Strawberry flowering the possibility of an everbearing cultivar

A study published in the Plant Biotechnology Journal has experimentally confirmed that the expression of a particular gene, called TERMINAL FLOWER1, is intricately involved in the repression of flowering in strawberry cultivars, including commercial varieties.

Background

Different strawberry cultivars will flower and produce fruit at different temperatures and different day lengths. Some cultivars require short day lengths, others require short days and a certain temperature in order to produce fruit, while others require longer days. When the day length and/or the temperature is not right, the plant will repress flowering and will instead sprout runners for reproduction. There are some everbearing varieties but they tend to not be good at reproducing themselves.

The study looked at expression levels of TERMINAL FLOWER1, a gene that has been studied in other plant species and which has been shown to represses flowering when it is expressed. Other plant cultivars with everbearing characteristics have been found to have a lack-of-function mutation in this gene.

Previous studies have found that the TERMINAL FLOWER1 gene is controlled by a photoperiodic pathway; the gene is activated by an upstream gene product, which in turn is activated by a gene product found to be produced in leaf tissues when exposed to long days (assuming the plant produces flowers only in short days). When short days return, the expression of the first gene in the pathway is down-regulated, in turn down-regulating the expression of the two other genes in the pathway, and flowering is once again allowed to occur.

The study

The study performed a number of simultaneous experiments to elucidate the link between day length, temperature and flowering with the expression of this pathway of genes:

  1. Silencing the TERMINAL FLOWER1 gene

The experiment took a short day cultivar and transformed it using an RNA interfernce construct (see our previous article on RNAi) to knockout the gene. The researchers searched a genome database and found 3 homologous genes within the cultivar and tested whether their RNAi construct affected those genes as well, but couldn’t find any difference in their levels of expression between the wild and the transgenic plant lines. Therefore, the sole effect of the RNAi construct should only be to repress this one gene.

What they found was that transgenic lines started flowering 2 weeks before the wild type and produced many more flowers when they were both grown in cool temperatures in short days. The transgenic line continued flowering throughout the experimental period, essentially becoming an everbearing line. Further, the number of runners produced were the same in both the wild and transgenic lines, pointing to a possible solution to the problem with current everbearing lines.

RNAi strawberry

Figure from article – (a) FLOWER TERMINAL1 expression; (b) Gene expression linked to runner production; (c) flowers per plant; (d) number of runners per plant; and (e) wild-type and transgenic strawberry plants.

2. Day length responses in three cultivars with different flowering times

The second experiment took three varieties of strawberry plants with different flowering times, ‘Honeoye’, ‘Alaska Pioneer’ and ‘Polka’, subjected the three to short days and long days and measured the expression of the FLOWER TERMINAL1 (FaTFL1) expression and expression of the other two upstream genes (FaSOC1 and FaFUL1) in the photoperiodic pathway.

Looking at the FLOWER TERMINAL1 gene expression (the middle row of bar graphs in the figure below), the three cultivars had differing responses to the short and long day, the ‘Honeoye’ having a large initial expression under long day lengths that reduced over the period of the trial, the ‘Alaska Pioneer’ doing similar but reducing at a slower rate than the ‘Honeoye’ in long days, and the ‘Polka’ showing a consistent level of expression when exposed to long day light over the trial period.

Strawberry expression levels

Figure from article. Expression levels of the three genes in the photoperiodic pathway in ‘Honeoye’, ‘Alaska Pioneer’ and ‘Polka’ varieties.

3. Interaction of day length and temperature in ‘Elsanta’ and ‘Glima’

‘Elsanta’ and ‘Glima’ are commercial varieties of strawberries with differing flowering habits; ‘Glima’ flowers in long days at temperatures below 21ºC while ‘Elsanta’ requires short days no matter the temperature. What the researchers found was that ‘Glima’ in fact flowered at any day length and only showed any repression when grown in long days at 21ºC. The ‘Elsanta’ plant, as predicted, didn’t grow any flowers in long days at any temperature, while short days resulted in flowers in all test plants with warmer temperatures resulting in earlier and a greater number of flowers.

Gene expression levels in the leaves and shoots of both cultivars were taken and showed that the expression of FLOWER TERMINAL1 was in line with the flowering of the plants but with a higher expression in ‘Elsanta’ than in ‘Glima’. This may explain why day length played a greater role in flowering habits in ‘Elsanta’. Further, the transcript levels in ‘Elsanta’ under long days was quite low at a growing temperature of 9ºC but still didn’t result in flowering.

strawberry Glima and Elsanta

Figure from article. Gene expression levels in ‘Glima’ and ‘Elsanta’ varieties under short and long days grown at three different temperatures.

Looking at the FaFTL1 expression levels in the above figure, the levels are quite similar in long day growing when ‘Glima’ was grown at 21ºC and ‘Elsanta’ was grown at 9ºC. Despite the similar levels, 60% of ‘Glima’ plants flowered while no ‘Elsanta’ plants flowered. This gives an insight into the differing effects of the gene expression in different cultivars.

What can we do with this information?

One take-away point for any gardener looking to grow some strawberries is that to grow and maximise your yield you must match the flowering requirements of the cultivar chosen with the local climate and day lengths. Living in a region with longer winters of shorter day lengths and colder climates can be matched with cultivars that will flower and produce strawberries for a longer period, reproducing over the summer period, while warmer climates with shorter winters and more long days than short will require an appropriate long day cultivar.

The RNAi experiment opens up the commercial possibility of developing a cultivar that has repressed FLOWER TERMINAL1 expression under all conditions, resulting in longer or everbearing plants without affecting vegetative reproduction.

Finally, as the researchers wrote, we don’t completely understand this photoperiodic pathway and how it interacts with temperature-related genes. Further research may identify temperature-sensitive genes which interact with the photoperiodic pathway and this may result everbearing qualities being engineered or bred in such a way as to allow any cultivar to successfully grown in a changing climate.

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