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In some rhizobia, the nif genes are located on plasmids called 'sym plasmids' (sym = symbiosis) which contain genes related to nitrogen fixation and metabolism, while the chromosomes contain most of the housekeeping genes of the bacteria. Regulation of the nif genes is at the transcriptional level and is dependent on colonization of the plant host.
Plants that contribute to nitrogen fixation include those of the legume family—Fabaceae— with taxa such as kudzu, clover, soybean, alfalfa, lupin, peanut and rooibos. [45] They contain symbiotic rhizobia bacteria within nodules in their root systems, producing nitrogen compounds that help the plant to grow and compete with other plants. [58]
Nitrogen is the most commonly limiting nutrient in plants. Legumes use nitrogen fixing bacteria, specifically symbiotic rhizobia bacteria, within their root nodules to counter the limitation. Rhizobia bacteria convert nitrogen gas (N 2) to ammonia (NH 3) in a process called nitrogen fixation.
Actinorhizal plants are distributed within three clades, [1] and are characterized by nitrogen fixation. [2] They are distributed globally, and are pioneer species in nitrogen-poor environments. Their symbiotic relationships with Frankia evolved independently over time, [ 3 ] and the symbiosis occurs in the root nodule infection site.
Understanding the structure and function of nitroplasts opens up possibilities for genetic engineering in plants. [1] By incorporating genes responsible for nitroplast function, researchers aim to develop crops capable of fixing their own nitrogen, potentially reducing the need for nitrogen-based fertilizers and mitigating environmental damage. [1]
The symbiosis between nitrogen fixing rhizobia and the legume family has emerged and evolved over the past 66 million years. [30] [31] Although evolution tends to swing toward one species taking advantage of another in the form of noncooperation in the selfish-gene model, management of such symbiosis allows for the continuation of cooperation. [32]
The nitrogen-fixing clade consists of four orders of flowering plants: Cucurbitales, Fabales, Fagales and Rosales. [a] This subgroup of the rosids encompasses 28 families of trees, shrubs, vines and herbaceous perennials and annuals. The roots of many of the species host bacteria that fix nitrogen into compounds the plants can use. [4] [5]
Leghemoglobin (also leghaemoglobin or legoglobin) is an oxygen-carrying phytoglobin found in the nitrogen-fixing root nodules of leguminous plants. It is produced by these plants in response to the roots being colonized by nitrogen-fixing bacteria, termed rhizobia, as part of the symbiotic interaction between plant and bacterium: roots not colonized by Rhizobium do not synthesise leghemoglobin.