Salomon Eitan, Bar-Eyal Leeat, Sharon Shir, Keren Nir
Department of Plant and Environmental Sciences, The Hebrew University of Jerusalem, Jerusalem, Israel.
Biochim Biophys Acta. 2013 Mar;1827(3):340-7. doi: 10.1016/j.bbabio.2012.11.010. Epub 2012 Nov 29.
Nitrogen limitation forces photosynthetic organisms to reallocate available nitrogen to essential functions. At the same time, it increases the probability of photo-damage by limiting the rate of energy-demanding metabolic processes, downstream of the photosynthetic apparatus. Non-diazotrophic cyanobacteria cope with this situation by decreasing the size of their phycobilisome antenna and by modifying their photosynthetic apparatus. These changes can serve two purposes: to provide extra amino-acids and to decrease excitation pressure. We examined the effects of nitrogen limitation on the form and function of the photosynthetic apparatus. Our aim was to study which of the two demands serve as the driving force for the remodeling of the photosynthetic apparatus, under different growth conditions. We found that a drastic reduction in light intensity allowed cells to maintain a more functional photosynthetic apparatus: the phycobilisome antenna was bigger, the activity of both photosystems was higher and the levels of photosystem (PS) proteins were higher. Pre-acclimating cells to Mn limitation, under which the activity of both PSI and PSII is diminished, results in a very similar response. The rate of PSII photoinhibition, in nitrogen limited cells, was found to be directly related to the activity of the photosynthetic apparatus. These data indicate that, under our experimental conditions, photo-damage avoidance was the more prominent determinant during the acclimation process. The combinations of limiting factors tested here is by no means artificial. Similar scenarios can take place under environmental conditions and should be taken into account when estimating nutrient limitations in nature.
氮素限制迫使光合生物将可用氮重新分配到基本功能中。与此同时，它通过限制光合机构下游能量需求代谢过程的速率，增加了光损伤的可能性。非固氮蓝细菌通过减小其藻胆体天线的大小和改变其光合机构来应对这种情况。这些变化可以达到两个目的：提供额外的氨基酸并降低激发压力。我们研究了氮素限制对光合机构形态和功能的影响。我们的目的是研究在不同生长条件下，这两个需求中的哪一个作为光合机构重塑的驱动力。我们发现，光照强度的大幅降低使细胞能够维持更具功能的光合机构：藻胆体天线更大，两个光系统的活性更高，光系统（PS）蛋白的水平也更高。使细胞预先适应锰限制，在此条件下PSI和PSII的活性均降低，会导致非常相似的反应。在氮限制的细胞中，发现PSII光抑制的速率与光合机构的活性直接相关。这些数据表明，在我们的实验条件下，避免光损伤是适应过程中更突出的决定因素。这里测试的限制因素组合绝非人为设定。类似的情况可能在环境条件下发生，在估计自然界中的养分限制时应予以考虑。
