Subcellular organization of N2-fixing nodules of cowpea (Vigna unguiculata) supplied with silicon

Abstract

Provision of silicon (0, 0.048, 0.096, 0.24, 0.48, and 0.96 g/l) in the form of silicic acid (H4SiO4) to nodulated cowpea plants (Vignia unguiculata [L.] Walp.) grown in liquid culture resulted in considerable changes in the internal organization of nodule structure. Compared to the control plants which received no added silicate, bacteroid numbers increased significantly (P < or = 0.05) at silicate concentrations of both 0.096 and 0.48 g/l. The number of symbiosomes also increased by 3.2-fold at the silicate concentration of 0.96 g/l compared to the control. In contrast, the size of bacteroids and symbiosomes decreased significantly (P < 0.05) inside nodules of silicate-treated plants. The peribacteroid space was also decreased considerably (P < 0.05) with the application of 0.096 and 0.96 g of silicate per liter to plants. However, the size of intercellular spaces adjacent to infected and uninfected interstitial cells within the nodule medulla increased significantly (P < or = 0.05) at 0.096 g of silicate per liter followed by a sharply marked (P < or = 0.05) decrease with each subsequent increase in silicate application. The result was a large decrease (P < 0.05) in the area of bacteria-infected tissue occupied by intercellular space at the highest silicate concentration, which was caused by a significant (P < or = 0.05) increase in cell wall thickness. Our findings show that the positive effects of silicon on N2 fixation might actually be due to an increased number of bacteroids and symbiosomes.

Elemental distribution in tissue components of N2-fixing nodules of Psoralea pinnata plants growing naturally in wetland and upland conditions in the Cape Fynbos of South Africa

“…In that study, there was a Si-induced increase in the number of bacteroids and symbiosomes in infected cells, which increased N 2 fixation (Nelwamondo et al 2001). So, the higher concentration of Si found in infected cells and in the medulla region of Psoralea nodules in this study (Tables 2, 3 and 4) directly confirms its role in symbiotic functioning.…”

Silicon‐induced root nodulation and synthesis of essential amino acids in a legume is associated with higher herbivore abundance

“…They also found that silicon increased cell wall thickness of root nodules which appeared to decrease intercellular spaces which should, in theory, reduce N 2 fixation because of lower gaseous diffusion (Nelwamondo, Jaffer & Dakora ). The fact that N 2 fixation actually increased led Nelwamondo, Jaffer & Dakora () to speculate that silicification reduced the need for lignin, which is costly in terms of carbon; carbon could then be channelled into bacteroid respiration to increase N 2 fixation. It is also possible that carbon could be channelled towards enhanced nodule organogenesis.…”

“…It is also possible that carbon could be channelled towards enhanced nodule organogenesis. Nelwamondo, Jaffer & Dakora () also proposed that silicon supplementation had parallels with phosphorus fertilisation in terms of stimulating root nodulation, including increased production of compounds that up‐regulate nodulation genes (Dakora & Nelwamondo ). This has yet to be tested.…”

“…In a follow-up study, Nelwamondo, Jaffer & Dakora (2001) showed that silicon supplementation increased the abundance of bacteroids and symbiosomes (the plantderived membrane that encases the bacteroids) in root nodules, which they suggested could explain enhanced N 2 fixation. They also found that silicon increased cell wall thickness of root nodules which appeared to decrease intercellular spaces which should, in theory, reduce N 2 fixation because of lower gaseous diffusion (Nelwamondo, Jaffer & Dakora 2001).…”

History and Introduction of Silicon Research

“…potassium (K), phosphorus (P) and iron (Fe) defi ciency; nitrogen (N), P and boron (B) excess; etc. ]; (17) promotion of nodule formation in legume plants and hence promotion of N 2 fi xation (Nelwamondo and Dakora 1999 ;Nelwamondo et al 2001 ;Mali and Aery 2008 );and (18) promotion of formation of phytolithoccluded carbon and hence having implications in carbon bio-sequestration of atmospheric CO 2 and global climate change (Song et al 2012.…”