January 15-19, 2011
Town & Country Convention Center
San Diego, CA
Andrew Groover , Juan Du , Eriko Miura , Marcel Robischon
Class III HD ZIP transcription factors are evolutionarily ancient, and have acquired numerous functions during the evolution of vascular plants. We describe characterization of two Populus Class III HD ZIPs, POPCORONA (PCN) and POPREVOLUTA (PRE), which demonstrates another function for Class III HD ZIPs in regulating secondary growth in woody stems. PCN is expressed broadly in the cambial zone and secondary xylem, with pronounced expression in rays. Transgenic Populus trees expressing miRNA-resistant PCN showed delayed lignification of xylem and phloem fibers during secondary growth, while transgenics expressing a synthetic miRNA targeting PCN transcripts showed abnormal lignification of cells in the pith. These phenotypes suggest a role for PCN in regulating cell differentiation. This possibility is supported by coordinated changes in expression of genes in PCN mutants within a transcriptional network regulating cell differentiation and cell wall biosynthesis, and hormone-related genes associated with fiber differentiation. In contrast, PRE is expressed unevenly around the circumference of the stem in the cambial zone, secondary xylem and phloem fibers during secondary growth. Transgenic Populus expressing a miRNA-resistant form of PRE present unstable phenotypic abnormalities affecting both primary and secondary growth. Surprisingly, phenotypic changes include abnormal formation of cambia within cortical parenchyma that can produce secondary vascular tissues in reverse polarity. Genes misexpressed in PRE mutants include transcription factors and auxin-related genes previously implicated in Class III HD ZIP functions during primary growth. Together these results indicate that members of Class III HD ZIPs have acquired specific functions associated with cell differentiation and tissue patterning functions during secondary growth, at least in part through direct cooption. We also discuss follow-on experiments designed to comprehensively define the transcriptional networks through which Class III HD ZIPs regulate secondary growth.