Cysteine proteases in nodulation and nitrogen fixation.

The cysteine proteinases or cysteine endopeptidases (EC 3.4.22) are known to occur widely in plant cells. They are involved in almost all aspects of plant growth and development including germination, circadian rhythms, senescence and programmed cell death. They are also involved in mediating plant cell responses to environmental stress (such as water stress, salinity, low temperature, wounding, ethylene, and oxidative conditions) and plant-microbe interactions (including nodulation). In the development and function of legume root nodules, cysteine proteases could be involved in several important processes:-(i) a defence response to root invasion by microorganisms; (ii) protein turnover required during the formation of new tissue; (iii) cellular homeostasis and metabolism; (iv) adaptation of host cells to physiological stresses; (v) control of nodule senescence. Because of their central importance to plant physiology, cysteine proteases could serve as important targets for the study of nodule development and functioning at the molecular level. Because of their widespread occurrence in nodulating plants they could also serve as candidate genes for targeted plant breeding programmes.