Functional analysis of a RING domain ankyrin repeat protein that is highly expressed during flower senescence

A gene encoding a RING zinc finger ankyrin repeat protein (MjXB3), a putative E3 ubiquitin ligase, is highly expressed in petals of senescing four o'clock (Mirabilis jalapa) flowers, increasing >40,000-fold during the onset of visible senescence. The gene has homologues in many other species, and the Petunia homologue is strongly up-regulated in senescing Petunia corollas. Silencing the expression of this gene in Petunia, using virus-induced gene silencing, resulted in a 2 d extension in flower life. In Mirabilis, a 2 kb promoter region, 5' upstream of the MjXB3 gene, was isolated. The promoter sequence included putative binding sites for many DNA-binding proteins, including the bZIP, Myb, homeodomain-leucine zipper (HD-Zip), MADS-box, and WRKY transcription factors. The construct containing a 1 kb promoter region immediately upstream of the MjXB3 gene drove the strongest expression of the beta-glucuronidase (GUS) reporter gene in a transient expression assay. In Petunia, GUS expression under the control of this heterologous promoter fragment was specific to senescing flowers. The Mirabilis promoter GUS construct was tested in other flower species; while GUS activity in carnation petals was high during senescence, no expression was detected in three monocotyledonous flowers--daylily (Hemerocallis 'Stella d'Oro'), daffodil (Narcissus pseudonarcissus 'King Alfred'), and orchid (Dendrobium 'Emma White').