Yi Keke, a professor at the Institute of Agricultural Resources and Regional Planning in the Chinese Academy of Agricultural Sciences, published a research paper entitled "Loss of two families of SPX domain-containing proteins required for vacuolar polyphosphate accumulation coincides with the transition to phosphate storage in green plants" in association with Zhu Yiyong, a professor at Nanjing Agricultural University, in the journal Molecular Plant recently.

During their research, Yi and Zhu unveiled the evolution and transition mechanisms behind the storage and use of vacuolar polyphosphate in the terrestrialization of green plants, providing a theoretical foundation to further enhance the efficiency of plants in absorbing phosphorus.

Phosphorus is an essential nutrient for all living organisms on the earth. A phosphate transporter plays an important role in helping land plants absorb phosphates and transport them for future utility. Vacuole is a major organelle for green plants to store phosphorus. SPX-MFS proteins will mediate phosphates in cytoplasm to deposit them in vacuole when there is sufficient phosphorus in the outside world. VPE proteins are expected to discharge phosphates from vacuoles and deliver them to cytoplasm for recycling when there is a shortage of phosphorus from the outside. Different from land plants, polyphosphoric acids are the major form of phosphorus in the vacuole of lower aquatic chlorophyte plants. It is still unclear about the mechanisms behind the storage and transportation of phosphorus in the vacuole of chlorophyte plants and how and when polyphosphoric acids grow into phosphates during the terrestrialization of aquatic plants.

Researchers found that two SPX domain-containing proteins, SPX-VTC and SPX-SLC, are required for the accumulation of vacuolar polysphosphate. In chlamydomonas reinhardtii. CrPTC1 (SPX-SLC) can mediate the discharge of vacuolar phosphorus and the loss of proteins' vitality accompanies the disappearance of polyphosphoric acids stored in the vacuole of chlamydomonas. It is thus concluded that both SPX-VTC and SPX-SLC proteins in chlamydomonas are responsible for maintaining a balance of polyphosphoric acids in vacuoles. Further evolution analysis shows that SPX-SLC, SPX-VTC, and SPX-MFS proteins were present in the common ancestor of green plants or Viridiplantae.

An illustration of phosphate transporter in vacuole and the transformation of vacuolar phosphorus's storage forms during the terrestrialization of green plants.

According to a systematic evolution analysis of the storage form of vacuolar phosphorus and SPX proteins, SPX-VTC and SPX-SLC proteins are conserved among species that store phosphorus as polyphosphoric acids and are absent from genomes of plants that store phosphorus as vacuolar phosphates during the evolution of streptophytes. The transformation of the storage form of vacuolar phosphorus from polyphosphoric acids to phosphates has been preserved from the terrestrialization of green plants. Although polyphosphoric acids perform better than phosphates in the storage efficiency of vacuolar phosphorus, they are likely to consume lots of adenosine triphosphates (ATPs) during their formation and require water to hydrolyse them into phosphates. Therefore, the storage of polyphosphoric acids is not advisable for green plants to accommodate the changes in the supply of phosphorus from the outside. The transformation of the storage form of vacuolar phosphorus from polyphosphoric acids to phosphates is an important development during the terrestrialization of green plants.

To sum up, SPX-VTC and SPX-SLC proteins are responsible for maintaining a balance of polyphosphoric acids in the vacuoles of chlamydomonas. The loss of SPX-VTC and SPX-SLC genes in streptophytes is an important preparatory and adaptable mechanism during the terrestrialization of aquatic plants.

The research has been funded by the National Key R&D Program of China, the National Plant Nutrition and Fertilizers Technologies System, and the Technological Innovation Project associated with the Chinese Academy of Agricultural Sciences.

Web link: https://doi.org/10.1016/j.molp.2021.01.015