How a ferroxidase equips plant roots for phosphate-deficient soil.
Phosphate is an essential nutrient for the plant. However, in the soil, metals such as iron interact with the scarce nutrient to form poorly soluble complexes, further limiting phosphate availability to the root. IPB scientists are investigating how plant cells respond to the antagonistic nutrients phosphate and iron and how they adapt their growth accordingly.
Until now, it was known that in phosphate-deficient soils the growth of the primary root is inhibited in favor of lateral root formation. This allows for a larger root surface area and better uptake of the coveted nutrient from the upper soil layers. A key molecular player in the response to local phosphate levels around the roots is the gene LPR1. Several years ago, IPB scientists found that lpr1 mutants no longer respond with inhibited downward root growth to low-phosphate conditions. However, exactly how LPR1 mediates the phosphate deficiency response remained unclear. The researchers assumed that LPR1 interferes with the redox balance of iron ions, because first, the LPR1 gene sequence strongly resembled those of multicopper oxidases, and second, wild-type plants in experiments showed a phosphate deficiency response with inhibited primary root growth only in the presence of iron.
A team led by IPB scientists has now shown for the first time that LPR1 actually exhibits ferroxidase enzyme activity. It binds Fe2+ as a substrate with high specificity and oxidizes it to Fe3+. In this way, it contributes to the redox cycling of iron in the intercellular spaces of the root tip, resulting in generating reactive oxygen species as possible by-products. These in turn lead to clogging callose deposits in the root apical meristem and eventually to root growth arrest. Upon closer analysis of the protein structure of LPR1, the scientists found striking similarities to a bacterial enzyme. Detailed phylogenetic analyses of multicopper oxidases and LPR1-like enzymes from plants, animals, bacteria and archaea led the researchers to hypothesize that plant LPR1 may have been transferred by horizontal gene transfer from a soil bacterium to a land plant progenitor. This is supported by the presence of LPR1-related genes in some algae, which are considered sister clades of land plants. The researchers speculate that a precursor of LPR1 ferroxidase arose during early bacterial land colonization and was then transferred several times to different groups of organisms, where it diversified further each time.
With their study, the Halle scientists could show that plants can respond to phosphate deficiency with the help of the ferroxidase LPR1, which converts ferrous ions available in the soil. They present indications that during evolution, plants acquired this ability from bacteria and were thus equipped for terrestrialization.
Original publication: Naumann, C., Heisters, M., Brandt, W., et al. (2022) Bacterial-type ferroxidase tunes iron-dependent phosphate sensing during Arabidopsis root development. Current Biology. Available at: http://dx.doi.org/10.1016/j.cub.2022.04.005.
