The researchers studied wild-type and Arabidopsis thaliana mutants with off-cycle circadian rhythms. The plants that had a matching circadian clock to the light-dark environment cycles had increased growth and survival. For example, these plants fixed more carbon and had more chlorophyll.
Circadian clocks are the body's biological clock that works to match external light-dark cycles. Circadian clocks give an advantage to organisms. In plants, circadian clock rhythm genes regulate opening of the stomata and reproduction. Circadian resonance occurs when the internal clock matches the external environment. When the cycles match, plants gain an advantage. Plants can optimize the timing of biological functions based on their accurate clock. T cycles are the length of an external cycle.
First, the researchers used wild-type Columbia-0 (Col-0) plants that had an internal cycle of 24 hours. This wild-type plant was grown in various environments. The external days included lengths of 20 hours (T20), 24 hours (T24), and 28 hours (T28). Half of these days are light and half of these days are dark. The mutant ztl-1 had an internal clock longer than normal (27.1-32.5 hours). The mutant toc1-1 had an internal clock shorter than normal (20.7 hours). Another mutant CCA1-ox overexpressed CCA1. CCA1 is a gene involved in creating the cycles of the circadian clock. So, CCA1-ox had irregular circadian rhythms. The authors studied the plants during vegetative growth, which is when a plant is growing but not flowering.
Results and Discussion
Leaves of the wild-type plant had the most chlorophyll in T24 after 30 days compared to plants grown in T20 and T28. The mutant ztl-1 had more chlorophyll in T28 than in T20. The mutant toc1-1 had more chlorophyll in T20 than in T28 (Figure 1). Thus, plants have more chlorophyll if their circadian clocks match the external cycles. The pigments that harvest light are relatively uniform throughout time. But, there are proteins that are degraded by light as a regulation. So, if there are longer cycles of light, more proteins will be degraded than anticipated.
Carbon fixation is when plants take carbon dioxide to create organic compounds for their own use. The mutants ztl-1 and toc1-1 fixed more carbon when the external cycles matched their circadian cycles. The mutant CCA1-ox fixed less carbon than the wild-type plant. In continuous light, the wild-type plant fixed carbon in a cyclic manner. But, the mutant CCA1-ox fixed carbon constantly and in an increasing manner. When the external environment was cyclic, the stomata of both the wild-type plant and CCA1-ox opened and closed. But, the wild-type plant could anticipate darkness or “dusk.” So, its stomata closed earlier. The wild-type plant conserved more water as a result (Figure 2).
Plants with circadian resonance grow the most. The researchers grew the plants for 32 days.The wild-type plant grown in T20 had 47% less aerial biomass than the plant grown in T24. The wild-type plant grown in T28 had 42% less aerial biomass than the plant grown in T24. The mutants ztl-1 and toc1-1 had the most aerial biomass and leaf surface area when grown in T28 and T20, respectively. The mutant CCA1-ox had less biomass and leaf surface area than the wild-type (Figure 3). Thus, circadian resonance increases aspects of plant fitness.
The researchers did not find a significant difference in seed production between plants and mutants. However, flowering depends on circadian clocks. So, the mutant genes may ambiguously affect seed production.
Plants with circadian resonance have a competitive advantage against other plants. The researchers created a lawn combined with the ztl-1 and toc1-1. When the lawn was grown in T20, toc1-1 outgrew ztl-1 in terms of biomass. The mortality of ztl-1 was higher than toc1-1. The reverse was true for T28. In a second experiment, the researchers looked at two more mutants. The additional mutants help control for any other alleles that may provide a competitive advantage in the first experiment. One was toc1-2, which has an internal clock of about 20 hours. The other was ztl-27, which has an internal clock of about 28 hours. When the lawn was in T20, toc1-2 outgrew ztl-27. The mortality of ztl-27 was higher than toc1-2. The reverse was true for T28 (Figure 4). The competitive advantage was likely due to the increased ability for photosynthesis.
ConclusionThe circadian clock can help the Arabidopsis thaliana grow up to twice as much as a mutant. The clock allows the plant to anticipate the external light-dark cycles. Because of the competitive advantage, the circadian clock was selected for during evolution. The researchers urge that selective breeding of crops should be done carefully. Keeping the circadian clock genes can increase plant production significantly.