The generation time of an organism varies from organism to organism. One of the defining features of model species is their short generation times, allowing fast analysis of, for example, gene mutations on phenotype (think of mice, zebrafish and the fruit fly). The shorter the generation time, the greater the number of experiments and analysis which can be performed within a set period of time.
In the world of plant science, Arabidopsis thaliana is generally considered the model organism and many papers on genetics and gene technologies use A. thaliana as the subject plant from which study results are drawn.
But conclusions drawn from a model organism doesn’t always translate to similar results in another organism – think of promising medical studies in mice that haven’t had the same effect in humans. Accordingly, model organisms may not always be appropriate for use to draw conclusions about other organisms.
Such is the case with the translation of studies in A. thaliana in some important crops such as wheat and barley. The problem with studying genetic traits in such crops is that the generation time of the plant may be long, significantly impeding important research.
Introducing Speed Breeding
Given the problem with generation times, a small army of researchers led by Lee Hickey of the Queensland Alliance for Agriculture and Food Innovation (and recent recipient of the Queensland Young Tall Poppy Scientist of the Year – congratulations Lee) developed a new protocol for speed breeding and tested a number of important crop plants for their response to the new protocol.
The protocol is pretty simple. Instead of using a standard photoperiod used when growing plants under study, the researchers increased the day length to 22 hours and reduced the length of time the plant is in darkness to 2 hours.
In fact, the paper specifies three protocols that all use this photoperiod, each of the protocols having a slight variation in the other growth conditions including the third protocol which is meant to be a less expensive set up able to generate similar results to that found in laboratory conditions.
Testing the Protocol
The first testing of the protocol took place in Norwich, United Kingdom, and compared the growth rates of bread wheat, durum wheat, barley and a model grass Brachypodium distachyon when grown using the speed breeding protocol or grown under usual greenhouse conditions with no supplemental lighting or heating.
The crops grown in the modified setting consistently flowered earlier than the control plants, taking approximately half the time to begin flowering with B. distachyon taking an average of 26 days and wheat taking on average 37 to 39 days. A significant change in the amount of time required to reproduce a target number of generations.
To ensure that the faster flowering plants didn’t suffer from loss of seed viability, the researchers tested whether the speed-bred plants were able to generate the following generation of plants. The researchers quantified effects on three phentoypic variables related to viability, including:
- wheat seed counts per spike – a decrease compared to control;
- spikes per plant – healthy number in the speed-bred plants; and
- viable seed production – no difference.
Moving back from Norwich to Queensland, the researchers tested a second but very similar protocol with spring wheat, barley, canola and chickpea cultivars. This protocol used high pressure sodium lamps to produce the 22-hour day for these plants and compared the effect of this protocol to the same plants grown in glass house with a 12 hour day/night cycle. This time, both treatment and control groups were subject to the same temperatures for the day and night portions.
Similar to the first test, the time to anthesis was significantly less under the speed breeding conditions, but this time the wheat plants produced more spikes, grain number didn’t show the same reduced effect as it did the first protocol and the flowering times of the different crop species were more uniform than in the control. The researchers point out that this last effect, crops flowering at the same time, is advantageous when genotype crossing is to be performed. Seed viability was retained even when harvested 14 days post-anthesis and subsequently subjected to cold treatment for 4 days, indicating that further time savings can be made in the production of generation times.
In canola and chickpea there was no significant difference in seed production between the treatment and control groups.
Figure 1 from article demonstrating the photoperiod and temperature ranges under the speed breeding (left) and control (right) conditions and the number of generations which can be produced in wheat, barley, chickpea and canola under each condition.
Overall, the number of generations that can be produced under speed breeding conditions in a year were:
- wheat – 5.7 generations;
- barley – 5.4 generations;
- canola – 3.8 generations;
- chickpea – 4.5 generations.
The possibility of using the speed breeding technique in conjunction with single seed descent breeding and research programs was the basis of a seed viability analysis in wheat and barley. Using 100-cell trays, seed viability was confirmed for both genotypes at 80% two weeks after anthesis and 100% at four week post-anthesis. Accordingly, single seed descent programs can also be used in conjunction with speed breeding.
A low-cost speed-breeding protocol was also tested. In this set-up, LEDs were used exclusively in conjunction with a conventional split-system air conditioner to create speed breeding conditions and garnered similar results.
What about effects other than generation times?
Although faster generation times are extremely useful, if the effect of the speed breeding is to alter the crops’ phenotype, research programs would be plagued by unwanted confounding factors and breeding programs would be rendered ineffective.
The research team therefore mutated the awn supressor B1 locus and the Reduced height (Rhd) genes in wheat and obtained the expected phenotypes under the faster generated crops.
Testing any variation in disease responses was performed by inoculating wheat spikes with the bacteria which causes fusarium head blight, one set of spikes belonging to a susceptible cultivar and the second belonging to a head blight-resistant cultivar. The speed-bred cultivars showed the expected resistant and susceptible phenotypes.
Finally, the use of plant breeding in genetic engineering programs was addressed by looking at transformation efficiency of barley seeds grown under the protocol and, again, found no difference when compared to conventionally grown transformed plants.
Hickey’s team has successfully tested a protocol that will greatly assist research efforts into these studied plants and whose results will likely be replicable in many other plants. They cite sunflower, pepper and radish as plants that have shown similar responses to being grown in extended daylight conditions.
The ability quickly test, for example, the effect of knocking out or over-expressing a particular gene will certainly lead to more efficient test results and, hopefully will be another stepping stone to overcoming some of the limits to efficient research we currently endure.