Abstract
Seed storage proteins must be hydrolyzed by proteases to deliver the amino acids essential for embryo growth and development. Several groups of proteases involved in this process have been identified in both the monocot and the dicot species. This review focuses on the implication of proteases during germination in two cereal species, barley and wheat, where proteolytic control during the germination process has considerable economic importance. Formerly, the participation of proteases during grain germination was inferred from reports of proteolytic activities, the expression of individual genes, or the presence of individual proteins and showed a prominent role for papain-like and legumain-like cysteine proteases and for serine carboxypeptidases. Nowadays, the development of new technologies and the release of the genomic sequences of wheat and barley have permitted the application of genome-scale approaches, such as those used in functional genomics and proteomics. Using these approaches, the repertoire of proteases known to be involved in germination has increased and includes members of distinct protease families. The development of novel techniques based on shotgun proteomics, activity-based protein profiling, and comparative and structural genomics will help to achieve a general view of the proteolytic process during germination.
Highlights
Barley is considered a model organism for the investigation of the cereal germination process
Cysteine proteases (CysProt) of the C1A family, which are known as papain-like, and the C13 family, alternatively called legumains or vacuolar processing enzymes (VPEs), are the main proteases involved in the germination of both dicot and monocot species [1,2,4]
In the case of barley and wheat, many advances have been made in understanding the role of proteases in the grain due to their potential value for the brewing industry and their relationship with the celiac disease
Summary
Barley is considered a model organism for the investigation of the cereal germination process. Other than CysProt from the papain family (C1A) and the legumain family (C13), members of the S10 serine carboxypeptidases (SCP) have been implicated in the germination process in cereal grains. The OsVPE-1 protease was described in the degradation of stored proteins in the rice grain [13], and the REP-2 rice legumain was suggested as an activator of other CysProt during rice germination [14]. In this process, the SCP46 serine carboxypeptidase from rice regulates grain filling and seed germination upon hormonal induction [15,16]. The participation of proteases in the germination processes of barley and wheat will be widely described in following sections
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