The concentration of inorganic phosphate (Pi) in the chloroplast stroma must be maintained within narrow limits to sustain photosynthesis and to direct the partitioning of fixed carbon. However, it is unknown if these limits or the underlying contributions of different chloroplastic Pi transporters vary throughout the photoperiod or between chloroplasts in different leaf tissues. To address these questions, we applied live Pi imaging to Arabidopsis (Arabidopsis thaliana) wild-type plants and two loss-of-function transporter mutants: triose phosphate/phosphate translocator (tpt), phosphate transporter 2; 1 (pht2; 1), and tpt pht2; 1. Our analyses revealed that stromal Pi varies spatially and temporally, and that TPT and PHT2; 1 contribute to Pi import with overlapping tissue specificities. Further, the series of progressively diminished steady-state stromal Pi levels in these mutants provided the means to examine the effects of Pi on photosynthetic efficiency without imposing nutritional deprivation. ΦPSII and nonphotochemical quenching (NPQ) correlated with stromal Pi levels. However, the proton efflux activity of the ATP synthase (gH+) and the thylakoid proton motive force (pmf) were unaltered under growth conditions, but were suppressed transiently after a dark to light transition with return to wild-type levels within 2 minutes. These results argue against a simple substrate-level limitation of ATP synthase by depletion of stromal Pi, favoring more integrated regulatory models, which include rapid acclimation of thylakoid ATP synthase activity to reduced Pi levels.

中文翻译：

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基因操纵叶绿体基质磷酸盐水平改变光合作用效率

叶绿体基质中无机磷酸盐 (Pi) 的浓度必须保持在狭窄的范围内，以维持光合作用并指导固定碳的分配。然而，尚不清楚这些限制或不同叶绿体 Pi 转运蛋白的潜在贡献是否在整个光周期或不同叶组织中叶绿体之间发生变化。为了解决这些问题，我们将实时 Pi 成像应用于拟南芥 (Arabidopsis thaliana) 野生型植物和两种功能丧失的转运蛋白突变体：磷酸丙糖/磷酸转运蛋白 (tpt)、磷酸转运蛋白 2； 1 (pht2; 1) 和 tpt pht2; 1.我们的分析表明基质Pi在空间和时间上发生变化，并且TPT和PHT2； 1 有助于具有重叠组织特异性的 Pi 输入。此外，这些突变体中一系列逐渐降低的稳态基质 Pi 水平提供了在不造成营养剥夺的情况下检查 Pi 对光合效率的影响的方法。 ΦPSII 和非光化学猝灭 (NPQ) 与基质 Pi 水平相关。然而，ATP 合酶 (gH+) 的质子流出活性和类囊体质子动力 (pmf) 在生长条件下没有改变，但在从暗到亮的转变后受到短暂抑制，并在 2 分钟内恢复到野生型水平。这些结果反对通过基质 Pi 消耗而对 ATP 合酶进行简单的底物水平限制，有利于更综合的调节模型，其中包括快速适应类囊体 ATP 合酶活性以降低 Pi 水平。