HvPap a member of the cathepsin F like family
HvPap-1, a member of the cathepsin F-like family, was identified in germinating barley seeds (Sreenivasulu et al., 2008). This enzyme participates in the mobilization of stored proteins (mainly hordeins) during the last phase of grain germination, and its activity is modulated by a propeptide or/and an inhibitor from the cystatin family (Cambra et al., 2012). The comprehensive analyses of several proteases from the C1A family, in the akt inhibitor at 24h after imbibition, have shown that changes in the expression of HvPap-1 were associated with variation in the expression of other enzymes of this family (members of L-, B-, H-, F-) and trypsin-like serine protease (Diaz-Mendoza et al., 2016). Cysteine proteases of the legumain family (C13) also play important roles in the growth of cereal seeds. Two types of legumains, also called vacuolar-processing enzymes (VPEs), exist, the vegetative VPEs and the seed-type VPEs (Hara-Nishimura et al., 1998). All VPEs are synthesized as inactive prolegumains and are then transferred to vacuoles or to the cell wall. When these enzymes reach those compartments under acidic pH conditions, they undergo activation through self-cleavage in their amino and C- termini. Then, they become active legumains to play their biological role in plant tissues (Dall and Brandstetter, 2012). Seed-type VPEs might be responsible for the post-translational processing of many proteins that contribute to the biosynthesis of vacuolar components. During germination and seedling growth, legumains may activate other cysteine proteases, which degrade storage proteins (Cambra et al., 2010). A legumain of rice seeds (REP-2) activates other cysteine proteases during rice germination (Kato et al., 2003), whereas βVPE and OsVPE-1 are key players in the correct processing of stored proteins during seed development in Arabidopsis and rice, respectively (Gruis et al., 2004, Wang et al., 2009). The high-level expression of barley legumain (HvLeg-2) that occurs during the early stages of endosperm development has been related to the processing of storage proteins, similar to the case for rice legumain OsVPE-1 (Wang et al., 2009). Legumain HvLeg-2 is also expressed during germination, which is induced by GA. Therefore, this enzyme may also have a role in mobilizing storage proteins, either by direct proteolytic activity or by processing and activating other proteases, as was suggested previously (Julián et al., 2013, Kato et al., 2003, Zakharov et al., 2004). Therefore, the joint activity of legumain and papain-like cysteine proteases probably is required for the accumulation and mobilization of storage proteins in cereal seeds. Another important role played by CPs is associated with PCD processes in developing seeds. In animals, PCD is controlled by specialized cysteine proteases called caspases (Watanabe and Lam, 2005). No caspase homologues have been identified in plants (Bonneau et al., 2008), but proteases of legumain family can exert caspase-like activity (Coffeen and Wolpert, 2004, Julián et al., 2013). The barley legumains, HvLeg-2, -3, -7, which are seed-type VPEs, are involved in nucellar PCD, whereas a member of the second group of VPEs (vegetative VPEs), HvLeg-5 (HvVPE4), likely participates in PCD in the pericarp (Linnestad et al., 1998, Radchuk et al., 2011). In situ hybridization studies revealed that HvVPE4 mRNA is located exclusively in the pericarp at 6, 8, and 10days after flowering but not in the endosperm, integuments or the vascular bundle. These results suggest that the legumain HvVPE4 is involved in only the PCD events occurring in the pericarp (Radchuk et al., 2011). The expression of a cysteine protease was also observed in the nucellus during the early stages of seed development (Domínguez and Cejudo, 1998), suggesting that this enzyme is involved in nucellar-cell degeneration. The legumain δVPE of Arabidopsis participates in cell death during seed coat formation. The highly specific expression of this enzyme in particular cellular layers of the seed coat is different from the patterns of expression of other Arabidopsis VPE homologs; αVPE and γVPE are expressed in the vegetative organs and βVPE is expressed in the developing embryo (Kinoshita et al., 1999). Although both δVPE and βVPE accumulate in developing seeds, the tissues in which they are expressed are completely different, with δVPE being expressed in maternal tissues and βVPE being expressed in the tissues of the next generation (the embryo) (Nakaune et al., 2005). The results of Nakaune et al. (2005) showed that δVPE has caspase-1-like activity, as do the VPEs of tobacco (Hatsugai et al., 2004). There is no sequence similarity between the VPEs and caspases, but VPE recognizes a caspase-1 substrate and VPE activity is inhibited by a caspase-1 inhibitor (Nakaune et al., 2005).