The common symbiotic signaling pathway (CSSP) is a signaling cascade in plants that seen to be activated in both NOD-factor perception (for nodule forming rhizobia), as well as found in MYC-factor perception that are released from Arbuscular mycorrhizal fungi. The pathway is distinguished from the pathogen recognition pathways, but may have some common receptors involved in both pathogen recognition as well as CSSP. A recent work [1] by Kevin Cope and colleagues shown that possibly other type of mycorrhizae may involve the CSSP components such as Myc-factor recognition.
The AMF colonization requires the following chain [2] of events that can be roughly divided into following steps - 1: The Pre-Contact Signaling,
1: The Pre-Contact Signaling,
2: The CSSP
2: A: Perception
2: B: Transmission
2: C: Transcription
3: The Accommodation program
To accurately recognize the infection thread of a different species of organism, and to establish a mutually beneficial association requires robust signaling. [3] AM fungi are also fatty acid auxotrophs; [4] [2] therefore they depend on plant for supply of fatty acid supply. [5]
At the pre-symbiotic signaling; both symbionts release chemical factors in their surroundings so that the partners can find each other. [6] ' Plant root exudates play role in complex microbial interaction, [7] by releasing a lot of versatile materials. [7] [8] [9] among which strigolactone has been identified to facilitate both AMF colonisation and pathogen infection. [8]
It is seen that phosphate starvation in plant induces strigolactone production as well as AMF colonisation. [8] Plants release strigolactone, a class of caroteinoid-based plant hormone which also attracts the fungal symbionts and stimulate the fungal oxidative metabolism and also growth and branching of the fungal partner [2] Strigolactone promotes hyphal branching in germinating AMF spores [9] It plays role in intense ramification of the AMF hyphae at the vicinity of root and then colonization [10]
The common symbiosis signalling pathway is called so because it has common components for fungal symbiosis as well as rhizobial symbiosis. The common signalling pathway probably evolved when the existing pathway for arbuscular mycorrhizae was exploited by rhizobia,. [2] [11]
The perception happens when fungal Myc factor is detected by plant. Myc factors are comparable to rhizobial nod factors . The chemical nature of Myc factor has recently been revealed as lipo-chito-oligosaccharide (Myc-LCOs) and chito-oligosaccharides (Myc-COs) that work as symbiotic signal, [10] [12] [13]
Presence of Strigolactone enhances the production of Myc-CO production by AMF [10]
Myc factor receptor (MFR) is still putative, however However, another protein DMI2 (or SYMRK) that have prominent role in perception process and it is thought to be a co-receptor of MFR. Rice plant probably show a different mechanism using OsCERK1 and OsCEBiP which probably detect chitin oligomers [2] [14] [15]
The transmission happens when the signal transmitted after detection to the gene expression stage. This process is mediated by two nucleoporins NUP85 and NUP133, [11] Alternatively, another hypothesis says HMG-CoA reductase is activated on perception, which then converts HMG-CoA into mevalonate, this mevalonate acts as a second messenger and activates a nuclear K+ cation channel (DMI-1 or Pollux). [2] [16] The transmission stage ends by creating a ‘calcium spike’ into the nucleus [17]
The transcription stage starts when a Calcium and Calmodulin dependent kinase (CCaMK) is activated. [2] then it stimulates a target protein CYCLOPS. [2] CCaMK and CYCLOPS probably forms a complex that along with DELLA protein, regulates the expression of RAM1 (Reduced Arbuscular Mycorrhyza1) gene expression. [2]
The accommodation process is the extensive remodelling of host cortical cells. This includes invagination of host plasmalemma, proliferation of endoplasmic reticulum, golgi apparatus, trans-golgi network and secretary vesicles. Plastids multiply and form “stromules”. Vacuoles also goes through extensive reorganization [11]
Chemical signalling starts prior to two symbionts come into contact. From the host plant's side, it synthesizes and releases a range of caroteinoid based phytohormone, called strigolactones. [2] They have a conserved tricyclic lactone structure also known as ABC rings. [18] Strigolactone biosynthesis occurs mainly in plastid, [19] where D27 (Rice DWARF 27; Arabidopsis ortholog ATD27), an Iron binding beta-carotene isomerase works at upstream of strigolactone biosynthesis [19] Then carotenoid cleavage dioxygenase enzyme CCD7 and CCD8 modifies the structure, which has following orthologs:
Gene name | Localization | function | Rice ortholog | Pea ortholog | Petunia ortholog | Arabidopsis ortholog |
---|---|---|---|---|---|---|
CCD7 | Plastid proteins | involved in strigolactone biosynthesis | D17/ HTD1 | RMS5 | DAD3 | MAX3 |
CCD8 | Plastid proteins | involved in strigolactone biosynthesis | D10 | RMS1 | DAD4 | MAX4 |
Alpha/Beta fold hydrolase | Nuclear proteins | involved in strigolactone perception | D14 | RMS3 | DAD2 | ? |
The alpha/beta fold hydrolase D3 and also D14L (D14-Like) (Later one has Arabidopsis ortholog KAI2, or KARRIKIN INSENSITIVE-2) is reported to have important roles in mycorrhizal symbiosis [3], notably, D3, D14 and D14L are localised in nucleus. [2]
NOPE1 or 'NO PERCEPTION 1', newly discovered transporter protein in Rice (Oryza sativa) and Maize (Zea mays), also required for the priming stage for colonisation by the fungus. NOPE1 is a member of Major Facilitator Super family of transport proteins, capable of N-acetylglucosamine transport. Since nope1 mutant's root exudates fail to elicited transcriptional responses in fungi, it strongly seems NOPE1 secretes something (not yet characterised) that promotes fungal response [2]
There are two main type of root symbiosis; one is root nodule symbiosis by Rhizobia (RN-type) and another is Arbuscular Mycorrhiza (AM-type). There are common genes involved in between these two pathways. [20] these key common components, form the Common Symbiosis pathway (CSP or CSSP). [20] It has been proposed that, RN symbiosis has originated from AM symbiosis. [11] The perception of presence of fungal symbiont, takes place mainly through fungal chemical secretions generally termed as Myc factors. Receptors for Myc factors are yet to be identified. However, DMI2/SYMRK probably acts as a co-receptor of Myc factor receptor (MFR). The AM fungal secreted materials relevant to symbiosis are Myc-LCOs, Myc-Cos, N-Acetylglucosamine [2] [21]
Myc factor | Plant protein it mainly act on |
---|---|
Myc-LCOs | LYS11 in Lotus japonicus |
Short chain chitin oligomers (COs) | OsCERK1 and OsCEBiP in rice |
N-acetylglucosamine | NOPE-1 in maize |
Like Rhizobial LCOs (Nod factors); Myc-LCOs play important role in perception stage. They are kind of secreted materials from AM fungi, mainly mixtures of lipo-chito-oligosaccharides (Myc-LCOs) . In Lotus japonicus , LYS11, a receptor for LCOs, was expressed in root cortex cells associated with intra-radical colonizing arbuscular mycorrhizal fungi [21]
AM host plants show symbiotic-like calcium wave upon exposure to short chain chitin oligomers. It has been reported that production of these molecules by AM fungus Rhizophagus irregularis , gets strongly stimulated upon exposure to strigolactones [2] This gives hint to a model that plants secrete strigolactones and as a reply to it, this fungus increases short chain chitin oligomer, which in turns elicits the plant response to accommodate the fungus. The lysine motif OsCERK1 and OsCEBiP is thought to be involved with perception of short chain chitin oligomers. [2]
NOPE-1 transporter has been described already. NOPE-1 also shows a strong N-acetylglucosamine uptake activity, and is thought to be associated with recognition of presence of fungal symbiont. [2]
Some plant proteins suspected to recognise Myc-factors, Rice OsCERK1 Lysin motif (LysM) receptor-like kinase, is one of them. [15]
There are multiple families of pattern recognition receptors and co-receptors involved in recognition of microbial pathogens and symbionts. Some of the relevant families involved in CSSP, are Membrane bound LysMs (LYM), Soluble LysM Receptor like Protein, LYK (LysM receptors with active Kinase domain), LYR (LysM proteins with inactive kinase domain), etc. (ref)
Seemingly, different combinations of a LYK and LYR generates differential signals, such as some combinations generate a pathogen recognition signal whereas some combinations rise to the symbiotic signal [22] [23] [24] [25]
DMI2/ SYMRK is a receptor like kinase, an important protein in endosymbiosis signal perception, reported in several plants (Mt-DMI2 or Mt-NORK in Medicago trancatula; Lj-SYMRK in Lotus japonicas; Ps-SYM19 in Pisum sativum; OsSYMRK in Rice). OsSYMRK lacks an N terminal domain and exclusively regulate AM symbiosis, does not work for RN symbiosis. [26] Notably, it has been found that a Nodulation-factor inducible gene, MtENOD11 get activated in presence of AMF exudates; Little is known about this phenomenon. [27] [28]
Lysin Motif (LysM) receptor-like kinase are a subfamily related to membrane bound Receptor like kinase (RLKs) with an extracellular region consisting of 3 Lysine motifs. They have some important orthologs in different plants, that vary in their function. (In some plant they are involved in AM symbiosis, in some plants they are not). Tomato (Solanum lycopersicum), a non-legume dicot, also have a similar LysM receptor, SlLYK10 that Promotes AM symbiosis. There are some co-receptors of Myc-factor receptor viz., OsCEBiP in Rice, a LysM membrane protein can function as a co-receptor of OsCERK1 but it works in a different pathway. [29] [30] [31]
Most of these kinases are Serine/Threonine kinase, some are Tyrosine kinase . [32] Also they are type-1 transmembrane proteins, that indicates their N-terminal domain towards the outside of the cell, and the C-terminal domain is towards inside of the cell. [24]
Medicago truncatula | Lotus japonicus | Pisum sativum (pea) | Prunus persica | Arabidopsis thalliana | Brassica rapa | Solanum lycopersicum (Tomato) | Brachypodium distachyon | Oryza sativa (Rice) | |||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Lysine Motif Receptor-Like Kinase and Lysine Motif Receptor like Protein | LYM | LYMI | LYM1 | PpLYM1 | AtLYM1 AtLYM3 | SlLYM1 | BdLYM1 BdLYM3 | OsLYP6 OsLYP5, OsLYP4 | |||||
LYMII | LYM2 | PpLYM3 PpLYM2 | AtLYM2 | SlLYM3 SlLYM2 | BdLYM2 BdLYM4 | OsCEBiP OsLYP3 | |||||||
LYR | LYR 1 | LYRIA | MtNFP MtLYR1 | LjNFR5 LjLYS11 | PpLYR1 | SlLyk10 | Bd LYR1 | OsNFR5 | |||||
LYRIB | MtLYR8 | PpLYR2 | SlLYK9 | Bd LYR2 | |||||||||
LYR 2 | LIRIIA | MtLYR10 | LjLYS16 | PpLYR6 | AtLYK2 | SlLYK2 | |||||||
LYRIIB | MtLYR9 | LjLYS15 | PpLYR7 | SlLYK15 | |||||||||
LYR 3 | LYRIIIA | MtLYR3 | LjLYS12 | PpLYR3 | AtLYK4 | SlLYK4 | Bd LYR4 | OsLYK6 | |||||
LYRIIIB | MtLYR2 | PpLYR4 | SlLYK7 SlLYK6 | ||||||||||
LYRIIIC | MtLYR4 MtLYR7 | LjLYS13 LjLYS14 | AtLYK5 | Bd LYR3 | OsLYK3 OsLYK2, OsLYK4 | ||||||||
LYR 4 | LYRIV | MtLYR5 MtLYR6 | LjLYS20 | PpLYR5 | |||||||||
LYK | LYKI | LYK1, LYK4, LYK5, LYK6, LYK7, LYK2, LYK3, LYK9, LYK8 | LjLYS2 LjLYS1, LjNFR1, LjLYS6, LjLYS7 | PpLYK2 PpLyk1 | AtLYK1/ AtCERK1 | SlLYK13 SlLYK1/ SlBti9, SlLYK12, SlLYK11 | BdLYK1 | OsCERK1 | |||||
LYKII | LYK10 | LjLYS3/ EPR3 | PpLYK3 PpLYK4 | ||||||||||
LYKII | PpLYK5 | AtLYK3 | SlLYK3 | BdLYK3 | |||||||||
Receptor like Kinase | RLK | Mt-DMI2/ Mt-NORK | Lj-SYMRK | Ps-SYM19 | OsSYMRK |
The transmission of signal cascades into nucleus is not well understood. However, this transmission includes carrying the message up to the nuclear membrane and generation of a calcium wave. [33] Some elements involved in this process are as follows:
Lotus japonicas Nucleoporins LjNUP85 and LjNUP133 has potential role in transmission of the signal. [34] Lj-NENA is another important nucleoporin that plays role in AM symbiosis. [35]
It has been proposed that the enzyme 3-hydroxy-3-methylglutaryl-CoA reductase (HMG CoA reductase or HMGR) has potential role in the transmission stage. The enzyme is activated by SYMRK/DMI2, and forms mevalonate. [36] [37] This mevalonate acts as a second messenger, and activates a nuclear potassium channel, DMI1 or pollux. [37]
Nuclear envelope Protein | Function | Rice | Lotus japonicus | Medicago truncatula | Pisum |
CNGC15 | Cyclic-nucleotide gated Calcium-channel | Mt-CNGC15 | |||
Castor | Potassium cation channel | Os-Castor | Lj-Castor | ||
POLLUX or DMI1 | Potassium cation channel | OsPOLLUX | LjPOLLUX | Mt-DMI1 | Ps-SYM8 |
The nuclear calcium channel CNGC15, which is cyclic nucleotide gated ion channel; mediates the symbiotic nuclear Ca2+ influx, and it is countered by K+ efflux by DMI1 [36]
Protein | Function | Name of the Plant | |||
Rice | Lotus japonicus | Medicago Truncatula | Pisum sativum | ||
CCamK | Calcium calmodulin-dependent kinase with role in AMF symbiosis | Os-DMI3 or Os-CCaMK | Lj-CCaMK | Mt-DMI3 | Ps-SYM9 |
CYCLOPS | Coiled coil domain containing proteins that respond to CCamK and promote AMF symbiosis | Os-CYCLOPS | Lj-CYCLOPS | Mt-IPD3 | Ps-SYM33 |
DELLA | Promote AMF symbiosis | Os-SLR1 | Mt-DELLA1 Mt-DELLA2 | Ps-LA Ps-CRY |
Calmodulin is a widespread regulatory protein that functions along with Ca2+ in various biological processes. In AM symbiosis signalling it modulates the CCaMK. [36] CCaMK or DMI3 is a calcium-and-calmodulin-dependent kinase (CCaMK) is thought to be a key decoder of Ca2+ oscillation and an important regulatory kinase protein. Nuclear Ca2+ spiking promotes binding of Ca2+ calmodulin with CCaMK. [36] Binding of Ca2+ calmodulin with CCaMK causes conformational change of CCaMK that stimulates a target protein CYCLOPS which has different orthologs. [36] CYCLOPS is a coiled coil domain containing protein [36] possibly form a complex with CCaMK [36] that works along with DELLA proteins. DELLA proteins are kind of GRAS-domain protein and were originally identified as repressors of Gibberellin signalling pathway, however now it is seen that DELLA provides a mechanism for crosstalk between many signalling pathways. [39] There are two DELLA proteins in Medicago trancatula and Pisum sativum play role in symbiosis whereas in rice plant only one DELLA protein fulfils this task. [36] Reduced Arbuscular Mycorrhiza or RAM1 [36] is a GRAS [40] protein whose gene is directly regulated by DELLA and CCaMK/ CYCLOPS. [36] Using Chromatin immune precipitation assay it has been shown that RAM1 binds to RAM2 gene promoter. [36] RAM1 also play role in many of the fungal accommodation genes directly or indirectly.
A bunch of GRAS proteins play role in AM symbiosis whose roles are not yet fully understood. These includes RAD1 (REQUIRED FOR ARBUSCLE DEVELOPMENT 1), MIG1 (MYCORRHIZA INDUCED GRAS1), NSP1, NSP2 etc. [36] WRKY transcription factor genes are thought to play very important roles in establishment of mycorrhizal symbiosis and they perhaps work through regulating plant defense genes. [41]
Huge reorganization of cortex cells takes place in order to make accommodation for the fungal endosymbiont. The pre-penetration apparatus (PPA) like structure and the peri-arbuscular membrane have to form and the cytoplasm have to retract, [34] so, the vacuole retract in size, the nucleus and nucleolus enlarge in size and chromatin decondense indicating heightened transcriptional activity, [34] Plastids multiply and stay connected with “stromulus”. [34] Furthermore, it was suggested that the apoplastic longitudinal hyphal growth is probably regulated by plant genes such as taci1 and CDPK1. [42]
Although various proteins have been identified which may play role on how this accommodation process happens, the detailed signalling cascade is not well understood. Some of the proteins and machineries involved in the deposition on peri-arbuscular membrane are EXOCYST complex, EXO70 subunit, a symbiosis-specific splice variant of SYP132, VAPYRIN, two variants of VAMP721 etc. [36] Plant enzymes FatM and RAM2 [43] and ABC transporter STR/STR2 is involved in synthesis and supplying of a lipid 16:0 β-monoacylglycerol to the AM fungi. [43] [44]
The protein composition of peri-arbuscular membrane is very different from plasma membrane. It includes some special transporters such as phosphate transporter (Mt-PT4, Os-PT11, Os-PT13), Ammonium transporter (Mt-AMT2 and 3) etc. and ABC transporters such as lipid transporter STR/STR2 [36] [45]
AM fungi and plants co-evolved and developed a very complex interaction that allow the plant accommodate the AM-fungal host [46] [47] [48] It has been proposed that, RN symbiosis has originated from AM symbiosis [34] [35]
An endosymbiont or endobiont is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον endon "within", σύν syn "together" and βίωσις biosis "living".) Examples are nitrogen-fixing bacteria, which live in the root nodules of legumes, single-cell algae inside reef-building corals and bacterial endosymbionts that provide essential nutrients to insects.
A mycorrhiza is a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant's rhizosphere, its root system. Mycorrhizae play important roles in plant nutrition, soil biology, and soil chemistry.
Nod factors, are signaling molecules produced by soil bacteria known as rhizobia in response to flavonoid exudation from plants under nitrogen limited conditions. Nod factors initiate the establishment of a symbiotic relationship between legumes and rhizobia by inducing nodulation. Nod factors produce the differentiation of plant tissue in root hairs into nodules where the bacteria reside and are able to fix nitrogen from the atmosphere for the plant in exchange for photosynthates and the appropriate environment for nitrogen fixation. One of the most important features provided by the plant in this symbiosis is the production of leghemoglobin, which maintains the oxygen concentration low and prevents the inhibition of nitrogenase activity.
An arbuscular mycorrhiza (AM) is a type of mycorrhiza in which the symbiont fungus penetrates the cortical cells of the roots of a vascular plant forming arbuscules. Arbuscular mycorrhiza is a type of endomycorrhiza along with ericoid mycorrhiza and orchid mycorrhiza.
Root hair, or absorbent hairs, are outgrowths of epidermal cells, specialized cells at the tip of a plant root. They are lateral extensions of a single cell and are only rarely branched. They are found in the region of maturation, of the root. Root hair cells improve plant water absorption by increasing root surface area to volume ratio which allows the root hair cell to take in more water. The large vacuole inside root hair cells makes this intake much more efficient. Root hairs are also important for nutrient uptake as they are main interface between plants and mycorrhizal fungi.
The rhizosphere is the narrow region of soil or substrate that is directly influenced by root secretions and associated soil microorganisms known as the root microbiome. Soil pores in the rhizosphere can contain many bacteria and other microorganisms that feed on sloughed-off plant cells, termed rhizodeposition, and the proteins and sugars released by roots, termed root exudates. This symbiosis leads to more complex interactions, influencing plant growth and competition for resources. Much of the nutrient cycling and disease suppression by antibiotics required by plants, occurs immediately adjacent to roots due to root exudates and metabolic products of symbiotic and pathogenic communities of microorganisms. The rhizosphere also provides space to produce allelochemicals to control neighbours and relatives.
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Glomus is a genus of arbuscular mycorrhizal (AM) fungi, and all species form symbiotic relationships (mycorrhizae) with plant roots. Glomus is the largest genus of AM fungi, with ca. 85 species described, but is currently defined as non-monophyletic.
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Nitrogen nutrition in the arbuscular mycorrhizal system refers to...
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