Plant Genomics and Plant Science

Phalaenopsis aphrodite (moth orchid) is one of the most important ornamental crops in international trade. In nature, this epiphytic orchid grows on the surface of trees. It utilizes crassulacean acid metabolism photosynthesis that takes up CO2 at night. High light environment might cause photodamage in orchid. Plants are sessile; therefore, they have evolved mechanisms to regulate growth and developed to cope with ever-changing environments. Phototropins are blue light receptors in plants that function in chloroplast movement, stomatal opening and affect plant growth and development. Full-length cDNAs of two PHOT genes, PaPHOT1 and PaPHOT2 were cloned from Phalaenopsis aphrodite and their functions in chloroplast movement were investigated in this study. Phalaenopsis orchid responds to Blue Light (BL). Slit assay indicated that chloroplasts did not move at 20 μmole m-2s-1; however, in low BL (<15 μmole m-2s-1) orchids accumulated more chloroplasts in the periclinal cell walls. Chloroplasts started moving at BL>25 μmole m-2s-1. Significant chloroplast avoidance movement was observed at BL>100 μmole m-2s-1. Orchids consistently expressed higher levels of PaPHOT1 and PaPHOT2 under low BL. while, PHOT2 was up-regulated under high BL regimes. To verify the biological function of phototropins in chloroplast movement, Virus-Induced Gene Silencing (VIGS) was used. VIGS-treated orchid leaves showed decreasing gene expression of PHOTs and reduced chloroplast movement phenomena under high BL. In addition, we showed heterologously overexpressing PaPHOT1 and PaPHOT2 in Arabidopsis mutants recovered chloroplast movement. In conclusion, we successfully isolated and characterized two PHOTs from P. aphrodite and showed that they function in blue light-induced chloroplast movement.