Carbon partitioning in tissues of a gain-of-function mutant (MYB75/PAP1-D) and a loss-of-function mutant (myb75-1) inArabidopsis thaliana

Abstract

The role of myeloblastosis (MYB) transcription factors in regulation of carbon flow among the various products of the phenylpropanoid pathway is still unclear. Carbon partitioning in Arabidopsis thaliana (L.) Heynh. was studied in a gain-of-function mutant (MYB75/PAP1-D) and a loss-of-function mutant (myb75-1). Determination of anthocyanins and total phenolics levels in leaf, peduncle, and root tissues of these mutants and their wild-type (WT) parental lines was assessed, as well as13C partitioning between “extractives” (predominantly cytoplasm) and “nonextractives” (predominantly cell wall) of these tissues. The amounts of anthocyanins and phenolics in the leaves and peduncles of MYB75/PAP1-D mutant were higher than in its WT line.13C excess in the extractives of leaves and peduncles of the mutant was also higher than that found in the WT line, indicating that the transcription factor MYB75 positively regulated carbon flow to flavonoids biosynthesis in the phenylpropanoid pathway. The myb75-1 mutant had higher distribution of13C in both extractives and nonextractives of leaves and peduncles, especially in nonextractives of the peduncles, and lower levels of anthocyanins compared with its WT. This suggests that the knockout of MYB75 inhibits biosynthesis of anthocyanins and regulates carbon flow from cytoplasm to cell wall components by activating the biosynthesis of monolignols in cytoplasm, which, in turn, are transported and deposited to the secondary cell wall, resulting in secondary cell wall thickening. Patterns of anthocyanin level and13C partitioning in roots were different from that seen in leaves and peduncles, suggesting that regulation of the phenylpropanoid pathway by MYB75 may be tissue specific.