The plant-type phosphoenolpyruvate carboxylase Gmppc2 is developmentally induced in immature soy seeds at the late maturation stage: a potential protein biomarker for seed chemical composition

Phosphoenolpyruvate carboxylase (PEPC) is a carbon-fixing enzyme with critical roles in seed development. Previously we observed a positive correlation between PEPC activity and protein content in mature seeds among soybean cultivars and varietal differences of PEPC activity in immature seeds, which is concordant with seed protein accumulation. Here, we report a PEPC isoform (Gmppc2) which is preferentially expressed in immature soybean seeds at the late maturation stage. Gmppc2 was co-expressed with enzyme genes involved in starch degradation: α-amylase, hexokinase, and α-glucan phosphorylase. Gmppc2 was developmentally induced in the external seed coats, internal seed coats, hypocotyls, and cotyledons at the late maturation stage. The expression of Gmppc2 protein was negatively regulated by the application of a nitrogen fertilizer, which suppressed nodule formation. These results imply that Gmppc2 is involved in the metabolism of nitrogen originated from nodules into seeds, and Gmppc2 might be applicable as a biomarker of seed protein content. Abbreviations: PEP: phosphoenolpyruvate; PEPC: phosphoenolpyruvate carboxylase; RNA-Seq: RNA sequencing; PCA: principal component analysis; SE: standard error