Root growth in Orchid Dendrobium cv. Sonia requires shade avoidance response of phytochromes along with regulation of auxin pathway genes

Abstract

Orchid Dendrobium Sonia has wide demand in floral market throughout the world due to its vibrant coloured flowers, flowering recurrence and dense inflorescence. Effects of different monochromatic lights (red, far-red, blue, white) on different developmental and growth responses such as seed gemination, shoot and root growth, chlorophyll and carotenoid accumulation were studied in Dendrobium Sonia. Asymbiotic seed germination was highest under blue or white light (80%, p < 0.001) and least under far-red (55%, p < 0.001) suggesting that it could either be controlled by an exceptional novel function of Cryptochrome 1 or the blue wavelengths perceived by PHYA or PHYB in Dendrobium Sonia. All analyses were done in 6-months-old plants till 1 year of age. Shoot length increased significantly in white or red light (3.7-fold, 3.5-fold respectively) while was least under far-red (decreased by 1.6-fold) indicating the major regulatory role of PHYB for shoot growth. Root growth in red light in Dendrobium Sonia was more similar to hypocotyl adventitious root (HAR) formation considering the PHYB transcript expression. We suggest that the root growth (HAR) under red light in Dendrobium Sonia is partly attributed to PHYB , negatively regulated with HY5 and positively associated with auxin biosynthesis and accumulation gene BABY BOOM (BBM2) and efflux carriers such as LIKE AUXIN RESISTANT 2 and 3 ( LAX2, LAX3) . Highest chlorophyll content under far-red and blue might be a hyper-response of shade avoidance response under far-red light in Dendrobium Sonia . Taxonomic tree analysis finds Dendrobium Sonia closer to Phalaenopsis and Dendrobium catenatum . The three phytochromes and one cryptochrome sequences were similar to PHYA, PHYB, PHYC and CRY1 respectively. CRY1 was retrieved with one more isoform CRY1_X2, with all these sequences more similar to those of Oryza sativa . Further study can clarify the indicating reason of a probable gene loss which is evident from the absence of any sequence similar to CRY2 in root RNA isolates of Dendrobium Sonia.

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