Plastid terminal oxidase requires translocation to the grana stacks to act as a sink for electron transport

The plastid terminal oxidase (PTOX) has been shown to be an important sink forphotosynthetic electron transport in stress tolerant plants. However, overexpression studies in stress sensitive species have previously failed to induce significant activity of this protein. Here we show that overexpression of PTOX from the salt tolerant brassica species Eutrema salsugineum does not, alone, result in activity, but that over-expressing plants show faster induction and a greater final level of PTOX activity once exposed to salt stress. This impliesthat an additional activation step is required before activity is induced. We show that that activation involves the translocation of the protein from the unstacked stromal lamellae to the thylakoid grana and a protection of the protein from trypsin digestion. This represents an important and novel activation step and opens up new possibilities in the search for stress tolerant crops.