Early flowering caused by faulty biological clock

A change in its internal clock helps barley adjust to cultivation in northern environments with short summers

May 14, 2012

According to scientists from the Max Planck Institute for Plant Breeding Research in Cologne and the John Innes Centre in Norwich, a spring barley variety used in Scandinavia has a defective internal clock, but is still very productive. Its ploy is that it curbs its biological timekeeping with a special mutation: even when daylight hours are short, this allows it to kick-start a metabolism which is linked to long days and short nights in the indigenous summer barley varieties. It therefore flowers much earlier.

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There are several phases in the development of barley flowers. First, vegetative growth tissue forms the flower (left). B. From this is formed the spike, which develops into fertile florets as the stem elongates (third from left). The duration of each of these phases varies independently of the others and depends on both the environment and the genetic makeup of the plant. Each developmental phase affects yield in a different way: the number of spikes per plant is determined at the early developmental stage before the spike is completely developed. The number of grains per spike is determined primarily during stem elongation.

There are several phases in the development of barley flowers. First, vegetative growth tissue forms the flower (left). B. From this is formed the spike, which develops into fertile florets as the stem elongates (third from left). The duration of each of these phases varies independently of the others and depends on both the environment and the genetic makeup of the plant. Each developmental phase affects yield in a different way: the number of spikes per plant is determined at the early developmental stage before the spike is completely developed. The number of grains per spike is determined primarily during stem elongation.

Wild barley was domesticated some 10,000 years ago in the Middle East, a region known as the Fertile Crescent. It is a long long-day plant and needs at least twelve hours of light a day to form flowers. Barley has both winter and spring varieties, with winter barley being the original form. It is sown in autumn, grows during the winter and, after a sufficient cold stimulus, flowers in the spring once the days lengthen. Spring barley, as its name suggests, is sown in the spring. It needs no cold stimulus and only flowers with some delay, i.e. not until the days get longer. In Germany, spring barley is used to brew beer, and winter barley as animal feed.

Barley is highly adaptable and thrives in the dry regions of the Middle East, in the Tibetan highlands, in the subtropical regions and at the polar circle. An early flowering variant for use in cold regions with their short growing season has been available for decades, bred selectively from the late-flowering spring barleys. This variety is very productive and well adapted to North European localities. Maria von Korff and her colleagues have now carried out studies to investigate the genetic basis of this early flowering and to find out how this variety, adapted to high latitudes, differs from the late-flowering varieties in Central Europe. The geneticists were able to show that this North-adapted variety contains a mutation that damages its internal clock. The mutated protein carries the cryptic name EAM8. The link between this protein and the internal clock was demonstrated through a comparison with the model plant Arabidopsis, commonly known as mouse-ear cress. EAM8 is the barley’s genetic counterpart to a clock protein in Arabidopsis, known as the ELF3 protein. “While the mutation in this genetic counterpart probably hasn’t caused a complete timekeeping breakdown”, commented von Korff on the findings, “the clock no longer works properly. It has been curbed, and runs faster than usual.”

During evolution, animals and plants have developed an internal clock allowing them to adapt their behaviour and key metabolic functions optimally to diurnal changes in environmental conditions. This internal pacemaker thus helps plants look into the future and make predictions about day length, temperature and patterns of light and shade. As a result, plants can use their biological clock to improve the robustness and the number of offspring they produce. “As far as we know, it has never previously been shown that an impaired internal clock function can lead to better adaptation and higher yields in cultivated plants”, says von Korff.

The scientists from Cologne and Norwich were also able to demonstrate how this variety, adapted to the far north pulls off this feat. It seems that the flowering metabolism linked to long day length conditions is activated in this variety.  The EAM8 mutation tricks the plant into “believing” that the days are longer when that is not actually the case. As a consequence, the transition from vegetative growth to the flower formation is speeded up, and the barley flowers much earlier than other spring barley varieties. In addition, von Korff and her colleagues showed that the EAM8 mutation causes increased production of the protein HvFT1 under short daylight conditions.  HvFT1 also has a counterpart in Arabidopsis, known as florigen. This protein carries the “It’s time to flower” signal from the leaves, where it is produced, to the end of the shoot axis, where the flowers are then formed. The appearance of the EAM8-induced HvFT1 protein at the end of the shoot axis initiates flower formation in this variety adapted to northern regions.

The Cologne-based group’s work is of interest for the breeding of new varieties adapted to extreme conditions. Future climate change will continue to shift cultivation areas further, creating a constant need to establish new areas. This mutant is therefore also interesting for Mediterranean growing areas, which are suffering increasingly from heat and a shortage of water. Early-flowering barley varieties are better adapted to the resultant shorter growing periods.

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