Flower development of Phalaenopsis orchid involves functionally divergent SEPALLATA-like genes.
Bottom Line: The tepal became a leaf-like organ when PeSEP3 was silenced by virus-induced silencing, with alterations in epidermis identity and contents of anthocyanin and chlorophyll.Silencing of the E-class genes PeSEP2 and PeSEP3 resulted in the downregulation of B-class PeMADS2-6 genes, which indicates an association of PeSEP functions and B-class gene expression.These findings reveal the important roles of PeSEP in Phalaenopsis floral organ formation throughout the developmental process by the formation of various multiple protein complexes.
Affiliation: Department of Life Sciences, National Cheng Kung University, Tainan, 701, Taiwan.Show MeSH
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Mentions: By studying loss-of-function mutants, several species of dicots have shown a general conservation of SEP-like gene functions in the specification of floral organ identity. The Arabidopsis sep1sep2sep3 triple mutants have a phenotype of all sepals, petals, stamens and carpels converted to sepal-like organs, whereas the sep1sep2sep3sep4 mutant has the most striking changes, with all floral organs replaced by leaf-like organs (Pelaz et al., 2000; Ditta et al., 2004). Downregulation of SEP1/2-like genes in tomato (LeTM29) and petunia (FBP2 and FBP5) produces sepallata-like flowers (Ampomah-Dwamena et al., 2002; Vandenbussche et al., 2003; Matsubara et al., 2008). Similar phenotypes were found when silencing ripening-related genes: FaMADS9 in strawberry and MaMADS8 and MaMADS9 in apple (Seymour et al., 2010; Ireland et al., 2013). The Petunia PhAGL6 shows similar expression patterns, but also redundant functions, to FBP2/FBP5 (Rijpkema et al., 2009). In monocots, grass species have been analyzed for loss of function of SEP-like genes in floral organ identity. Simultaneous silencing of rice OsMADS1,OsMADS5,OsMADS7 and OsMADS8 transformed all floret organs into leaf-like structures (Cui et al., 2010). Our analysis of PeSEP silencing is the first species investigated other than Poaceae. Although functions of orchid SEP-like genes have been characterized by ectopic expression in Arabidopsis in two species, Oncidium and Dendrobium (Yu & Goh, 2000; Chang et al., 2009), VIGS of PeSEP genes in Phalaenopsis allows for a much more comprehensive and detailed understanding of their functions. We found that the PeSEP-silenced perianth was unable to maintain its distinctive nature as an elaborate floral tepal, but rather converted to an organ with leaf-like appearance. Silencing of the PeSEP genes was sufficient to promote homeotic transformation with reduced flower size and delayed perianth senescence in leaf-like perianth. PeSEPs may trigger a change in the original cell identity from a leaf to perianth via broad changes in cellular levels by cell expansion, inhibition of chlorophyll accumulation, anthocyanin pigmentation, cutin formation and extracellular structures (Fig.12).
Affiliation: Department of Life Sciences, National Cheng Kung University, Tainan, 701, Taiwan.