Laboratory of Biophysics, Wageningen University, Wageningen, The Netherlands.
Institute for Integrative Biology of the Cell (12BC), CEA, CNRS, Université Paris-Sud, Université Paris-Saclay, 91198 Gif-sur-Yvette, France.
Plant Physiol. 2021 May 27;186(1):569-580. doi: 10.1093/plphys/kiab063.
State transitions are a low-light acclimation response through which the excitation of Photosystem I (PSI) and Photosystem II (PSII) is balanced; however, our understanding of this process in cyanobacteria remains poor. Here, picosecond fluorescence kinetics was recorded for the cyanobacterium Synechococcus elongatus using fluorescence lifetime imaging microscopy (FLIM), both upon chlorophyll a and phycobilisome (PBS) excitation. Fluorescence kinetics of single cells obtained using FLIM were compared with those of ensembles of cells obtained with time-resolved fluorescence spectroscopy. The global distribution of PSI and PSII and PBSs was mapped making use of their fluorescence kinetics. Both radial and lateral heterogeneity were found in the distribution of the photosystems. State transitions were studied at the level of single cells. FLIM results show that PSII quenching occurs in all cells, irrespective of their state (I or II). In S. elongatus cells, this quenching is enhanced in State II. Furthermore, the decrease of PSII fluorescence in State II was homogeneous throughout the cells, despite the inhomogeneous PSI/PSII ratio. Finally, some disconnected PBSs were resolved in most State II cells. Taken together our data show that PSI is enriched in the inner thylakoid, while state transitions occur homogeneously throughout the cell.
态转变是一种低光照适应反应，通过该反应平衡光系统 I (PSI) 和光系统 II (PSII) 的激发；然而，我们对蓝藻中的这一过程的理解仍然很差。在这里，使用荧光寿命成像显微镜 (FLIM) 对蓝藻聚球藻 (Synechococcus elongatus) 进行了叶绿素 a 和藻胆体 (PBS) 激发后的皮秒荧光动力学记录。使用 FLIM 获得的单个细胞的荧光动力学与使用时间分辨荧光光谱法获得的细胞群体的荧光动力学进行了比较。利用它们的荧光动力学绘制了 PSI 和 PSII 以及 PBS 的全局分布。在分布中发现了径向和侧向异质性。在单细胞水平上研究了状态转变。FLIM 结果表明，所有细胞（无论其状态为 I 还是 II）都发生 PSII 猝灭。在 S. elongatus 细胞中，这种猝灭在状态 II 中增强。此外，尽管 PSI/PSII 比值不均匀，但在状态 II 中 PSII 荧光的降低在整个细胞中是均匀的。最后，在大多数状态 II 细胞中解析出一些不连续的 PBS。总之，我们的数据表明 PSI 富集在内层类囊体中，而状态转变在整个细胞中均匀发生。
