Johan Sjölander standing in the greenhouse at UPSC inbetween young aspen trees holding his PhD thesis in the hand.PhD student Johan Sjölander discovered that modifying the internal clock in hybrid aspen can increase growth (photo: Anne Honsel).

The internal clock coordinates the plant growth and adaptation to daily and seasonal changes. It is among others acting via plant hormones. These small molecules are present in low concentrations in plants and control plant growth. Johan Sjölander focussed his PhD studies on the connection between the clock and plant hormones, particularly gibberellin. He discovered that modifying clock components altered the levels of this plant hormone and increased growth in hybrid aspen.

Environmental conditions during day and night, as well across different seasons, vary significantly. Without light, photosynthesis is impossible, and extreme temperatures such as those in winter can harm plants. The plant’s internal clock helps coordinate growth and adaptation to these recurring changes. It is regulated by a complex molecular network and interacts with many other developmental and growth processes like those ones controlled by plant hormones.

“Plant hormones play important roles in regulating almost every aspect of plant development”, says Johan Sjölander, PhD student in Maria E. Eriksson’s group at Umeå Plant Science Centre and Umeå University. “The clock acts as a master regulator, ensuring optimal timing and hormone balance throughout the day and seasons, which ultimately increases the fitness and adaptation of the plant.”

To investigate how the internal clock interacts with plant hormones, Johan Sjölander and his colleagues created hybrid aspen with reduced activity in one clock component. When measuring plant hormone levels, they observed that the levels of the plant hormone gibberellin were altered, along with increased growth. While gibberellin is known to stimulate cell elongation and promote plant height, seeing this effect when perturbing the internal clock was unusual.

“Generally, these kinds of alterations of the internal clock led to trees that are out-of-sync with their environment, which ultimately hamper their ability to grow”, explains Johan Sjölander. “Our research indicates that targeting specific clock components can have the opposite effect. This illustrates how complex the relationship between the internal clock and plant hormones such as gibberellin is.”

Johan Sjölander’s supervisor Maria E. Eriksson previously demonstrated that trees producing more gibberellins struggled to adjust their growth to seasonal changes. The trees did not stop to grow when the days became short. Johan Sjölander and colleagues now used an adjusted strategy. He created hybrid aspen in which the increase of gibberellins was controlled by the internal clock. These trees grew better, produced more biomass and did not lose their ability to stop growth under short days.

“These findings imply that the internal clock is a promising target for enhancing tree productivity without sacrificing seasonal adaptability”, says Johan Sjölander. “When starting my PhD, one of the selling points for me was that my findings might help improve efficiency and sustainability in agriculture and forestry. There is of course still a lot to do but it is rewarding to see the potential of this research for improving future breeding strategies.”

About the public defence:

Johan Sjölander, Umeå Plant Science Centre, Department of Plant Physiology, Umeå University, defended his PhD thesis on Friday, 14th of June 2024. Faculty opponent was Anthony Hall, Head of Plant Genomics, Earlham Institute, Norwich Research Park, Norwich, UK. The thesis was supervised by Maria E. Eriksson.

Title of the thesis: Timing is everything: Exploring the role of the circadian clock in plant growth and adaptation

Link to Johan Sjölander’s PhD thesis

For more information, please contact:

Johan Sjölander
Umeå Plant Science Centre
Department of Plant Physiology
Umeå University
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