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We inadvertently omitted a double bond in the A ring of several flavonoid molecules in Figure 1, a mistake that was kindly brought to the attention of The Plant Cell by Dr. Guanghui Xu (National Maize Improvement Center of China). In addition, the Figure 1 legend has been corrected to refer to rhamnosylation at “C-2” of the Glc moiety (instead of “C-1”). For ease of comparison, the original and corrected versions of this figure are presented below, with the position of the error and correction marked with red arrows.
Figure 1.
Original: Maize 3-Deoxyflavonoid and Flavone Biosynthetic Pathways.
Figure 1.
Corrected: Maize 3-Deoxyflavonoid and Flavone Biosynthetic Pathways.
The condensation reaction between p-coumaroyl-CoA and malonyl-CoA, the first committed step in flavonoid formation, is catalyzed by chalcone synthase (CHS), resulting in naringenin chalcone (chalcone). This chalcone is converted to the flavanone naringenin by chalcone isomerase. Naringenin, the branching point of the pathway, is converted to apiforol (flavan-4-ol) by a dihydroflavonol reductase (DFR) to apigenin (flavone) by action of a F2H followed by a dehydration step (see Supplemental Figure 7C online). An FNS could also catalyze this step. Naringenin can also be converted to eriodictyol by a flavanone-3′-hydroxylase (F3′H) and serves as substrate for F2H/dehydration or FNS, forming the flavone luteolin. Dihydroflavonol reductase can also act on eriodictyol to generate the flavan-4-ol luteoforol. Apiforol and luteoforol polymerize to form the red phlobaphene pigments. The proposed steps for conversion of apigenin and luteolin into the C-glycosyl flavones apimaysin and maysin, respectively, involves at least three steps: glycosylation at C-6 by a CGT, followed by a rhamnosylation at C-1 of the Glc moiety possibly mediated by SM2, and finally a dehydration step mediated by SM1 (McMullen et al., 2004).
The condensation reaction between p-coumaroyl-CoA and malonyl-CoA, the first committed step in flavonoid formation, is catalyzed by chalcone synthase (CHS), resulting in naringenin chalcone (chalcone). This chalcone is converted to the flavanone naringenin by chalcone isomerase. Naringenin, the branching point of the pathway, is converted to apiforol (flavan-4-ol) by a dihydroflavonol reductase (DFR) to apigenin (flavone) by action of a F2H followed by a dehydration step (see Supplemental Figure 7C online). An FNS could also catalyze this step. Naringenin can also be converted to eriodictyol by a flavanone-3′-hydroxylase (F3′H) and serves as substrate for F2H/dehydration or FNS, forming the flavone luteolin. Dihydroflavonol reductase can also act on eriodictyol to generate the flavan-4-ol luteoforol. Apiforol and luteoforol polymerize to form the red phlobaphene pigments. The proposed steps for conversion of apigenin and luteolin into the C-glycosyl flavones apimaysin and maysin, respectively, involves at least three steps: glycosylation at C-6 by a CGT, followed by a rhamnosylation at C-2 of the Glc moiety possibly mediated by SM2, and finally a dehydration step mediated by SM1 (McMullen et al., 2004).
Footnotes
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Editor’s note: the corrected figure and accompanying text were reviewed by members of The Plant Cell editorial board.