E2F and retinoblastoma related proteins may regulate GL1 expression in developing Arabidopsis trichomes

This is an addendum to our recent paper published in The Plant Journal (52:352-61). The major findings were: (1) trichomes on the leaves of gl3-sst sim double mutants developed as large multicellular clusters whereas wild type trichomes are composed of single cells; (2) ectopic CYCD3;1 expression in gl3-sst trichomes also resulted in trichome cluster formation; and (3) that GL1 expression is prolonged in the gl3-sst sim trichome clusters. This addendum shows that ectopic CYCD3;1 expression in gl3-sst also enhanced GL1 expression. An analysis of the GL1 promoter found two overlapping potential E2F binding sites in a region of the promoter known to be essential for GL1 function. This finding indicates that GL1 may be direcdy regulated by the activity of a CYCD3/CDKA complex that phosphorylates E2F-RB bound to the GL1 promoter.