Chapter 1 - Nodule Physiology and Proteomics of Stressed Legumes

Abstract

Abstract Symbiotic bacteria are harboured in the nodules of the nitrogen-fixing plants. The bacteria, collectively termed rhizobia, include genera such as Rhizobium, Sinorhizobium, Bradyrhizobium, Mesorhizobium and Azorhizobium, and form specialised organs within the plant. Nitrogen fixation occurs via the conversion of N2 into NH3 by bacterial nitrogenases. Knowledge of protein profile (structural and soluble) of bacteria and host may provide information useful in understanding the bacteria–host relationship and improving N2-fixation efficiency in leguminous plants. Although the majority of nitrogen-fixing plants belong to the family Fabaceae, a few non-leguminous plants (like actinorhizal plants) can also fix nitrogen. Proteomics is an ideal tool to study the protein profile and its correlation with nodule-associated metabolic and symbiotic processes. N2-fixing symbioses are well studied but not in terms of proteomic response to abiotic stress. Data obtained in some proteomic studies on Medicago trancatula and few other leguminous plants provide useful information on root nodules, their symbiotic bacteria and the proteins produced by both partners during their constant signal exchange and growth. Mass spectrometric analysis has helped in identifying several proteins, including those associated with molecular regulation, respiration and leghaemoglobin, proteases in the nodule. Differential expression of proteins under different abiotic stresses such as temperature, drought, salinity and toxic metals that affect the profile of nodule proteome is believed to be due to the production of oxidative stress, osmotic imbalance and other direct or secondary consequences of stress. However, nutrient stress also affects proteome profile as in iron deficiency. Iron-containing proteins play a key role in symbiotic nitrogen fixation (SNF) that occurs in the nodule. Several proteins like those related to SNF, predominant components of nitrogenase complexes such as nifD, nifH, nifK, nitrogen regulatory protein II (GlnB) and PIIA (PtsN) and urease accessory protein (UreE) are known to be affected by abiotic stress. Nodules are well equipped with antioxidant enzymes (superoxide dismutase, ascorbate peroxidase and glutathione reductase, etc.) which respond to stress conditions. This review introduces nodule physiology and examines critically the recent developments in the field of nodule proteomics, emphasising, in particular, upon changes brought about by abiotic stresses to the nodule proteome, provides up-to-date information on key metabolic proteins that help to combat stress and discusses the prospects of nodule proteomics.