Abstract
Almost all protease families have been associated with plant development, particularly senescence, which is the final developmental stage of every organ before cell death. Proteolysis remobilizes and recycles nitrogen from senescent organs that is required, for example, seed development. Senescence-associated expression of proteases has recently been characterized using large-scale gene expression analysis seeking to identify and characterize senescence-related genes. Increasing activities of proteolytic enzymes, particularly cysteine proteases, are observed during the senescence of legume nodules, in which a symbiotic relationship between the host plant and bacteria (Rhizobia) facilitate the fixation of atmospheric nitrogen. It is generally considered that cysteine proteases are compartmentalized to prevent uncontrolled proteolysis in nitrogen-fixing nodules. In addition, the activities of cysteine proteases are regulated by endogenous cysteine protease inhibitors called cystatins. These small proteins form reversible complexes with cysteine proteases, leading to inactivation. However, very little is currently known about how the cysteine protease-cysteine protease inhibitor (cystatin) system is regulated during nodule development. Moreover, our current understanding of the expression and functions of proteases and protease inhibitors in nodules is fragmented. To address this issue, we have summarized the current knowledge and techniques used for studying proteases and their inhibitors including the application of “omics” tools, with a particular focus on changes in the cysteine protease-cystatin system during nodule development.
Highlights
Nitrogen deficiency is a widespread problem in many soils requiring the use of nitrogen fertilizers to increase crop productivity
The data presented here have been used to illustrate that cysteine proteases are present at all stages of nodule development but that the composition of expressed forms is different in senescent nodules from that in young nodules
The data presented here indicate that legumain-like cysteine proteases, which are required to release and active papain-like cysteine proteases, are the major cysteine proteases that are expressed in senescent nodules, as illustrated in Figure 8, which provides a schematic overview of expression changes for the cysteine protease–cystatin system in during soybean nodule development
Summary
Nitrogen deficiency is a widespread problem in many soils requiring the use of nitrogen fertilizers to increase crop productivity. A less expensive nitrogen source is available to farmers in the form of the fixation of atmospheric nitrogen by root nodule bacteria (rhizobia) in food and forage legumes [1]. Grain legumes, such as peanuts, cowpeas, soybeans, and fava beans have high rates of nitrogen fixation, under conditions of low soil nitrogen. The tropical legume, soybean is often considered to be the most important grain legume in terms of its economic value. It is a vital source of vegetable protein for food and animal feed world-wide. Symbiotic nitrogen assimilation in soybean nodules produces ureides, which are exported to the plant tissues. The ureides are converted into amino acids, which are used for the synthesis of a wide range of essential molecules, such as nucleotides and proteins
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