Plant carbon nourishment of arbuscular mycorrhizal fungi

Plant carbon nourishment of arbuscular mycorrhizal fungi

Available online at ScienceDirect Plant carbon nourishment of arbuscular mycorrhizal fungi Ronelle Roth and Uta Paszkowski Reci...

534KB Sizes 6 Downloads 116 Views

Available online at

ScienceDirect Plant carbon nourishment of arbuscular mycorrhizal fungi Ronelle Roth and Uta Paszkowski Reciprocal nutrient exchange between the majority of land plants and arbucular mycorrhizal (AM) fungi is the cornerstone of a stable symbiosis. To date, a dogma in the comprehension of AM fungal nourishment has been delivery of host organic carbon in the form of sugars. More recently a role for lipids as alternative carbon source or as a signalling molecule during AM symbiosis was proposed. Here we review the symbiotic requirement for carbohydrates and lipids across developmental stages of the AM symbiosis. We present a role for carbohydrate metabolism and signalling to maintain intraradical fungal growth, as opposed to lipid uptake at the arbuscule as an indispensible requirement for completion of the AM fungal life cycle.

nutrients. Concomitantly, fungal growth also extends into the soil where large extraradical mycelia (ERM) develop that are involved in the uptake of minerals and the generation of spores.

Edited by Tzyy-Jen Chiou and Toru Fujiwara

In the absence of a host, AM fungal spores contain sufficient resources to support modest hyphal growth, whereas plant derived organic carbon (C) is thought to fuel intense fungal proliferation and spore formation [2]. Sugars as the key symbiotic C currency transferred from plant to fungus was suggested over 40 years ago [3]. Indeed, intraradical hyphal (IH) uptake of C in the form of hexoses, particularly glucose, was later confirmed by in vivo NMR spectroscopy, radiorespirometry and stable isotope labelling [4–6]. Once acquired by IRM, hexoses can be converted into glycogen and lipids for long distance transport and storage in vesicles and spores inside and outside mycorrhizal roots, respectively. An emerging body of evidence more recently proposes fatty acids (FA) as an additional form of plant-delivered C [7,8,38]. Here, we review knowledge on fungal carbon requirements across the developmental stages of the symbiosis and propose that uptake of FA at the arbuscules is required for the completion of the fungal life-cycle.

A role for carbohydrates in sustaining fungal growth during AM symbiosis

Address Department of Plant Sciences, Downing Street, Cambridge CB2 3EA, United Kingdom Corresponding author: Paszkowski, Uta ([email protected])

Current Opinion in Plant Biology 2017, 39:50–56 This review comes from a themed issue on Cell signalling and gene regulation

1369-5266/Crown Copyright ã 2017 Published by Elsevier Ltd. All rights reserved.

Introduction The symbiotic success in arbuscular mycorrhizal (AM) symbioses for over 400 million years has involved host– fungal transactions, underpinned by a tightly regulated reciprocal nutrient exchange based on mutual rewards. For AM fungi, forming a successful symbiosis with plants is an obligate requirement to complete their life-cycle, manifested by the production of daughter spores [1]. During the association, extensive hyphal growth occurs in the intercellular space of root epidermis and cortex tissue (intraradical mycelia, IRM), accompanied by the development of highly branched haustoria, the arbuscules, inside cortex cells. Arbuscules form by dichotomous hyphal branching while invaginating the cortex cell membrane. On a micrometer scale, an enormous symbiotic interface is created for the exchange of signals and Current Opinion in Plant Biology 2017, 39:50–56

AM fungi acquire sugar in the form of hexoses, predominantly glucose [6,9,10]. The first sugar transporter reported from a glomeromycotan fungus was the monosaccharide transporter from Geosiphon pyriformis, GpMST1, which engages in symbiosis with the cyanobacterium Nostoc punctiforme [11]. The genome of Rhizophagus irregularis (formerly Glomus intraradices) contains related Monosaccharide Transporters (RiMSTs) with different spatial expression patterns and substrate specificity. While RiMST2 is highly expressed in planta, RiMST5 and RiMST6 transcripts accumulated preferentially in germinated spores (Figure 1, Table 1A, [12,13]). RiMST5 and RiMST6 are proton co-transporters with high specificity for glucose, indicating the possibility of glucose acquisition during spore germination [13]. In contrast, RiMST2 has promiscuous substrate specificity for hexoses with preference for xylose and is present in extraradical mycelium (ERM), around IH and arbuscules during the interaction with potato and M. truncatula [12], indicating that sugar uptake in AM fungi might involve IH in addition to arbuscules, which is supported by earlier radiotracer-based observations (Figure 1, [6]). Furthermore, addition of xylose induced RiMST2 expression in

Plant carbon nourishment of arbuscular mycorrhizal fungi Roth and Paszkowski 51

Figure 1



Host Gene Expression Carbohydrates Lipids

Rhizodermis Exodermis Outer Cortex IH







Sucrose Synthase



Vasculature MST5/6 Glc


MST2 Hexoses

Sucrose Current Opinion in Plant Biology

Summary of fungal carbon nourishment during AM symbiosis. AM fungal colonization results in increased expression of source-to-sink metabolizing genes Sucrose Transporters (SUTs) and Sugars Will Eventually Be Exported Transporters (SWEETs) as well as genes encoding sucrose metabolizing enzymes Sucrose Synthase and Invertases. Fungal uptake of apoplastic hexoses are likely mediated by Monosaccharide Transporter2 (MST2) that is induced around intra-radical hyphae (IH) and arbuscules. Upregulation of MST5/6 in spores and extra-radical mycelia (ERM) suggest that AM fungi may also be able to take up glucose (Glc) from their surrounding. AM-conserved genes, RAM2 and FatM are induced in arbuscule-containing cells and are required for synthesis of the C16:0 fatty acid, b-monoacylglycerol (b-MAG). ABC transporters STR1/STR2 that localize to the peri-arbuscular membrane (PAM) might play a role in the transport of b-MAG into the symbiotic interface from where it is taken up by the fungus and utilized for arbuscule formation.

ERM, and may therefore also represent the trigger for RiMST2 expression in planta [12]. Consistently, fungal xylose reductase genes, required for xylose catabolism, were induced during AM colonization. Functional analysis of RiMST2 by knocking down RiMST2 using hostinduced gene silencing (HIGS) led to severely compromised fungal colonisation and abnormal arbuscule morphology [12]. This confirmed the importance of plant carbohydrates for the maintenance of IH and arbuscule

growth and moreover suggested cell wall monosaccharides as a source of organic C for AM fungi. During fungal colonisation of roots, an increased sourceto-sink flux occurs through redirection of sucrose from leaves to roots. Sucrose in- and efflux, monosaccharide uptake, or also sucrose cleaving Sucrose Synthase (SucS) and cell wall Invertases (cwInv) all regulate aspects of sugar partitioning. It is well established that distinct

Table 1A Fungal carbohydrate transporters induced during AM symbiosis Fungal carbohydrate transporters induced during AM symbiosis

Gene expression

Mutant description


IH and arbucules [12]



Spores and ERM [13]


AM symbiosis phenotype: quantitative Reduced colonization compared to WT n.d.

Mutant phenotype: arbuscule

Senescent n.d.

Current Opinion in Plant Biology 2017, 39:50–56

52 Cell signalling and gene regulation

members of gene families encoding these proteins are differentially regulated in response to mycorrhizal colonization, frequently being transcriptionally induced not only in but also next to arbuscule-containing cortex cells and furthermore adjacent to intraradical hyphae [14,15–18] (Figure 1, Table 1B, for recent review see Refs. [14,15–18]). A large body of literature documents their tight spatio-temporal control, and indicates a finely tuned activation of sink metabolism in plant cells surrounding hyphal structures. Interestingly, most of the characterized sugar transporters mediate uptake, possibly reflecting a certain degree of plant-fungal competition for sugars in colonized areas of the root. In contrast, members of the SWEET (Sugars Will Eventually be Exported Transporter) proteins are involved in monosaccharide or disaccharide export or import and can be targets of microbial effectors that redirect sugar fluxes towards the invader (reviewed in Ref. [19]). Promoters of genes encoding distinct members of potato SWEETs were recently shown to be active within and next to arbuscule-hosting cells (Figure 1, Table 1B, [20]). In mycorrhizal roots, SWEET activity may lead to the release of sugars into the interface between the plant and the fungal membrane (the peri-arbuscular space, PAS) and thereby fine-tune sugar fluxes and availability in colonized and adjacent non-colonized cortex cells. Although comprehensive functional analyses of the SWEET genes are required to confirm their function

during AM symbiosis their expression pattern is consistent with contributing to sustained fungal growth inside the host root.

Regulating AM symbiosis development by linking carbohydrate metabolism with signalling It is conceivable that the higher sink strength of mycorrhizal roots leads to sugar availability becoming a limiting factor for symbiosis development. However, the increased availability of hexoses due to overexpression of invertases in tobacco or M. truncatula did not yield increased fungal colonization levels, thereby arguing against a shortage of carbohydrates under normal growth conditions [21]. However, reducing acid invertase activity or interfering with phloem loading caused a significant reduction in fungal colonization, demonstrating that below a critical threshold of available sugars fungal abundance was affected [21]. Conversely, knockdown of the tomato sucrose transporter SUT2, but not SUT1 or SUT4, led to increased mycorrhizal colonization and abolished the positive growth response to AMS in tomato, intuitively pointing towards a fungal advantage in the competition for carbohydrates in the absence of functional SUT2 (Figure 1, Table 1B, [22]). However, although SUT2 is regulated by sucrose, predominantly in sink tissue, direct sucrose transport activity could not be confirmed but instead SUT2 shares features with yeast sugar sensors [23]. Co-localization

Table 1B Plant genes involved in sugar partitioning and induced in AM symbiosis Plant gene name MtSucS1

LeLin6/ Invertases SlSWEETs





Gene expression Upregulated in vasculature, cortex cells surrounding IH and arbuscules [17] Induced [20] Upregulated in vasculature, cortex cells containing IH and arbuscules [19] Induced [21] Upregulated in vasculature, cortex cells containing IH and arbuscules [21] Upregulated in cortex cells containing IH and arbuscules; Protein localizes to PM and PAM [21] Upregulated in cortex cells containing IH and arbuscules [45] Induced [15]

Mutant description

Mutant phenotype: quantitative

Mutant phenotype: arbuscule

Anti-sense KD [44]

Reduced colonization compared to WT


Enzyme Activity Inhibited [20]

Reduced colonization compared to WT


n. d.



Anti-sense KD [21] Anti-sense KD [21]

none none

none none

Anti-sense KD [21]

Increased colonization compared to WT








[44] Baier et al., Plant Physiology 2010, 152:1000–1014. [45] Harrison Plant Journal 1996, 9:491–503.

Current Opinion in Plant Biology 2017, 39:50–56

Plant carbon nourishment of arbuscular mycorrhizal fungi Roth and Paszkowski 53

experiments suggested SUT2 might regulate tissue sucrose fluxes by a combination of sensing sucrose and modulating the activity of other high and low affinity sucrose transporters [23]. In Funneliformis mosseae colonized tomato roots, SUT2 localized to both the plasma membrane (PM) and the peri-arbuscular membrane (PAM) and was shown to physically interact with components of brassinosteroid (BR) biosynthesis and signalling, namely a BRI1-Associated receptor Kinase (BAK1)-like receptor kinase, a membrane steroid-binding protein (MSBP) and the BR biosynthetic sterol reductase DIMINUTO/DWARF [22]. Consistent with a role of BR signalling in AM symbiosis, mutation of either M. truncatula MSPB or tomato and rice DIMINUTO resulted in reduced AM fungal colonization [22,24,25]. A plausible scenario is therefore that SUT2 function directly or indirectly negatively regulates fungal expansion, a constraint which is removed by BR signalling [22]. Induced expression of source-sink components, cwInv, SUTs and SWEETS during AM symbiosis also occurs in response to plant pathogens (reviewed in Ref. [18]). Thus profound modification of source-sink relationships may additionally provoke the activation of host defence signalling and thus regulation during AM symbiosis [26,27]. For some pathogens such as the obligate biotrophic fungus Erysiphe cichoracearum upregulation of hexoses in the host apoplast represents a valuable source of energy allowing for enhanced pathogen growth and reproduction [28]. Sugar transporters of the SWEET family that function as facilitators of sugar efflux have similarly been targeted by pathogens to acquire sugars for their own growth [29]. Interestingly the Arabidopsis homologue of the AM-induced M. truncatula Hexose 1 transporter (MtHex1), AtSTP13, is also induced during plant-pathogen interactions, and important in basal plant resistance [30]. AtSTP13 interacts with both the pattern recognition receptor (PRR) flagellin-sensing 2 (FLS2) and its coreceptor BAK1 upon ligand binding (flg22), resulting in an increased hexose uptake from the apoplast [31]. Upregulation of MtHex1 during AM colonization raises the intriguing possibility that similar sugar signalling mechanisms, operating through PRR complexes such as e.g. chitin-receptors [32,33], may be functionally relevant during AM symbiosis.

AM fungi are fatty acid heterotrophs As oleaginous organisms AM fungal hyphae accumulate most of their dry weight as lipids, stored in extraradical spores and intraradical vesicles from where they can be used as an energy source to sustain anabolism during spore germination (reviewed in Ref. [34]). Mass spectrometry based membrane lipid profiling of R. irregularis confirmed that lipids in ERM consist predominantly of triacylglycerol (TAG, >90%), containing 16:0 (palmitic acid) and 16:1v5 (palmitvaccenic acid) acyl groups [7]. In fungi, cytosolic multi-domain fatty acid (FA) synthases

(Type I FAS) catalyse the de novo synthesis of FAs. Surprisingly, the genome of both R. irregularis and Gigaspora rosea lack genes encoding Type I FAS [7,35,36]. Nevertheless, the enzymatic machinery for the full FA desaturation and elongation pathway is present in R. irregularis and corresponding transcripts are upregulated in IRM [7]; hence substrate availability correlated with intraradical fungal structures and suggested that AM fungi acquire FA from their hosts. Indeed, a recent phylogenomics approach in M. truncatula identified 12 lipid biosynthesis genes that are only present in host plants of AM fungi, thereby possibly reflecting the specific evolutionary adaptation of the symbiotic system to FA requirements [37]. Among these, the acyl-ACP thioesterase (FatM, Fat required for AM symbiosis) is specifically induced in arbuscule-containing cells [38]. Importantly, mutation of FatM led to significantly reduced levels of AM fungal colonization and collapsed arbuscular fine branches [37,38]. FatM belongs to a gene family that includes three additional Fat genes, FatA, FatB and FatC in Medicago, which show partial overlap in their in vitro biochemical activities including their ability to hydrolyze C16:0-ACP [38]. Consequently, all three Fat genes could to some extend restore arbuscule morphology when expressed in fatm mutants, however, none of them was upregulated during AM symbiosis. FatM function however, appeared to be the result of timing and heightened expression culminating in the increased release of C16:0 free fatty acids from plastids during AM symbiosis (Figure 1, [38]). This points to a fungal requirement of FatM for arbuscule development or for supplying lipids building the PAM [37]. In addition to FatM, the earlier reported Reduced Arbuscular Mycorrhization 2 (RAM2) also belongs to the phylogenetically conserved AM-specific genes [37]. RAM2 encodes a sn-2 glycerol-3-phosphate acyltransferases (sn2-GPAT), homologous to GPAT6 in A. thaliana that incorporates modified acyl groups into glycerol-3-phosphate (G3P) to generate cutin monomers and, via its phosphatase activity, also removes phosphate groups from lysophosphatidic acid (LPA) to generate sn-2 monoacylglycerol (b-MAG) (Yang et al 2012 Pl Ph, Yang et al 2010 PNAS). RAM2 was specifically induced in arbusculecontaining cells of M. truncatula and ram2 mutants displayed a strong morphological arbuscule phenotype resembling that of fatm mutants (Figure 1, [8]). Based on related lipid profiles of fatm and ram2, it was proposed that induced FatM expression during AM symbiosis results in an increase in 16:0 FA synthesis in plastids, which RAM2 inside the endoplasmic reticulum converts into 16:0 b–MAG [38]. Cellular export of FAs is mediated by homodimerizing or heterodimerizing half-size ATP-binding cassette (ABC) Current Opinion in Plant Biology 2017, 39:50–56

54 Cell signalling and gene regulation

transporters of the ABCG clade [39]. Specific to mycorrhizal plant species are two ABCG transporters STR1/STR2 that specifically reside and interact within the PAM [40]. Interestingly, mutation of either gene yielded defective arbuscules that closely resemble that of fatm and ram2 mutants suggesting that STR1/STR2 could be required for FA transport across the PAM (Figure 1, [40,41]). However, lipid profiling showed reduced 16:0 b–MAG in fatm and ram2 relative to WT but not in str where 16:0 b–MAG levels were comparable to that of WT despite decreased FatM and RAM2 transcript levels [38]. Although to date, the substrate(s) for STR1/STR2 remain unknown, the overlapping phenotypes of str1/str2, ram2 and fatm in M. truncatula together with lipid profiling speak in collective favour of a link between arbuscule development, lipid biosynthesis, and an involvement of STR1/STR2 in delivering FAs across PAM and into the symbiotic interface.

FA uptake at the arbuscule correlates with fungal completion of life cycle The correlation between arbuscule and spore formation [2] suggested that functional arbuscules are an essential requirement for completion of the fungal life cycle. Arbuscule development requires the DELLA type of GRAS transcription factors as M. truncatula della1/della2 mutants completely lack arbuscules [42]. Interestingly, despite the absence of arbuscules della1/della2 mutants support significant IH growth within the root cortex, suggesting that IH growth in della1/della2 mutants was sustained by hyphal uptake of carbohydrates which may even be available at increased levels due to derepressed GA signalling [42]. The signalling mechanisms underpinning arbuscule development are complex and beyond the scope of this article. Interestingly however, in contrast to DELLA, the GRAS protein RAM1 represents an AM-specific transcription factor that interacts with DELLA and additional GRAS proteins in a complex and is central for arbuscule development in M. truncatula, L. japonicus and petunia, once again reflecting functional conservation across phylogenetically distant mycorrhizal plant species (Figure 1, [43,44–46]). It is noteworthy that the different mutant phenotypes share the commonality that the fungus is able to continue intraradical spreading despite a lack of vesicle and spore production. These observations support a hypothesis whereby carbohydrate uptake promote intraradical fungal proliferation, however, for completion of the fungal life cycle alternative C sources such as FAs are required that are essential for arbuscule development. Importantly, RAM1 is strictly necessary for the activation of RAM2 and Str1/Str2 gene expression, and in M. truncatula overexpression of RAM1 in the absence of the fungus resulted in the upregulation of RAM2 and Str/ Str2 transcripts [43], thus pointing to RAM1 as the Current Opinion in Plant Biology 2017, 39:50–56

master regulator of arbuscule development, possibly by modulating lipid biosynthesis and allocation.

Concluding remarks Our understanding of AM fungal nourishment has progressed immensely since the first observations of sugar uptake by AM fungi. Although a large body of evidence supports altered carbohydrate flux during AM symbiosis, how AM fungi manipulate sugar flux during AM symbiosis remains an open question. Exciting new insights have emerged for the nourishing and possible signalling role of fatty acids for arbuscule development and for the completion of the AM fungal life cycle with production of spores and vesicles. Future labelling experiments should confirm transfer of FAs by STR/STR2 across the PAM and confirm the identity of lipids taken up by the fungus. Recent phylogenomic analyses have identified a plethora of novel candidates involved in lipid biosynthesis and metabolism. Uncovering their function and regulation during AM symbiosis will unleash new insights into organic C nurture of AM fungi and into ancient host-fungal transactions that drive AM symbiosis.

Acknowledgements Ronelle Roth is supported by the BBSRC grant BB/N008723/1 to Uta Paszkowski.

References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as:  of special interest  of outstanding interest 1.

Smith S, Read D: Mycorrhizal Symbiosis. London: Academic Press; 2008.


Douds D: Relationship between hyphal and arbuscular colonization and sporulation in a mycorrhiza of Paspalum notatum flugge. New Phytol. 1994, 126:233-237.


Ho I, Trappe JM: Translocation of C from Festuca plants to their endomycorrhizal fungi. Nat. New Biol. 1973, 244:30-31.


Bago B, Zipfel W, Williams RM, Jun J, Arreola R, Lammers PJ, Pfeffer PE, Shachar-Hill Y: Translocation and utilization of fungal storage lipid in the arbuscular mycorrhizal symbiosis. Plant Physiol. 2002, 128:108-124.


Shachar-Hill Y, Pfeffer PE, Douds D, Osman SF, Doner LW, Ratcliffe RG: Partitioning of intermediary carbon metabolism in vesicular–arbuscular mycorrhizal leek. Plant Physiol. 1995, 108:7-15.


Solaiman M, Saito M: Use of sugars by intraradical hypahe of arbuscular mycorrhizal fungi revealed by radiorespirometry. New Phytol. 1997, 136:533-538.


Wewer V, Brands M, Dormann P: Fatty acid synthesis and lipid metabolism in the obligate biotrophic fungus Rhizophagus irregularis during mycorrhization of Lotus japonicus. Plant J. 2014, 79:398-412. Significant study that describes lipid profiling in Rhizophagus irregularis to characterize lipid metabolism during AM symbiosis. Genomic and transcriptome data mining illustrates the lack of de novo fatty acid synthase in AM fungi.


Wang E, Schornack S, Marsh JF, Gobbato E, Schwessinger B, Eastmond P, Schultze M, Kamoun S, Oldroyd GE: A common signaling process that promotes mycorrhizal and oomycete colonization of plants. Curr. Biol. 2012, 22:2242-2246.

Plant carbon nourishment of arbuscular mycorrhizal fungi Roth and Paszkowski 55


Pfeffer PE, Douds DD Jr, Becard G, Shachar-Hill Y: Carbon uptake and the metabolism and transport of lipids in an arbuscular mycorrhiza. Plant Physiol. 1999, 120:587-598.

10. Douds D, Pfeffer P, Shachar-Hill Y: Application of in vitro methods to study carbon uptake and transport by AM fungi. Plant Soil 2000, 226:255-261. 11. Schussler A, Martin H, Cohen D, Fitz M, Wipf D: Characterization of a carbohydrate transporter from symbiotic glomeromycotan fungi. Nature 2006, 444:933-936. 12. Helber N, Wippel K, Sauer N, Schaarschmidt S, Hause B,  Requena N: A versatile monosaccharide transporter that operates in the arbuscular mycorrhizal fungus Glomus sp. is crucial for the symbiotic relationship with plants. Plant Cell 2011, 23:3812-3823. Functional analysis of the Rhizophagus irregularis AM fungal monosaccharide transporter MST2 by host-induced gene silencing shows fungal gene induction by xylose, required for IH growth and arbuscule function during AM symbiosis. 13. Lahmidi NA, Courty P-E, Brule D, Chatagnier O, Arnould C, Doidy J, Berta G, Lingua G, Wipf D, Bonneau L: Sugar exchanges in arbuscular mycorrhiza: RiMST5 and RiMST6, two novel Rhizophagus irregularis monosaccharide transporters, are involved in both sugar uptake from the soil and from the plant partner. Plant Physiol. Biochem. 2016, 107:354-363. 14. Garcia K, Doidy J, Zimmermann SD, Wipf D, Courty PE: Take a trip through the plant and fungal transportome of mycorrhiza. Trends Plant Sci. 2016, 21:937-950. 15. Gaude N, Bortfeld S, Duensing N, Lohse M, Krajinski F: Arbuscule-containing and non-colonized cortical cells of mycorrhizal roots undergo extensive and specific reprogramming during arbuscular mycorrhizal development. Plant J. 2012, 69:510-528.

transporter SlSUT2 regulate the formation of arbuscular mycorrhiza. Plant Signal. Behav. 2014, 9:e970426. 25. Kuhn H, Kuster H, Requena N: Membrane steroid-binding protein 1 induced by a diffusible fungal signal is critical for mycorrhization in Medicago truncatula. New Phytol. 2010, 185:716-733. 26. Herbers K, Meuwly P, Frommer WB, Metraux JP, Sonnewald U: Systemic acquired resistance mediated by the ectopic expression of invertase: possible hexose sensing in the secretory pathway. Plant Cell 1996, 8:793-803. 27. Roitsch T, Balibrea ME, Hofmann M, Proels R, Sinha AK: Extracellular invertase: key metabolic enzyme and PR protein. J. Exp. Bot. 2003, 54:513-524. 28. Fotopoulos V, Gilbert MJ, Pittman JK, Marvier AC, Buchanan AJ, Sauer N, Hall JL, Williams LE: The monosaccharide transporter gene, AtSTP4, and the cell-wall invertase, Atbetafruct1, are induced in Arabidopsis during infection with the fungal biotroph Erysiphe cichoracearum. Plant Physiol. 2003, 132: 821-829. 29. Chen LQ, Hou BH, Lalonde S, Takanaga H, Hartung ML, Qu XQ, Guo WJ, Kim JG, Underwood W, Chaudhuri B et al.: Sugar transporters for intercellular exchange and nutrition of pathogens. Nature 2010, 468:527-532. 30. Lemonnier P, Gaillard C, Veillet F, Verbeke J, Lemoine R, CoutosThevenot P, La Camera S: Expression of Arabidopsis sugar transport protein STP13 differentially affects glucose transport activity and basal resistance to Botrytis cinerea. Plant Mol. Biol. 2014, 85:473-484.

16. Gomez SK, Harrison MJ: Laser microdissection and its application to analyze gene expression in arbuscular mycorrhizal symbiosis. Pest Manag. Sci. 2009, 65:504-511.

31. Yamada K, Saijo Y, Nakagami H, Takano Y: Regulation of sugar  transporter activity for antibacterial defense in Arabidopsis. Science 2016, 354:1427-1430. Important study that demonstrates the interaction between A. thaliana Sugar Transporter Protein13 (AtSTP13) and pattern recognition receptor (PRR) flagellin-sensing 2 (FLS2) and its co-receptor BAK1 upon flg22 ligand binding.

17. Hohnjec N, Perlick AM, Puhler A, Kuster H: The Medicago truncatula sucrose synthase gene MtSucS1 is activated both in the infected region of root nodules and in the cortex of roots colonized by arbuscular mycorrhizal fungi. Mol. Plant Microbe Interact. 2003, 16:903-915.

32. Miyata K, Kozaki T, Kouzai Y, Ozawa K, Ishii K, Asamizu E, Okabe Y, Umehara Y, Miyamoto A, Kobae Y et al.: The bifunctional plant receptor, OsCERK1, regulates both chitintriggered immunity and arbuscular mycorrhizal symbiosis in rice. Plant Cell Physiol. 2014, 55:1864-1872.

18. Doidy J, Grace E, Kuhn C, Simon-Plas F, Casieri L, Wipf D: Sugar transporters in plants and in their interactions with fungi. Trends Plant Sci. 2012, 17:413-422.

33. Zhang X, Dong W, Sun J, Feng F, Deng Y, He Z, Oldroyd GE, Wang E: The receptor kinase CERK1 has dual functions in symbiosis and immunity signalling. Plant J. 2015, 81:258-267.

19. Chen LQ, Cheung LS, Feng L, Tanner W, Frommer WB: Transport of sugars. Annu. Rev. Biochem. 2015, 84:865-894.

34. Bago B, Pfeffer PE, Shachar-Hill Y: Carbon metabolism and transport in arbuscular mycorrhizas. Plant Phys. 2000, 124: 949-957.

20. Manck-Gotzenberger J, Requena N: Arbuscular mycorrhiza  symbiosis induces a major transcriptional reprogramming of the potato SWEET sugar transporter family. Front. Plant Sci. 2016, 7:487. SWEETs gene expression in and around arbuscule-containing cells suggests SWEET Glc/sucrose efflux in cortex cells surrounding the fungus. 21. Schaarschmidt S, Gonzalez M-C, Roitsch T, Strack D, Sonnewald U, Hause B: Regulation of arbuscular mycorrhization by carbon: the symbiotic interaction cannot be improved by increased carbon availability accomplished by root-specifically enhanced invertase activity. Plant Physiol. 2007, 143:1827-1840. 22. Bitterlich M, Krugel U, Boldt-Burisch K, Franken P, Kuhn C: The  sucrose transporter SlSUT2 from tomato interacts with brassinosteroid functioning and affects arbuscular mycorrhiza formation. Plant J. 2014, 78:877-889. Significant study that provides evidence for the sucrose transporter 2 (SlSUT2) in tomato as an inhibitor of AM symbiosis acting through components of brassinosteroid (BR) biosynthesis and signaling. 23. Barker L, Kuhn C, Weise A, Schulz A, Gebhardt C, Hirner B, Hellmann H, Schulze W, Ward JM, Frommer WB: SUT2, a putative sucrose sensor in sieve elements. Plant Cell 2000, 12: 1153-1164. 24. Bitterlich M, Krugel U, Boldt-Burisch K, Franken P, Kuhn C: Interaction of brassinosteroid functions and sucrose

35. Tisserant E, Malbreil M, Kuo A, Kohler A, Symeonidi A, Balestrini R, Charron P, Duensing N, Frei dit Frey N, Gianinazzi-Pearson V et al.:  Genome of an arbuscular mycorrhizal fungus provides insight into the oldest plant symbiosis. Proc. Natl. Acad. Sci. U. S. A. 2013, 110:20117-20122. Comprehensive analysis of the 153-Mb haploid genome and gene repertoire from the AM fungus, Rhizophagus irregularis. 36. Tang N, San Clemente H, Roy S, Becard G, Zhao B, Roux C: A survey of the gene repertoire of Gigaspora rosea unravels conserved features among Glomeromycota for obligate biotrophy. Front. Microbiol. 2016, 7:233. 37. Bravo A, York T, Pumplin N, Mueller LA, Harrison MJ: Genes  conserved for arbuscular mycorrhizal symbiosis identified through phylogenomics. Nat. Plants 2016, 2:15208. Milestone phylogenomics analysis idendifying 138 AM-host specific genes in M. truncatula of which 12 are involved in lipid biosynthesis including FatM. Reverse genetic analysis of FatM confirms its importance for arbuscule development. 38. Bravo A, Brands M, Wewer V, Dormann P, Harrison MJ:  Arbuscular mycorrhiza-specific enzymes FatM and RAM2 fine-tune lipid biosynthesis to promote development of arbuscular mycorrhiza. New Phytol. 2017, 214:1631-1645. Provides compelling evidence for link between plant fatty acid synthesis and symbiosis development, involving FatM and RAM2, thereby strongly supporting plant carbon nourishment of the fungus in the form of fatty acids. Current Opinion in Plant Biology 2017, 39:50–56

56 Cell signalling and gene regulation

39. Pighin JA, Zheng H, Balakshin LJ, Goodman IP, Western TL, Jetter R, Kunst L, Samuels AL: Plant cuticular lipid export requires an ABC transporter. Science 2004, 306:702-704. 40. Zhang Q, Blaylock LA, Harrison MJ: Two Medicago truncatula half-ABC transporters are essential for arbuscule development in arbuscular mycorrhizal symbiosis. Plant Cell 2010, 22:1483-1497. 41. Gutjahr C, Radovanovic D, Geoffroy J, Zhang Q, Siegler H, Chiapello M, Casieri L, An K, An G, Guiderdoni E et al.: The halfsize ABC transporters STR1 and STR2 are indispensable for mycorrhizal arbuscule formation in rice. Plant J. 2012, 69: 906-920. 42. Floss DS, Levy JG, Levesque-Tremblay V, Pumplin N,  Harrison MJ: DELLA proteins regulate arbuscule formation in arbuscular mycorrhizal symbiosis. Proc. Natl. Acad. Sci. U. S. A. 2013, 110:E5025-E5034. Important paper showing the importance of DELLA proteins intersected with GA signalling for arbuscule formation. Analysis of Mtdella1/Mtdella2 mutants shows a lack of arbuscule development whilst maintaining intraradical hyphal growth.

Current Opinion in Plant Biology 2017, 39:50–56

43. Park HJ, Floss DS, Levesque-Tremblay V, Bravo A, Harrison MJ: Hyphal branching during arbuscule development requires  reduced arbuscular mycorrhiza1. Plant Physiol. 2015, 169: 2774-2788. Significant study showing the requirement and sufficiency of RAM1 for STR1/STR2 and RAM2 gene induction. 44. Rich MK, Schorderet M, Bapaume L, Falquet L, Morel P, Vandenbussche M, Reinhardt D: The Petunia GRAS transcription factor ATA/RAM1 regulates symbiotic gene expression and fungal morphogenesis in arbuscular mycorrhiza. Plant Physiol. 2015, 168:788-797. 45. Xue L, Cui H, Buer B, Vijayakumar V, Delaux PM, Junkermann S, Bucher M: Network of GRAS transcription factors involved in the control of arbuscule development in Lotus japonicus. Plant Physiol. 2015, 167:854-871. 46. Gobbato E, Wang E, Higgins G, Bano SA, Henry C, Schultze M, Oldroyd GE: RAM1 and RAM2 function and expression during arbuscular mycorrhizal symbiosis and Aphanomyces euteiches colonization. Plant Signal. Behav. 2013, 8.