Evidence That Putrescine Modulates the Higher Plant Photosynthetic Proton Circuit

The light reactions of photosynthesis store energy in the form of an electrochemical gradient of protons, or proton motive force (pmf), comprised of electrical (Delta psi) and osmotic (Delta pH) components. Both components can drive the synthesis of ATP at the chloroplast ATP synthase, but the Delta pH component also plays a key role in regulating photosynthesis, down-regulating the efficiency of light capture by photosynthetic antennae via the q(E) mechanism, and governing electron transfer at the cytochrome b(6)f complex. Differential partitioning of pmf into Delta pH and Delta psi has been observed under environmental stresses and proposed as a mechanism for fine-tuning photosynthetic regulation, but the mechanism of this tuning is unknown. We show here that putrescine can alter the partitioning of pmf both in vivo (in Arabidopsis mutant lines and in Nicotiana wild type) and in vitro, suggesting that the endogenous titer of weak bases such as putrescine represents an unrecognized mechanism for regulating photosynthetic responses to the environment.