Mutant Microbe Improves Its Plant Nodulation Despite Fat Production Changes

Mary Jones
12th February, 2024

Mutant Microbe Improves Its Plant Nodulation Despite Fat Production Changes

Sinorhizobium meliloti

Photo adapted from: Jared Shorma / CC BY (Source)
The symbiotic relationship between certain bacteria (rhizobia) and legume plants, like alfalfa, is crucial for agriculture, as it allows plants to access nitrogen from the atmosphere. This process relies on the formation of root nodules, specialized structures where the bacteria reside and convert atmospheric nitrogen into a usable form for the plant. Phosphatidylcholine (PC), a type of fat molecule found in cell membranes, is believed to be essential for this symbiosis. However, understanding how PC contributes to this complex interaction has been a challenge. Researchers at UNAM – Centro de Ciencias Genómicas[1] investigated this by studying a mutant of Sinorhizobium meliloti, a rhizobia species, that lacked PC. This mutant produced excessive amounts of a sticky substance called succinoglycan, couldn’t swim effectively, and crucially, couldn’t form functional nodules on alfalfa roots. The overproduction of succinoglycan is a known issue in PC-deficient rhizobia, hindering their ability to properly interact with the plant. The team then sought to identify genetic changes – mutations – that could suppress these defects in the PC-deficient mutant, essentially restoring its ability to function normally. They found that mutations in several different genes – ExoS, ChvI, FabA, and RpoH1 – could all revert the mutant back to a state where it could swim and, importantly, initiate nodule formation. This suggests these genes play a role in compensating for the lack of PC. Interestingly, while the suppressor mutations allowed nodules to form, these nodules were defective. They remained white (normal nodules turn pink due to a molecule called leghemoglobin) and couldn’t fix nitrogen, meaning the symbiotic relationship wasn’t fully functional. This indicates that PC is not just important for nodule formation, but also for the later stages of symbiosis, specifically nitrogen fixation. Further investigation focused on the FabA gene. Mutations in FabA led to the production of fatty acids with shorter chains and fewer unsaturated bonds. Fatty acids are key components of cell membranes, and their composition influences membrane fluidity and function. This finding connects to earlier research showing that altering acyl chain length can compensate for the loss of PC in yeast[2]. In yeast, reducing the length of the fatty acid chains helped maintain membrane fluidity when PC was absent. The UNAM researchers propose a similar mechanism is at play in S. meliloti: altering the fatty acid composition partially restores membrane homeostasis, compensating for the missing PC and allowing the bacteria to swim and initiate nodule formation. The ExoS and ChvI genes are particularly interesting because they are part of a two-component regulatory system[3][4]. These systems act like a cellular communication network, where one protein (ExoS) senses a signal and relays it to another protein (ChvI) which then triggers a response. The R. meliloti ExoS-ChvI system regulates succinoglycan production[4]. The fact that mutations in both ExoS and ChvI can suppress the PC deficiency phenotype suggests a link between this regulatory system, succinoglycan production, and the need for PC during symbiosis. The RpoH1 gene encodes a sigma factor, a protein involved in regulating gene expression in response to stress[5]. The study found that RpoH1 mutations also suppressed the PC deficiency defects, highlighting the role of stress response pathways in compensating for the loss of this essential lipid. The research builds on previous findings that show S. meliloti responds to environmental stresses, like acidic pH, by altering gene expression via sigma factors like RpoH1[5].

BiotechGeneticsPlant Science

References

Main Study

1) Phosphatidylcholine-deficient suppressor mutant of Sinorhizobium meliloti, altered in fatty acid synthesis, partially recovers nodulation ability in symbiosis with alfalfa (Medicago sativa).

Published 11th February, 2024

https://doi.org/10.1111/tpj.16661

Related Studies

2) Shortening of membrane lipid acyl chains compensates for phosphatidylcholine deficiency in choline-auxotroph yeast.

https://doi.org/10.15252/embj.2021107966

3) Receiver domain structure and function in response regulator proteins.

https://doi.org/10.1016/j.mib.2010.01.015

4) Succinoglycan production by Rhizobium meliloti is regulated through the ExoS-ChvI two-component regulatory system.

Journal: Journal of bacteriology, Issue: Vol 180, Issue 1, Jan 1998

5) The role of sigma factor RpoH1 in the pH stress response of Sinorhizobium meliloti.

https://doi.org/10.1186/1471-2180-10-265


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