Starch is an agronomically important insoluble polyglucan synthesized by plants as a means to store photoassimilates for subsequent use in metabolism and growth. The accumulation of starch in floral organs plays an important role in the growth and development of reproductive tissues. Starch branching enzymes SBEI and SBEIIb cloned from maize endosperm were constitutively expressed in a starchless line of Arabidopsis, lacking endogenous SBEs, and also in wildtype. Both ZmSBEI and ZmSBEIIb were able to individually restore starch synthesis in the null line, and transgenic lines showed a starch excess phenotype at the end of the dark period. Effects on starch structure were observed, with transgenic lines exhibiting altered amylopectin chain length distribution and starch granule morphology. Correlations were found between reproductive tissue development and intensity of branching enzyme complementation, suggesting branching enzyme activity in reproductive tissues may be a critical factor in determining seed yield in transgenic Arabidopsis plants.