Abstract
Lysine is the most deficient essential amino acid in cereal grains. A bifunctional lysine degrading enzyme, lysine ketoglutarate reductase/saccharopine dehydrogenase (LKR/SDH), is one of the key regulators determining free lysine content in plants. In rice (Oryza sativa. L), a bifunctional OsLKR/SDH is predominantly present in seeds. Here, we show that OsLKR/SDH is directly regulated by RISBZ1 and RPBF, major transcriptional regulators of seed storage protein (SSP) genes. OsLKR/SDH was highly expressed in the aleurone and subaleurone layers of the endosperm. RPBF-recognizing prolamin box and the RISBZ1-recognizing GCN4 motif act as important cis-elements for proper expression of OsLKR/SDH, like SSP genes. Reducing either RISBZ1 or RPBF decreased OsLKR/SDH levels, resulting in an increase in free Lys content in rice grain. This result was in contrast with the fact that a significant reduction of SSP was observed only when these transcription factors were simultaneously reduced. The same combinations of TF and cis-elements are involved in the regulation of OsLKR/SDH and SSP genes, but there is a distinct difference in their regulation mechanisms.
