Biological complexity: gene control of nitrogen binding (Introduction)

by David Turell @, Tuesday, April 02, 2024, 17:12 (19 days ago) @ David Turell

A study of legumes:

"Legume plants are unique in their ability to produce specialised root nodules, which host bacteria called rhizobia that convert atmospheric nitrogen into nutrients. Previous research showed that a genetic program for initiating the development of lateral – or secondary – roots also underpins the same process that triggers the formation of these nodules. But the question remained around the additional genetic factors that confer nodule identity as distinct from lateral roots.

"By gene expression profiling and imaging the model legume Medicago truncatula, research carried out as part of the Enabling Nutrient Symbioses in Agriculture (ENSA) project showed that two members of the LIGHT-SENSITIVE SHORT HYPOCOTYL (LSH) family of genes determine the identity of bacterial induced lateral root organs as nodules. This group of factors was previously predominantly known to define the organs and tissues that produce flowers and stems.

"We now understand that LSH1 and LSH2 are instrumental in forming a group of cells that are infectable and habitable by nitrogen-fixing bacteria early during nodule development."

The original paper:

"While nodules are unique structures associated with symbiotic bacterial N fixation, we have yet to see any evidence for de novo gene evolution associated with the emergence of nodulation. Rather, we find evidence for the re-networking of preexisting developmental pathways, facilitating the emergence of this novel form of root development. The neo-functionalization of the nodule-specific transcription factor NIN and the associated evolution of cis-regulatory DNA-binding sites in the promoter regions of its downstream targets led to the recruitment of a lateral root organ initiation program into the symbiotic interaction with rhizobial bacteria. Similarly, we hypothesize that further neo-functionalization of NIN provided the opportunity for recruiting a growth-regulatory network with pleiotropic functions in the shoot into the symbiotic root context, thereby promoting the expansion and diversification of the regulatory function of LSH1/LSH2 and their associated downstream regulatory subnetworks into nodule development. This notion is in line with the common principle of morphological evolution as proposed by Carroll,59 in which changes in the spatial and temporal gene expression of preexisting developmental regulators and their associated downstream networks lead to trait divergence and the diversification of novel organ forms and functions. The parallel recruitment of a root initiation program and primordium identity program from the shoot that dictate nodule form and function are essential in non-legume species that are targets for engineering N fixation." (my bold)

Comment: with the largest concentration of gas in the atmosphere, one would think nitrogen is easily obtainable. Unlike oxygen, which is extremely active, nitrogen is really inert. Once again it is specialized bacteria who come to the rescue. Not only does the symbiosis feed each plant, but nitrogen is spread into the soil, reducing the need to spread out fertilizer mixes. How did this evolve? Note my bold. The authors see adaptation of existing parts and processes. But I see it as not that simple. It involves recognizing the need for more nitrogen, then finding the right bacteria to fit into a newly created home, the extremely specialized nodule. Recognizing the need is the easy part. The rest is a very involved development of morphologic alterations which then involves attracting a specific bacterium. I see design, not a chance happening.

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