Protection of photosystem II against UV-A and UV-B radiation in the cyanobacterium Plectonema boryanum: The role of growth temperature and growth irradiance

Photochemistry and Photobiology, Dec 2000 by Ivanov, Alexander G, Miskiewicz, Ewa, Clarke, Adrian K, Greenberg, Bruce M, Huner, Norman P A

In addition to the effects on Fv/Fm, UV treatments were also associated with changes in the initial Fo fluorescence (Fig. 5). Exposure of cultures grown at either 29/150 or 15/ 6 to PAR and UV-A UV-B irradiance resulted in a timedependent increase in Fo, reaching 124 and 130% of the initial Fo values, respectively, after 2 h (Fig. 5A). In addition, exposure to UV radiation induced a 30-40% increase in the ratio of F658/F684 (Table 2). Cells acclimated to low temperature and moderate light (15/150) did not show any measurable changes in F^sub omicron^ during the same time interval. Similar trends were observed for cultures grown at 29/150, 15/150 and 15/6 and subsequently exposed to PAR and UV-A (Fig. 5B).

Effects of UV irradiation on pigment accumulation

Exposure of control 29/150 cultures to either PAR and UVA or PAR and UV-A UV-B resulted in a gradual, timedependent increase in the putative myxoxanthophyll level up to 138 and 160% after 2 h of treatment relative to levels prior to UV exposure (Fig. 6A,C). In contrast, the putative myxoxanthophyll content did not change significantly in cultures grown at either 15/150 or 15/6 (Fig. 6A,C). Exposure to PAR and UV-A resulted in minimal changes in the amount of P-carotene in all variants (Fig. 6B). However, exposure to PAR and UV-A UV-B caused -carotene content to decrease by 80-90% of initial values in both 29/150 and 15/6 cells after 30 min of treatment (Fig. 6D). In contrast, cells grown at 15/150 exhibited a minimal decline of the beta-carotene level even after 2 h of exposure to PAR and UV-A LV-B. The time of exposure of all cultures to either PAR or UV-A or PAR and UV-A UV-B did not affect the levels of Scy and OS-MAA (data not shown).

Immunodetection of Up proteins

It was previously shown (24) that Clp protease(s) play an important role in the acclimation of the cyanobacterium Synechococcus sp. PCC 7942 to UV radiation. To investigate further the higher capacity to survive UV exposure by P. boryanum cells grown at 15/150 compared to those grown at either 15/6 or 29/150, we determined the levels of the ATP-dependent Clp proteases in these cells. The highest levels of all Clp proteins tested were found in P. boryanum cells grown at 15/150. The abundance of ClpC and ClpP3 proteins in these cells increased by approximately 50%, while the content of ClpX increased as much as two-fold compared to cells grown at 29/150 (Fig. 7). However, when the growth irradiance was decreased to 6 wmol m-2 s^sup -1^ at 15 deg C (15/6) P. boryanum exhibited similar or lower levels of Clp proteins than cells grown at control conditions (Fig. 7).

DISCUSSION

Acclimation of the cyanobacterium, P. boryanum, to growth at low temperature and moderate irradiance (15/150) appears to induce a significant resistance to inhibition of PSII photochemistry compared to control cells (29/150) when exposed to either UV-A or UV-A UV-B (Fig. 4). However, this phenomenon cannot be a simple growth-temperature response since cells grown at 15/6 exhibited similar susceptibilities to UV radiation as control cells (29/150) (Fig. 4). Furthermore, this phenomenon also cannot be explained as a simple growth-irradiance response because cells grown at 29/150 were more susceptible to inhibition of PSII photochemistry by UV radiation than cells grown at 15/150. Thus, the increased resistance of PSII photochemistry to UV radiation appears to be a consequence of the combined effects of low growth temperature and moderate growth irradiance. These results are consistent with those reported for the resistance to photoinhibition induced in winter cereals and green algae as a consequence of growth under high excitation pressure (25). Thus, it appears that increased resistance to UV radiation may also be a consequence of growth under high excitation pressure which is sensed through changes in photosynthetic redox poise (23,25). However, since qP was higher in cells grown at 15/150 than control cells (29/150), the redox state of PSII measured under steady-state growth conditions does not appear to be a reliable estimate of excitation pressure in the prokaryote, P. boryanum, in contrast to that reported for plants and green algae (25). This may reflect the fact that the redox sensor for photoacclimation in P. boryanum appears to be downstream of the Cyt b^sub 6^/f complex (23) whereas the plastoquinone pool appears to act as the redox sensor in green algae (26,27) and other cyanobacteria (28).