Fig. 9

Proposed model of MdFRK2-regulated cellulose synthesis in apple and poplar. In stems, a direct metabolic pathway for the biosynthesis of cellulose is through the increased cleavage of Suc into UDPG via increased SUSY activity, which is an alternative pathway for UDPG production from Suc via NINV. One observation of this study is that overexpression of MdFRK2 resulted in increased cellulose content by shifting F6P or G6P towards UDPG formation. With increased FRK activity increases the sink strength overall so there is more carbohydrate available to fuel increased cambial activity and that resulted in more secondary phloem. Blue boxes indicate a reduction in soluble sugar concentrations. Yellow boxes indicate increases in soluble sugar or hexose phosphate concentrations, enzyme activity or relative mRNA expression of genes. White boxes, no change in relative mRNA expression of genes that are involved in the biosynthesis of lignin (PAL1, and C4H1). Purple boxes indicate the relative mRNA expression of MdFRK2