20:	Type A and Type B Phytochromes Have Distinct Functions in the Photoperiodic Control of Flowering in the LDP Nicotiana sylvestris.

Authors:	Zheng, Zhiliang(A)Yang, Zhenbiao(B)Jang, Jyan-Chyan(A)Metzger, James, D.(A)
Affiliations:	(A): Dept Horticulture & Crop Science, The Ohio State University (B): Dept Plant Biology, The Ohio State University
Presenter:	Metzger, James D., metzger.72@osu.edu

Phytochrome is the primary photosensor involved in assessing daylength information in the photoperiodic control of flowering, but it remains unclear what light cues are sensed by individual phytochromes. The obligate LDP Nicotiana sylvestris with a critical daylength of about 12h was used as a model to determine the functions of phyA and phyB in the photoperiodic control of flowering. The following transgenic lines were used to study the flowering responses under different daylength extensions following 8h natural light: PHY-A1 underproducer (SUA2), PHY-B1 underproducer (SCB35), PHY-B1 overproducer (SOB36), and the cross SUA2 x SCB35. Flowering in SUA2 was delayed compared to WT when plants were subjected to 4 or 8h daylength extensions with incandescent (inc) light (R/FR=1.0). There was a much smaller delay between the two genotypes when days were extended with 8h fluorescent (fluo) light (R/FR=7.4). FR alone did not promote flowering in any genotype suggesting it is a necessary, but not sufficient component in the daylength extension for maximum flower promotion and that a type-A phytochrome mediates the FR response. Under an 8h extension with inc, SCB35 flowered at the same time as WT, but was substantially delayed with 8h fluo. The effects of the two treatments were reversed when SOB36 and WT were compared. However SCB35 flowered much earlier, and SOB36 later than WT under a 4h inc extension. The cross SCB35 x SUA2 did not flower under any treatment. In total the results indicate the response to R in the daylength extension is mediated by a type-B phytochrome and that R is a necessary and sufficient component of the daylength extension. The results also indicate that PHYB-1 has a dual inhibitory/promotive function in the photoperiodic control of flowering.