A matter of quantity: The effect of chloroplast stromal phosphate levels on photosynthetic efficiency

Phosphorus (P) is an essential mineral nutrient for plants, playing central roles in development, growth and environmental responses.In the soil, P is available in the form of inorganic phosphate (Pi) which, once incorporated by plants, can be converted into nucleic acids, membrane lipids and sugars.Pi is also necessary for ATP synthesis during photosynthesis, and its concentration in the chloroplast is linked to photosynthetic regulation.Environmental fluctuations and metabolic changes can alter chloroplastic Pi homeostasis, triggering acclimation responses to maintain photosynthetic performance (Bouain et al., 2022).Stromal Pi levels need to be high enough to sustain photophosphorylation without inhibiting Calvin-Benson cycle reactions (Grotjohann and Gräber, 2002; Marcus et al., 2005).Therefore, plants strictly regulate Pi levels to avoid inhibition of photosynthesis.The concentration of Pi in the chloroplast stroma is determined by photophosphorylation rates, Pi recycling from the Calvin-Benson cycle and starch synthesis, and Pi import from the cytosol.The latter involves a series of molecular transporter families: plastidic phosphate translocator (pPT), PHOSPHATE TRANSPORTER 2 (PHT2), and PHT4 (Versaw and Garcia, 2017).In particular, the triose phosphate/Pi translocator (TPT, a member of the pPT family) couples Pi import to the stroma with triose phosphate export (produced by photosynthesis) to the cytosol (Fliege et al., 1978; Flügge et al., 1989) (Figure 1).Since TPT is fundamental for carbon export to the cytosol, it is expected to be one of the main pathways for Pi translocation to the chloroplast stroma.However, tpt mutants are not growth-impaired, suggesting that additional Pi transport mechanisms and alterations in starch metabolism might be involved in Pi chloroplast homeostasis and triose phosphate export (Walters et al., 2004).In this regard, PHT2 and PHT4 transporters could complement the deficiency of TPTmediated Pi transport in the tpt mutants.However, the contribution of these transporters to stromal Pi levels and their regulation is poorly understood.In this issue of Plant Physiology, Subramani Raju et al. 2024 used a FRET (Förster resonance energy transfer)-based Pi sensor to evaluate the role of TPT, PHT2;1 and PTH4;4 as chloroplastic Pi transporters in Arabidopsis thaliana (Figure 1).The authors estimated stromal Pi levels in Arabidopsis tpt, pht2;1 and pht4;2 mutants and the wild type (WT) line grown under Pi-replete conditions.The tpt and pht2;1 mutant showed reduced stromal Pi levels compared to WT plants, while even lower Pi levels were observed in the tpt pht2;1 mutant line.Since stromal Pi levels did not differ between the pht4;2 mutant and the WT, the authors focused on the tpt and pht2;1 line for further studies.