**Group 1: Symbiosis and Nitrogen Fixation**
– Nitrogen is commonly limiting in plants
– Legumes use rhizobia bacteria in root nodules for nitrogen fixation
– Rhizobia convert nitrogen gas to ammonia
– Ammonia is assimilated into essential plant components
– Symbiotic relationship between rhizobia and legumes involves sugar exchange
– Legume root nodules convert atmospheric nitrogen to ammonia
– Ammonia is assimilated into amino acids, nucleotides, and other cellular components
– Legumes with nitrogen-fixing ability require less nitrogen fertilizer
– Energy for nitrogen fixation in nodules comes from translocated sugars
– Legume nodules use leghemoglobin to facilitate oxygen diffusion
**Group 2: Leguminous and Non-leguminous Plants**
– Fabaceae family includes kudzu, clovers, soybeans, and more
– Symbiotic rhizobia bacteria are present in nodules of legumes
– Fixed nitrogen is released upon plant death, fertilizing the soil
– Legumes like clover are used in crop rotation for nitrogen benefits
– Some genera like Styphnolobium do not have nitrogen-fixing nodules
– Actinorhizal plants like alder and bayberry form nitrogen-fixing nodules
– Parasponia in the Cannabaceae family interacts with rhizobia
– Only a few genera in families like Rosaceae can fix nitrogen
– Nitrogen-fixing ability is not universal in certain plant families
– Some plants have evolved to switch partners in nitrogen fixation
**Group 3: Nodulation Process**
– Legumes release flavonoids to attract rhizobia
– Nod factors initiate root hair curling
– Nod factors trigger cell division for nodule formation
– Effective nodulation occurs approximately four weeks after planting
– Nodulation is controlled by external and internal factors
**Group 4: Nodule Classification and Structure**
– Legumes have determinate and indeterminate nodule types
– Determinate nodules are found in specific legume tribes
– Indeterminate nodules are common in legumes from all sub-families
– Indeterminate nodules have an active apical meristem for growth
– Different zones in indeterminate nodules mark various stages of development
– Root nodules evolved three times in Fabaceae
– Root structure influences propensity for nodulation
– Some fungi produce tuberculate ectomycorrhizae structures hosting nitrogen-fixing bacteria
**Group 5: Research and Molecular Mechanisms**
– Study by Foucher and Kondorosi on cell cycle regulation in nodule organogenesis
– Research by Monahan-Giovanelli et al. on infection thread networks in root nodules induced by Sinorhizobium meliloti
– Van de Velde et al.’s work on molecular view of nodule senescence in Medicago truncatula
– Shen et al.’s discovery of a homeotic mutation changing legume nodule ontogeny
– Eckardt’s research on flavonoids’ role in root nodule development and auxin transport
– Esseling et al.’s study on Nod factor-induced root hair curling
– Mergaert et al.’s research on eukaryotic control in the Rhizobium-legume symbiosis
– Adjei’s work on nitrogen fixation and inoculation of forage legumes
– Reid et al.’s exploration of molecular mechanisms controlling legume autoregulation of nodulation
Root nodules are found on the roots of plants, primarily legumes, that form a symbiosis with nitrogen-fixing bacteria. Under nitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known as rhizobia. This process has evolved multiple times within the legumes, as well as in other species found within the Rosid clade. Legume crops include beans, peas, and soybeans.
Within legume root nodules, nitrogen gas (N2) from the atmosphere is converted into ammonia (NH3), which is then assimilated into amino acids (the building blocks of proteins), nucleotides (the building blocks of DNA and RNA as well as the important energy molecule ATP), and other cellular constituents such as vitamins, flavones, and hormones.[citation needed] Their ability to fix gaseous nitrogen makes legumes an ideal agricultural organism as their requirement for nitrogen fertilizer is reduced. Indeed, high nitrogen content blocks nodule development as there is no benefit for the plant of forming the symbiosis. The energy for splitting the nitrogen gas in the nodule comes from sugar that is translocated from the leaf (a product of photosynthesis). Malate as a breakdown product of sucrose is the direct carbon source for the bacteroid. Nitrogen fixation in the nodule is very oxygen sensitive. Legume nodules harbor an iron containing protein called leghaemoglobin, closely related to animal myoglobin, to facilitate the diffusion of oxygen gas used in respiration.