Research using model species leads to fundamental discoveries in biology, and this holds true of our favorite model legume Medicago truncatula as well. Findings made over the past year made particularly significant contributions to what will soon be textbook knowledge. Here’s a roundup of three noteworthy lessons we learned last year from international groups working on Medicago.
1 Plants feed fats as well as sugars to associated beneficial fungi.
In exchange for soil nutrients such as phosphorous and nitrogen, plant hosts have long been thought to provide carbon in the form of sugars to their beneficial fungal partners. These beneficial “mycorrhizal” fungi such as Rhizophagus irregularis use different types of lipids to build essential cellular membranes. Curiously, however, mycorrhizal fungi themselves do not encode enzymes that help synthesize 16 Carbon containing saturated fatty acids (eg: Palmictic acid). In the June Issue of Science, Jiang et al. and Luginbuehl et al., use the Medicago truncatula ram1 and ram2 (REQUIRED FOR ARBUSCULAR MYCORRHIZATION) mutants amongst others to demonstrate that plants also provide lipids to fungus (1, 2). They further overexpress a thioesterase (Umbellularia californica lauroyl-ACP thioesterase) that increases levels of the 12-carbon lauric acid, normally absent in Medicago roots. Interestingly, this also leads to an almost 20-fold increase in lauric-acid-containing triglycerides present in the fungal spores! This finding also holds true in other legume species such as Lotus. In a meticulous study, Keymer et al., describe DIS (DISINTEGERATED ARBUSCULES) as another component involved in the pathway required for transfer of lipids to arbuscular mycorrhizal fungi (3). Thus this pathway, previously thought to only play a role in signaling to mycorrhiza, is also directly involved in feeding the microbes.
2 Cysteine-rich peptides can help discriminate inefficient nitrogen-fixing bacteria.
Nodule Cysteine Rich (NCR) peptides are present exclusively in “galegoid” legumes such as Medicago truncatula. “Galegoid” refers to legumes whose bacterial symbiotic partners undergo “terminal differentiation” before they start fixing nitrogen in host nodules. Terminal differentiation involves multiplication of the bacterial nuclear genome without cell division leading to formation of large non-motile rhizobia now called “bacteroids.” NCR peptides play a clear role in inducing terminal differentiation of compatible bacteria thus ensuring successful nitrogen fixation. In two articles published in the journal PNAS, scientists at the University of Kentucky and collaborators identify a new role for the NCR peptides. Yang et al., and Wang et al., show that some NCR peptides might also be involved in actively terminating bacterial interactions that form inefficient symbiosis with the host (4, 5). They describe two Medicago genes NITROGEN FIXATION SPECIFICTY (NFS) NFS1 and NFS2 identified in a screen of recombinant Inbred Lines (RIL) derived from a cross between Jemalong A17 and DZA315 genotypes. When nodulated with the Rhizobium meliloti strain, Rm41 DZA315 lines can form pink, fully functional nitrogen-fixing nodules (fix+) but Jemalong A17 plants cannot (fix-). The authors show that the presence of NFS1 and NFS2 in Jemalong A17 plants but not their allelic versions in DZA315 lines determines whether Rm41 is allowed to fix nitrogen in planta. This is advantageous to A17 plants because they can be colonized by more efficient nitrogen-fixing symbionts. In an article published in Plant Physiology, De Bang et al. present a comprehensive inventory of all peptides, including the NCRs, in Medicago truncatula (6).
3 Plants can fend off pathogens without discouraging symbionts.
Plants are surrounded by a multitude of microbes at any given time. To survive, they must distinguish between harmful and beneficial microbes around them. Plants have therefore evolved gene networks to encourage symbionts and deter pathogens. However, many of the signaling components involved in the recognition of both these types of bacteria are very similar at the protein level. Bozsoki et al., use two legumes, Lotus and Medicago, to show that LysM receptor kinases similar to the Nod factor receptors are required for recognition of long-chain chitin molecules (7). Mutants in Ljlys6/Mtlyk9 and Mtlyr4 are more easily infected by pathogens, but the interaction with beneficial rhizobia or mycorrhizal fungi is unaffected. This indicates that legumes use independent receptor complexes to recognize and initiate either plant immunity or symbiosis. A separate study conducted by Pfeilmeier et al. supports this idea by overexpressing the Arabidopsis EFR (ELONGATION FACTOR-THERMO UNSTABLE RECEPTOR) normally absent in Medicago, under the control of the CaMV35S promoter (8). This cross-species transfer is sufficient to confer enhanced pathogen resistance to Medicago without affecting the number of functional nitrogen fixing nodules that are formed.
- Y. Jiang et al., Plants transfer lipids to sustain colonization by mutualistic mycorrhizal and parasitic fungi. Science 356, 1172-1175 (2017).
- L. H. Luginbuehl et al., Fatty acids in arbuscular mycorrhizal fungi are synthesized by the host plant. Science 356, 1175-1178 (2017).
- A. Keymer et al., Lipid transfer from plants to arbuscular mycorrhiza fungi. Elife 6, (2017).
- S. Yang et al., Microsymbiont discrimination mediated by a host-secreted peptide in Medicago truncatula. Proc Natl Acad Sci U S A 114, 6848-6853 (2017).
- Q. Wang et al., Host-secreted antimicrobial peptide enforces symbiotic selectivity in Medicago truncatula. Proc Natl Acad Sci U S A 114, 6854-6859 (2017).
- T. C. de Bang et al., Genome-Wide Identification of Medicago Peptides Involved in Macronutrient Responses and Nodulation. Plant Physiol 175, 1669-1689 (2017).
- Z. Bozsoki et al., Receptor-mediated chitin perception in legume roots is functionally separable from Nod factor perception. Proceedings of the National Academy of Sciences, 201706795 (2017).
- S. Pfeilmeier et al., Heterologous expression of the immune receptor EFR in Medicago truncatula reduces pathogenic infection, but not rhizobial symbiosis. bioRxiv, (2017).