Plant Physiol, November 2002, Vol. 130, pp. 1414-1425

Stoichiometry of the Photosynthetic Apparatus and Phycobilisome Structure of the Cyanobacterium Plectonema boryanum UTEX 485 Are Regulated by Both Light and Temperature¹

Department of Biology, University of Western Ontario, London, Ontario, Canada N6A 5B7

The role of growth temperature and growth irradiance on the regulation of the stoichiometry and function of the photosynthetic apparatus was examined in the cyanobacterium Plectonema boryanum UTEX 485 by comparing mid-log phase cultures grown at either 29°C/150 µmol m² s¹, 29°C/750 µmol m² s¹, 15°C/150 µmol m² s¹, or 15°C/10 µmol m² s¹. Cultures grown at 29°C/750 µmol m² s¹ were structurally and functionally similar to those grown at 15°C/150 µmol m² s¹, whereas cultures grown at 29°C/150 µmol m² s¹ were structurally and functionally similar to those grown at 15°C/10 µmol m² s¹. The stoichiometry of specific components of the photosynthetic apparatus, such as the ratio of photosystem (PS) I to PSII, phycobilisome size and the relative abundance of the cytochrome b₆/f complex, the plastoquinone pool size, and the NAD(P)H dehydrogenase complex were regulated by both growth temperature and growth irradiance in a similar manner. This indicates that temperature and irradiance may share a common sensing/signaling pathway to regulate the stoichiometry and function of the photosynthetic apparatus in P. boryanum. In contrast, the accumulation of neither the D1 polypeptide of PSII, the large subunit of Rubisco, nor the CF₁ -subunit appeared to be regulated by the same mechanism. Measurements of P700 photooxidation in vivo in the presence and absence of inhibitors of photosynthetic electron transport coupled with immunoblots of the NAD(P)H dehydrogenase complex in cells grown at either 29°C/750 µmol m² s¹ or 15°C/150 µmol m² s¹ are consistent with an increased flow of respiratory electrons into the photosynthetic intersystem electron transport chain maintaining P700 in a reduced state relative to cells grown at either 29°C/150 µmol m² s¹ or 15°C/10 µmol m² s¹. These results are discussed in terms of acclimation to excitation pressure imposed by either low growth temperature or high growth irradiance.