Differential appearance of isoforms and cultivar variation in protein temporal profiles revealed in the maturing barley grain proteome

Proteome analysis of mature barley (Hordeum vulgare subsp. vulgare) seeds has led to the identification of proteins in about 450 spots on 2D-gels. To shed light on the role of some of these proteins, their temporal appearance was monitored over 5 weeks during grain-filling and maturation of field-grown barley. Appearance profiles are described for 105 proteins identified in 185 2D-gel spots in the overlapping pI ranges 4-7 and 6-11. Grouping of proteins according to appearance across functional categories revealed instances of differential regulation of protein forms. Thus, a single 1-cys-peroxiredoxin isoform was identified in three spots, one present throughout grain filling, one appearing during desiccation and one observed only in mature seeds. This suggested post-translational modification of the protein to different degrees during seed maturation. Distinct isoforms of several proteins were identified in spots with individual appearance profiles, indicating differential expression of isoforms. Three isoforms of β-1,3 endoglucanase, including one not previously observed, each had a different temporal appearance pattern probably reflecting involvement in diverse processes such as cell wall modification or defence against fungal pathogens. Comparison of two cultivars, Barke and Morex, led to identification of protein spots appearing earlier in Morex than Barke, reflecting the faster maturation of Morex seeds.