As a result of a collaborative effort between researchers from the HUN-REN Biological Research Centre, Szeged (HUN-REN BRC Szeged) and partners from the University of Edinburgh, the Max Planck Institute of Molecular Plant Physiology, and the Princeton University, a research paper has been published on the first phosphate transporter characterized in green algae in the prestigious journal Plant Physiology.
Phosphorus is an essential element for life, but it is also a limited resource that needs to be managed wisely. Microalgae, which are microscopic plants that live in water, can help us recover phosphorus from wastewater and use it as a fertilizer for crops. However, to do this, we need to understand how microalgae take up and distribute phosphorus inside their cells.
Phosphorus is found in every compartment of the plant cell. It is a structural component of nucleic acids and phospholipids, and is also indispensable for signal transduction and energy transfer reactions, including photosynthesis. Plants take up phosphorus from the soil in the form of inorganic phosphate. Despite its widespread occurrence in the environment, inorganic phosphate availability often limits plant growth, because of phosphate binding to metals and other substances in the soil. Fertilizers, derived from non-renewable rock phosphate, improve crop yields that otherwise are limited by inorganic phosphate availability, but the leaching of excess inorganic phosphate into aquatic ecosystems causes environmental problems such as eutrophication. For these reasons, studying inorganic phosphate uptake and transport inside plant cells is of high importance.
The scientists discovered a new protein that transports phosphate into the chloroplasts, which are the compartments of the cell where photosynthesis takes place. They found that this protein, called CrPHT4-7, is very important for the photosynthesis, growth and survival of microalgae, especially under challenging conditions such as high light.
Characterization of the PHT4-7 transporter of the green alga Chlamydomonas reinhardtii. Venus labeling proves that the transporter is located in the chloroplast membrane (upper left figure). The growth of pht4-7 mutants is extremely slow, regardless of the phosphate content of the culture medium (lower left figure). The pht4-7 mutants are highly sensitive to strong light (upper right figure). The PHT4-7 transporter is therefore located in the envelope membrane of the chloroplast and its task is to transport inorganic phosphate into the chloroplast (lower right figure).
To study the function of CrPHT4-7, the CRISPR/Cas12a gene editing technique was used to generate mutants that lack this protein in Chlamydomonas reinhardtii, a model green alga. They also expressed CrPHT4-7 in a yeast strain that is unable to transport phosphate, and measured the phosphate and ascorbate (also known as vitamin C) transport activities of CrPHT4-7 in yeast cells. They found that CrPHT4-7 transports phosphate, but not ascorbate, into the chloroplasts. This is surprising, because CrPHT4-7 is very similar to a protein from a land plant that does transport ascorbate.
This is the first detailed characterization of a phosphate transporter in green algae. The research conducted by Szilvia Zita Tóth’s group reveals new insights into the evolution and function of phosphate transporters in green algae, and it may help us to enhance the biotechnological applications of microalgae in the future. For example, by manipulating the expression of CrPHT4-7, it may become possible to increase the biomass and phosphate content of microalgae, and use them as a source of fertilizer.