Cool temperatures interfere with D1 synthesis in tomato by causing ribosomal pausing

Photodamage occurs when leaves are exposed to light in excess of what can be used for photosynthesis and in excess of the capacity of ancillary photoprotective as well as repair mechanisms. An important site of photodamage is the chloroplast encoded D1 protein, a component of the photosystem II (PSII) reaction center. Even under optimal growth irradiance, D1 is photodamaged necessitating rapid turnover to prevent the accumulation of photodamaged PSII reaction centers and consequent inhibition of photosynthesis. However, this on-going process of D1 turnover and replacement was impeded in the chilling-sensitive tomato (Solanum lycopersicum) plants when exposed to high-growth light at cool temperature. The decrease in D1 turnover and replacement was found not to be due to changes in the steady-state level of the psbA message. While the recruitment of ribosomes to psbA transcript, initiation of D1 translation, and the association of polysomes with the thylakoid membrane occurred normally, chilling temperatures caused ribosomal pausing during D1 peptide elongation in tomato. The pause locations were non-randomly located on the D1 transcript. The interference with translation caused by ribosomal pausing allowed photodamaged PSII centers to accumulate leading to the consequent inhibition of photosynthesis.