State transition is quiet around pyrenoid and LHCII phosphorylation is not essential for thylakoid deformation in Chlamydomonas 137c

Access to the Antenna System of Photosystem I via Single-Molecule Excitation-Emission Spectroscopy

In the development of single-molecule spectroscopy, the simultaneous detection of the excitation and emission spectra has been limited. The fluorescence excitation spectrum based on background-free signals is compatible with the fluorescence-emission-based detection of single molecules and can provide insight into the variations in the input energy of the different terminal emitters. Here, we implement single-molecule excitation-emission spectroscopy (SMEES) for photosystem I (PSI) via a cryogenic optical microscope. To this end, we extended our line-focus-based excitation-spectral microscope system to the cryogenic temperature-compatible version. PSI is one of the two photosystems embedded in the thylakoid membrane in oxygen-free photosynthetic organisms. PSI plays an essential role in electron transfer in the photosynthesis reaction. PSIs of many organisms contain a few red-shifted chlorophylls (Chls) with much lower excitation energies than ordinary antenna Chls. The fluorescence emission spectrum originates primarily from the red-shifted Chls, whereas the excitation spectrum is sensitive to the antenna Chls that are upstream of red-shifted Chls. Using SMEES, we obtained the inclining two-dimensional excitation-emission matrix (2D-EEM) of PSI particles isolated from a cyanobacterium, Thermosynechococcus vestitus (equivalent to elongatus), at about 80 K. Interestingly, by decomposing the inclining 2D-EEMs within time course observation, we found prominent variations in the excitation spectra of the red-shifted Chl pools with different emission wavelengths, strongly indicating the variable excitation energy transfer (EET) pathway from the antenna to the terminal emitting pools. SMEES helps us to directly gain information about the antenna system, which is fundamental to depicting the EET within pigment-protein complexes.

Genotype-dependent Strategies to "Overcome" Excessive Light: Insights into Non-Photochemical Quenching under High Light Intensity

High light (HL) intensities have a significant impact on energy flux and distribution within photosynthetic apparatus. To understand the effect of high light intensity (HL) on the HL tolerance mechanisms in tomatoes, we examined the response of the photosynthesis apparatus of 12 tomato genotypes to HL. A reduced electron transfer per reaction center (ET0 /RC), an increased energy dissipation (DI0 /RC) and non-photochemical quenching (NPQ), along with a reduced maximum quantum yield of photosystem II (FV /FM ), and performance index per absorbed photon (PIABS ) were common HL-induced responses among genotypes; however, the magnitude of those responses was highly genotype-dependent. Tolerant and sensitive genotypes were distinguished based on chlorophyll fluorescence and energy-quenching responses to HL. Tolerant genotypes alleviated excess light through energy-dependent quenching (qE ), resulting in smaller photoinhibitory quenching (qI ) compared to sensitive genotypes. Quantum yield components also shifted under HL, favoring the quantum yield of NPQ (ՓNPQ ) and the quantum yield of basal energy loss (ՓN0 ), while reducing the efficient quantum yield of PSII (ՓPSII ). The impact of HL on tolerant genotypes was less pronounced. While the energy partitioning ratio did not differ significantly between sensitive and tolerant genotypes, the ratio of NPQ components, especially qI , affected plant resilience against HL. These findings provide insights into different patterns of HL-induced NPQ components in tolerant and sensitive genotypes, aiding the development of resilient crops for heterogeneous light conditions.