Phosphoenolpyruvate Carboxylase during Maturation and Germination Sorghum Seeds: Enzyme Activity and Regulation

Resumo

Sorghum (Sorghum bicolor (L.) Moench) is a species of great socio-economic and ecological importance for countries in arid and semi-arid climate. In C₄ plants like sorghum, phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) plays a key role in seed development and germination. In this work, the PEPC activity shows an increase followed by a decrease at the early and later stages of maturation, respectively. In germinating seeds, the PEPC activity quickly increases after soaking. The L-malate test and the ratio of PEPC activity determined at pH 8.0 and 7.1, indicates, that PEPC is phosphorylated at the early stages of maturation then becomes dephosphorylated at the later stages and during seed germination, PEPC takes back its phosphorylated form. The determination of the affinity constant showed different K_M depending on the seed developmental stage. As there is no PEPC-C4 isoform in developing sorghum seeds, this result indicates that the different K_M observed during seed maturation could be a result of a post-translational regulation such as phosphorylation or ubiquitination of a pre-existing isoform. This regulation enhances the PEPC activity at early stages of seed development.