host range of P. palmivora also enables it to infect the workhorse species of time-effi cient plant cell biology and fast-forward genetics: Nicotiana benthamiana [34], a species that has also been studied in the context of AM fungal symbiosis [35] (Figure 2). Transformation of Phytophthora facilitates the generation of tracer lines and allows highlighting pathogen-formed haustoria components [31]. Transformability and also knowledge of the P. palmivora genome will enable us to functionally assess pathogen-encoded factors that contribute to colonization such as secreted eff ector proteins.

We need to know more about eff ectors
Historically, microbe-encoded eff ector proteins with functionality inside the host were a hallmark of bona fi de pathogens [36]. Cytoplasmic eff ectors enter the host cells and modify plant processes to suppress immunity and support pathogen colonization [7]. Notably, eff ectors that target plant root cell processes have recently also been reported from Laccaria bicolor and G. irregularis, which are both benefi cial mycorrhizal fungi [12,13]. Evidently, also in benefi cial plant-microbe interactions the microbe needs to suppress plant immunity to establish an interaction and prove its usefulness to the plant. Th e growing repertoire of characterized eff ectors and availability of numerous pathogen genomes (Table 1) will provide inroads to study common and contrasting eff ector-target ed processes of mutualistic and parasitic inter actions. Furthermore, eff ector functions have been studied mainly in leaves, but transformable fi lamentous microbes that colonize roots now give us the tools to study eff ector functions below ground, where principles might diverge from the known.
We have the tools and we will use them Th e described systems (Table 1), which are based on dual plant infections utilizing AM fungi and pathogens, will allow us to challenge fi ndings obtained previously in either pathogen or symbiosis research. Th is will give insights into the commonalities and diff erences of both types of interaction outcomes. Moreover, several biotrophic pathogens (M. oryzae, P. palmivora), endophytes (P. indica) and benefi cial AM fungi (G. irregularis) are directly applicable on rice, barley and other dicot crops, thereby bypassing the step to transfer knowledge of mechanisms from model plants into application.
Systems that form specialized intracellular structures will be integral in solving open questions on fi lamentous microbe interactions with plants. We need to elucidate how similar arbuscules and pathogen interfaces are [9], and whether they generally also serve as devices for nutrient uptake [37]. Knowledge on general and specifi c transport mechanisms between microbe and plant may be decisive for our ability to protect plants from pathogens while maintaining symbiosis. Common sets of regulated genes point to a large overlap in development processes during benefi cial and detrimental interactions [19]; however, we still need to clarify whether observed structural features, such as the pre-penetration apparatus formed during root colonization by AM fungi [38] or the typical tree-like branching of AM fungal arbuscules, are defi ned by the microbe species or the plant, or both.
Root-colonizing microbes are guided by chemical plant signals but we do not know the extent to which these signals overlap. For example, plant fl avonoids act as attrac tants for mobile oomycete zoospores and benefi cial nitrogen-fi xing root bacteria, while symbiotic fungi but also parasitic Striga hermontica plants perceive strigolactones released by the plant [39,40]. Whether some fi lamentous pathogens are consistently responsive to strigolactones remains to be clarifi ed [41].
Furthermore, it is important to identify contrasting principles in eff ector-mediated reprogramming and immune suppression between symbiotic microbes and pathogens. Th is might enable us to engineer the host processes they target in order to direct the outcome of an interaction towards the benefi cial side.

Competing interests
The authors declare that they have no competing interests.