Marja Rantanen

Mutation in TERMINAL FLOWER1 reverses the photoperiodic requirement for flowering in the wild strawberry, Fragaria vesca

Photoperiodic flowering has been extensively studied in the annual short-day and long-day plants rice (Oryza sativa) and Arabidopsis (Arabidopsis thaliana), whereas less is known about the control of flowering in perennials. In the perennial wild strawberry, Fragaria vesca (Rosaceae), short-day and perpetual flowering long-day accessions occur. Genetic analyses showed that differences in their flowering responses are caused by a single gene, SEASONAL FLOWERING LOCUS, which may encode the F. vesca homolog of TERMINAL FLOWER1 (FvTFL1). We show through high-resolution mapping and transgenic approaches that FvTFL1 is the basis of this change in flowering behavior and demonstrate that FvTFL1 acts as a photoperiodically regulated repressor. In short-day F. vesca, long photoperiods activate FvTFL1 mRNA expression and short days suppress it, promoting flower induction. These seasonal cycles in FvTFL1 mRNA level confer seasonal cycling of vegetative and reproductive development. Mutations in FvTFL1 prevent long-day suppression of flowering, and the early flowering that then occurs under long days is dependent on the F. vesca homolog of FLOWERING LOCUS T. This photoperiodic response mechanism differs from those described in model annual plants. We suggest that this mechanism controls flowering within the perennial growth cycle in F. vesca and demonstrate that a change in a single gene reverses the photoperiodic requirements for flowering.

Light quality regulates flowering in FvFT1/FvTFL1 dependent manner in the woodland strawberry Fragaria vesca

Control of flowering in the perennial model, the woodland strawberry (Fragaria vesca L.), involves distinct molecular mechanisms that result in contrasting photoperiodic flowering responses and growth cycles in different accessions. The F. vesca homolog of TERMINAL FLOWER1 (FvTFL1) functions as a key floral repressor that causes short-day (SD) requirement of flowering and seasonal flowering habit in the SD strawberry. In contrast, perpetual flowering F. vesca accessions lacking functional FvTFL1 show FLOWERING LOCUS T (FvFT1)-dependent early flowering specifically under long-days (LD). We show here that the end-of-day far-red (FR) and blue (B) light activate the expression of FvFT1 and the F. vesca homolog of SUPPRESSOR OF THE OVEREXPRESSION OF CONSTANS (FvSOC1) in both SD and LD strawberries, whereas low expression levels are detected in red (R) and SD treatments. By using transgenic lines, we demonstrate that FvFT1 advances flowering under FR and B treatments compared to R and SD treatments in the LD strawberry, and that FvSOC1 is specifically needed for the B light response. In the SD strawberry, flowering responses to these light quality treatments are reversed due to up-regulation of the floral repressor FvTFL1 in parallel with FvFT1 and FvSOC1. Our data highlights the central role of FvFT1 in the light quality dependent flower induction in the LD strawberry and demonstrates that FvTFL1 reverses not only photoperiodic requirements but also light quality effects on flower induction in the SD strawberry.

Dynamic control of supplemental lighting intensity in a greenhouse environment

The global increase in energy prices, the urgent need to reduce CO2 emissions to the atmosphere and the high energy usage are currently the major threats to the greenhouse industry. Optimised control of the lighting quality, quantity and periodicity can contribute to improvements in the productivity and energy efficiency of greenhouses. In this paper, the effects of dynamic control of supplemental lighting intensity on electricity consumption and fresh weight accumulation of lettuce plants are investigated. The use of the dynamic lighting control resulted in a 20% reduction in the electricity consumption in comparison to a similar lighting system operated under a discontinuous on–off regime. However, there was no statistically significant difference between both regimes in terms of plants’ average fresh weight accumulated per electrical energy unit consumed.

Strawberry homologue of terminal flower1 integrates photoperiod and temperature signals to inhibit flowering.

Photoperiod and temperature are major environmental signals affecting flowering in plants. Although molecular pathways mediating these signals have been well characterized in the annual model plant Arabidopsis, much less information is known in perennials. Many perennials including the woodland strawberry (Fragaria vesca L.) are induced to flower in response to decreasing photoperiod and temperature in autumn and they flower following spring. We showed earlier that, in contrast with Arabidopsis, the photoperiodic induction of flowering in strawberry occurs in short days (SD) when the decrease in FvFT1 (flowering locus T) and FvSOC1 (suppressor of the overexpression of constans1) expression leads to lower mRNA levels of the floral repressor, FvTFL1 (terminal flower1). By using transgenic lines and gene expression analyses, we show evidence that the temperature-mediated changes in the FvTFL1 mRNA expression set critical temperature limits for the photoperiodic flowering in strawberry. At temperatures below 13 °C, low expression level of FvTFL1 in both SD and long days (LD) allows flower induction to occur independently of the photoperiod. Rising temperature gradually increases FvTFL1 mRNA levels under LD, and at temperatures above 13 °C, SD is required for the flower induction that depends on the deactivation of FvSOC1 and FvTFL1. However, an unknown transcriptional activator, which functions independently of FvSOC1, enhances the expression of FvTFL1 at 23 °C preventing photoperiodic flowering. We suggest that the observed effect of the photoperiod × temperature interaction on FvTFL1 mRNA expression may allow strawberry to induce flowers in correct time in different climates.

Growth and cropping of primocane and biennial raspberry cultivars grown under a film absorbing far-red light

Summary In an attempt to control vegetative growth in raspberry (Rubus idaeus L.) in protected cultivation, we studied the effect of a far-red (FR)-absorbing greenhouse film on growth, cropping, and fruit quality in four raspberry cultivars. The primocane cultivars ‘Autumn Bliss’ and ‘Polka’ were grown in tunnels covered either with a clear film or a FR-absorbing film until the end of their first-year-harvest season. The biennial cultivars ‘Glen Ample’ and ‘Tulameen’ were treated similarly during their period of primocane growth. The FR-absorbing film increased the numbers of flowers in the primocane cultivars, whereas the numbers of flowers were reduced in the biennial cultivars during forcing in the following year. The numbers of flowers per plant were 185 in ‘Autumn Bliss’ and 171 in ‘Polka’ under the clear film; but 202 and 203, respectively, under the FR-absorbing film. The number of berries per plant was also increased in ‘Polka’. The FR-absorbing film reduced cane growth in ‘Polka’, due to a reduced number of internodes, and in ‘Tulameen’, due to reduced internode length. The modified light spectrum had no effect on the berry contents of total anthocyanins, sugars, most mineral components, or on fruit colour in the primocane cultivars. However, levels of ellagic acid were increased by the FR-absorbing film, while the soluble solids content:titratable acidity ratio was slightly reduced. FR-absorbing films may therefore be useful when growing primocane raspberry cultivars in order to increase their yield potential, the contents of bioactive compounds in their fruit, and to decrease vegetative growth. However, such films may not be recommended for growing primocanes of biennial-type raspberry cultivars due to impaired flower development.

Effects of LED light spectra on lettuce growth and nutritional composition

Year-round greenhouse production in northern latitudes depends on the use of artificial lighting. Light emitting diodes provide a promising means to save energy during cultivation as well as to modify the light spectrum to regulate the growth and quality of the crop. We compared the effects of light emitting diode lighting with different spectral compositions on the growth, development and nutritional quality of lettuce (Lactuca sativa L. ‘Frillice’). We show that warm-white and warm-white supplemented with blue spectra provide equal growth and product quality compared to conventional high-pressure sodium lighting in the absence and presence of daylight. Our data indicate that for biomass accumulation, the far-red component in the light spectrum is more critical than green light or the red/blue ratio. Furthermore, we demonstrate that a red + blue spectrum increases the concentration of several vitamins in lettuce. However, biomass accumulation using this spectrum was insufficient when daylight was excluded.