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UBC Theses and Dissertations

Functional analysis of homeodomain transcription factors in secondary cell wall formation in Arabidopsis thaliana Liu, Yuanyuan


The plant secondary cell wall is a composite network of complex polymers (cellulose, lignin, and hemicellulose) that provides protective and structural properties to the cell wall. The TALE homeodomain transcription factor KNOTTED ARABIDOPSIS THALIANA7 (KNAT7) is part of a regulatory network governing the commitment to secondary cell wall biosynthesis of Arabidopsis thaliana, where it contributes to negative regulation of this process. This thesis demonstrates that BLH6, a BEL1-LIKE HOMEODOMAIN protein, specifically interacts with KNAT7, and this interaction influences secondary cell wall development. BLH6 is a transcriptional repressor, and BLH6-KNAT7 physical interaction enhances KNAT7 and BLH6 repression activities. The overlapping expression patterns of BLH6 and KNAT7 and phenotypes of blh6, knat7, and blh6 knat7 loss-of-function mutants are consistent with existence of a BLH6-KNAT7 heterodimer that represses commitment to secondary cell wall biosynthesis in interfascicular fibers. BLH6 and KNAT7 overexpression results in thinner interfascicular fiber secondary cell walls, phenotypes that are dependent on the interacting partner. A major impact of the loss of BLH6 and KNAT7 function is enhanced expression of the HD-ZIP transcription factor REVOLUTA/INTERFASCICULAR FIBERLESS1 (REV/IFL1). BLH6 and KNAT7 bind to the REV promoter and repress REV expression, while blh6 and knat7 interfascicular fiber secondary cell wall phenotypes are suppressed in blh6 rev and knat7 rev double mutants, suggesting that BLH6/KNAT7 signaling acts through REV as a direct target. This thesis revealed the pleiotropic phenotypes induced by the overexpression of BLH6 and BLH10, which shed light on other biological functions of BLH6 and BLH10. This thesis also provided new information regarding the existence of a BLH6-KNAT7-OFP complex.

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