FTL2 expression preceding bud set corresponds with timing of bud set in Norway spruce under different light quality treatments

• FR day extension sustained growth and B treatment induced bud set in Norway spruce.
• In comparison, R delayed/reduced bud set, particularly in southern provenance.
• FTL2 expression preceding bud set correlated with bud set timing in these treatments.
• COL1, COL2, SOC1 expression decreased rapidly in bud-inducing light qualities.
• Expression of CRY and PHY light receptor genes increased after bud set.

Shoot elongation and bud set in temperate zone trees is under photoperiodic control, but light quality also plays a role with far-red (FR) light sustaining growth, and blue (B) affecting timing of bud set. This may suggest a role of B light receptors in addition to the photoperiod- and red (R)-FR-sensing phytochrome (phy) system. However, there is limited information on effect of light quality on genes thought to be involved in growth-dormancy cycling. In this study we have examined effect of light quality on expression of genes known to be affected by photoperiod as well as CRY and PHY genes, which were also characterised in this study. In contrast to growth-sustaining FR and reference LD treatment, bud-inducing R and B light and reference SD exposure resulted in accumulation of transcripts of PaFTL2, and reduction in PaCOL1, PaCOL2 and PaSOC1 before visible morphological changes associated with bud set. The light receptor transcripts (CRY1, CRY2, PHYN, PHYO, PHYP) also accumulated, but with the largest changes after growth cessation. Reduced or slightly delayed bud set under R compared to B light and SD, was in most cases reflected in changes in transcript levels. Of the studied genes, FTL2 expression preceding bud set correlated the most closely with timing of bud set in in the studied provenances under the different light quality treatments. These results demonstrate action of light quality and photoperiod through a common pathway in control of bud set in Norway spruce, with FTL2 as a key factor.