Their results were published recently in the journal Plant, Cell and Environment.
Sonali Ranade and María Rosario García-Gil have shown earlier that Norway spruce populations in Sweden are differently adjusted to the different local light conditions in Sweden. Norway spruce from the North tolerates shade much better than Norway spruce populations from Southern Sweden and this comes along with a better resistance to diseases. Now, the researchers have identified one underlying factor for this adaptation to the latitudinal different light conditions – variations in the genes for photoreceptors.
“Photoreceptors are light sensitive proteins. Plants use them to understand light quality and to sense daylength. Based on the perceived information, plants regulate processes and signalling pathways to adjust their development to their environment”, explains Sonali Ranade, first author of both studies. “We could show now that the genes for two photoreceptors - phytochromes and cryptochromes - genetically vary in Swedish Norway spruce populations and that this variation follows a gradient from North to South.”
The variation is located in functional parts of the photoreceptor
The researchers were comparing the DNA sequence of 1654 different Norway spruce trees that derived from different latitudes across Sweden. They divided the group into six smaller populations according to different latitudes and were looking for small variations in the DNA sequence of the different trees. To focus first on photoreceptors, especially on phytochromes, was obvious for them as they are central regulators of the light pathways in plants.
Phytochromes perceive red and far-red light which is at the extreme end of the visible light spectrum. Under shade conditions, the red to far-red light ratio is low because there is much more far-red light available than red light. Cryptochromes sense blue light and play an important role for adjusting the plant’s growth and development to the day-night cycle. The variations that the researchers identified in the DNA sequences of the phytochrome and cryptochrome genes of Norway spruce were all located in regions that contain information for functional parts of the photoreceptor.
Different populations have adapted to local light conditions
“We do not know how and if the function of the photoreceptors in the different Norway spruce populations is changed but we think that these modifications were probably important to adapt the tree’s growth and development to the latitudinally different light conditions in Sweden,” says Sonali Ranade. “The growing period in Northern Sweden is much shorter than in the South. The trees would not have survived if they would burst their buds when it is still freezing in spring or have not set their buds yet when the first snow arrives in autumn.”
Genetically, all populations of Norway spruce in Sweden derive from two different origins reflected in two main populations - a southern and central population and a northern population. The researchers were expecting that the genetic variation would reflect mainly these two main populations. Instead, they found that the gradient changed continuously from South to North, clearly following the changing light quality through the different latitudes across Sweden, meaning that different populations have adapted to local light conditions.
Light quality differs between Northern and Southern Sweden because the amount of far-red (FR) light increases towards the north reducing the ratio between red and far-red light. Photoreceptors sense light quality and researchers have now discovered that the photoreceptor genes of Norway spruce vary across populations from different latitudes across Sweden, revealing local adaptations to different light conditions (illustration: Sonali Ranade).
“Detecting the genetic basis of local adaptation to environmental conditions in coniferous tree species is difficult”, says María Rosario García-Gil, group leader at Umeå Plant Science Centre. “Often many genes of small effect contribute to a certain characteristic leading to a complex genetic architecture. These species also have a huge genome size compared to herbaceous plants like for example thale cress and their genome is still much less understood.”
The findings from the current study are interesting from an evolutionary perspective illustrating how populations adapt to the local environment but the researchers think that this is not all. Norway spruce is an economically important conifer species for the Swedish forest industry. Understanding how trees to adapt to local conditions - like in this case local light conditions - and knowing which genes are influencing a certain tree characteristic might be also useful to design novel strategies for breeding programmes for Norway spruce.