Repression of barley cathepsins, HvPap-19 and HvPap-1, differentially alters grain composition and delays germination

During barley germination, cysteine-proteases are essential in the mobilization of storage compounds providing peptides and amino acids to sustain embryo growth until photosynthesis is completely established. Knock-down barley plants, generated by artificial microRNA, for the cathepsins B- and F-like, HvPap-19 and HvPap-1 genes, respectively, showed less cysteine protease activities and consequently lower protein degradation. The functional redundancy between proteases triggered an enzymatic compensation associated with an increase in serine-protease activities in both knock-down grains, which was not sufficient to maintain germination rates and behaviour. Concomitantly, these transgenic lines showed alterations in the accumulation of protein and carbohydrates in the grain. While the total amount of protein increased in both transgenic lines, the starch content decreased in HvPap-1 knock-down lines and the sucrose concentration was reduced in silenced HvPap-19 grains. Consequently, phenotypes of HvPap-1 and HvPap-19 artificial microRNA lines showed a delay in the grain germination process. These data expand the potential of exploring the properties of barley proteases to be selectively modified and used for brewing or livestock feeding industry.