Senescence associated proteases: the key players in plant senescence

Senescence is the final stage of the plant development which leads to cell death. This process consists of a highly regulated, ordered series of events involving loss of photosynthetic capability, breakdown of proteins, loss of chlorophyll, disintegration of chloroplasts and export of all solubilised nutrients. The regulation and control of gene expression of senescence is governed by a set of Senescence Associated Genes (SAGs). They encode the enzymes which degrade the biomolecules during the leaf senescence and thus strictly associated with senescence in different plants. The enzymes involved in protein degradation for recycling of peptides are known as proteases. There are various categories of reported proteases, include cysteine, serine, aspartic, threonine and metalloprotease. Most of the proteases undergo the processing for their maturation and then activate the senescence process. The multiple roles of these proteases in plant defense include hypersensitive response (HR), a form of programmed cell death (PCD) that is associated with resistance to pathogens. Cysteine proteases are a major family of proteases which has been found to be associated with senescence thus playing a key role in plant defense and development.