Characterisation of seeds of a C4 phosphoenolpyruvate carboxylase-deficient mutant of Amaranthus edulis.

Seeds from the C-4 plant Amaranthus edulis were studied as part of the characterisation of a mutant (designated LaC4 2.16), which contains reduced amounts (5% of wild type) of the photosynthetic leaf form of phosphoenolpyruvate carboxylase (PEPC). On a per seed basis, the amount of PEPC activity was not significantly altered, while the weight and protein content of the mutant seeds were 34% lower than that of the wild type. Western gel blot analysis detected two PEPC polypeptides with molecular masses of 105 kDa (minor) and 100 kDa (major). The determination of in vitro phosphorylation in reconstituted assays revealed the presence of both calcium-dependent and calcium-independent PEPC-kinase activities in protein extracts of wild-type and mutant seeds. However, PEPC proteins were phosphorylated in dry seeds, and PEPC phosphorylation did not occur in vivo during seed imbibition in the presence of 32P-phosphate. In contrast, 32P-phosphate was incorporated into a range of proteins in wild-type seeds, but not in mutant seeds. In addition, ATP content was much reduced in germinating mutant seeds and this did not increase following the supply of phosphate. Collectively, these data suggest that the deficiency in C-4 PEPC in mutant A. edulis leaves has no effect on C-3-type PEPC content and phosphorylation state in seeds, but causes impairment of energy production, thereby accounting for the reduced germination of the mutant.