In vitro pollen growth assays in WT and homozygous nadp-me4 T-DNA insertion mutants. Credit: Nature Communications, University of Hong Kong

Application of energy biosensors in measuring pollen tube growth. Credit: Jinhong Liu, University of Hong Kong

The bioenergetics of rapid growth, metabolism in pollen tubes

10 January 2023

Biologists discover how the pollen tubes of the flower plant Arabidopsis gain energy to sustain its rapid growth. With such efficient and tremendous energy for metabolism and continuous synthesis, the source of this growth had previously been a mystery.

Pollen tubes are the fastest-growing plant cells known. Studies have shown that the growth rate of maize pollen tubes can reach up to 2.8 microns per second, while the growth rate of lily pollen tubes can also reach 0.2 to 0.3 microns per second. Its polarised growth process consumes a tremendous amount of energy, required for metabolism and continuous synthesis of the plasma membrane and cell wall. Researchers from the University of Hong Kong set out to find out where all this energy comes from.

Application of energy biosensors in measuring pollen tube growth. Credit: Liu Jinhong, University of Hong Kong

Adenosine triphosphate is the principal molecule for storing and transferring energy in cells. It is often referred to as the energy currency of the cell. A research team led by Dr Lim Boon Leong at the School of Biological Sciences developed practical biosensors able to measure real-time dynamic changes in the energy currency in living plant cells and organelles.

Unlike leaf cells, pollen tubes do not perform photosynthesis and rely on sugar supply from the style to generate energy molecules such as adenosine triphosphate, nicotinamide-adenine dinucleotide phosphate, and nicotinamide adenine dinucleotide to support pollen tube growth.

Fatty acids, the building blocks of the plasma membrane of the pollen tube, are synthesised in the pollen plastid, a precursor organelle of the chloroplast that does not contain chlorophyll. The synthesis of fatty acids consumes a large amount of energy molecules as well as acetyl coenzyme A, an important metabolic intermediate. There are multiple possible routes of supplying the molecules for this synthesis, but the exact mechanisms are obscure owing to the small size of pollen tubes and the lack of tools to measure the concentrations.

The team developed a method of introducing biosensors to the plastids and the cytosol of the Arabidopsis pollen tube, to reveal the bioenergetics of its growth – i.e., the pathways for Arabidopsis pollen tube growth and sources for fatty acid synthesis. This research not only developed more practical biosensors that measure real-time dynamic changes of energy molecules in living plant cells and organelles, but also reveals the exact biochemical routes of supplying them for fatty acid synthesis in pollen plastids.

Model of the bioenergetics of Arabidopsis pollen tube growth. Credit: Dr Lim Boon Leong, University of Hong Kong

“The in planta fluorescence protein sensors we developed are powerful tools for solving some key questions in plant bioenergetics,” said Liu Jinhong, the first author of the article and a PhD student of Lim’s group.

The research team published their findings in the journal Nature Communications.