HKU biologist throws new light on plant growth

2 November 2018

Plant biologist Dr Boon Leong Lim (Croucher Scholarship 1991) and his research group at the University of Hong Kong have used genetic engineering to introduce a fluorescent Adenosine triphosphate (ATP) sensor into the cystosol (liquid in cells) and chloroplast (where photosynthesis occurs) compartments of a model plant (Arabidopsis thaliana), enabling the team to visualise and monitor real-time changes in ATP concentration.

ATP plays a crucial role in photosynthesis, providing the energy needed for carbon fixation. The research sheds new light on plant growth and how this could be accelerated, with potential implications for increasing the energy efficiency and yield of plants. The findings have been published in the Proceedings of the National Academy of Sciences of the United States of America (PNAS).

Observations made by Lim’s HKU team, in collaboration with an international research team from Germany, Sweden and the US, indicated that chloroplasts – key energy harvesters and producers in plant cells – demand extremely high amounts of ATP. However, mature plant chloroplasts manage ATP largely in isolation from other cellular spaces, pinpointing how plants can use energy efficiently. This contrasted with a previous study that showed chloroplasts in unicellular diatoms (single-cell photosynthetic organisms) could import cytosolic ATP to support carbon fixation.

Live images of a plastid-localised ATP sensor in an Arabidopsis seedling. Red and green panels show the images of the ATP sensor at 470 nm – 507 nm and 526 nm – 545 nm in a three-day-old seedling. The ratio between both images, which corresponds to ATP concentration (higher level in red and lower level in green), is shown in the lower left panel. The lower right panel is the brightfield image of the seedling.

While the HKU team observed that chloroplasts of young developing leaves could import ATP from the cytosol, given ATP transporters on the chloroplast membrane are expressed in such leaves, ATP imports into mature chloroplasts were negligible. This appeared to originate from a plant mechanism to avoid wasteful energy consumption. While illumination could increase chloroplast ATP concentration instantly, it dropped to a basal level in only 30 seconds once the light ceased. Such a pattern of energy consumption suggests that mature chloroplasts have managed to formulate a strategy to maintain energy efficiency in the dark.

Lim explained the advantage for plants in restricting ATP imports into mature chloroplasts: “During their early developmental stage, young chloroplasts require exogenous ATP for biogenesis. When chloroplasts are fully developed and become self-sustaining, ATP transporters need to be down-regulated so that the energy harvested in daytime will not be wastefully consumed by chloroplasts at night.

“Unlike unicellular diatoms, which only need to manage their own energy usage, multicellular photosynthetic organisms need to manage ATP consumption in different cell types. The primary job of mature leaf chloroplasts is to harvest energy and export sugar in the light to support the growth of other cells while minimising wasteful energy consumption in the dark.” Lim added: “We believe chloroplasts could obtain ATP by catabolising starch accumulated during the daytime, providing a minimal level of ATP for metabolism at night.”

Looking ahead, Lim said: “We found that by modulating energy exchanges between chloroplasts and mitochondria, the sugar production capabilities of chloroplasts and ATP generation by mitochondria could be enhanced, promoting plant growth.” He plans to focus on this finding in future research, which he hopes can assist in solutions to global warming.

Co-authors of the PNAS article included Prof Markus Schwarzländer, Münster University; Dr Wayne K Versaw, Texas A&M University; and Prof Per Gardeström, Umeå University.

Dr Boon Leong Lim Wallace is Associate Professor of Biological Sciences at the University of Hong Kong. He is interested in plant energy and organelle biology. He was awarded a Croucher Scholarship in 1991 to study at the University of Oxford.

To view Dr Lim’s Croucher profile please click here