Biology subjects

Harimoto, S.

Publications and source records attributed to Harimoto, S..

4 recordsLinked to original sources

bioRxiv2026

Tissue-resolved photosynthetic responses in living leaves revealed by microscopic imaging-pulse-amplitude-modulation

Leaves of terrestrial plants possess heterogeneous anatomical structures composed of multiple cell layers. Although biochemical differences among leaf tissues have been inferred from protein analyses and anatomical studies, direct comparisons of dynamic photochemical responses among tissues while preserving their spatial context remain challenging. Leaves experience intrinsically heterogeneous environments because incident light enters primarily from above and propagates through complex internal leaf structures. Therefore, analyzing photosynthetic activity at the tissue and cellular levels is essential for understanding how photosynthesis operates within structurally heterogeneous leaves. Here, we combined live leaf-section imaging with microscopic Imaging-PAM chlorophyll fluorescence measurements to analyze photochemical responses at the tissue level in living leaf sections. In dorsiventral dicot leaves, palisade tissues exhibited a higher effective PSII quantum yield [Y(II)] and more rapid induction of regulated energy dissipation [Y(NPQ)] than spongy tissues, indicating higher photosynthetic capacity and photoprotective activity. In contrast, rice leaves, which lack palisade-spongy differentiation, showed uniform photochemical responses along the adaxial-abaxial axis. In the C4 plant finger millet, mesophyll and bundle sheath cells displayed distinct photochemical responses consistent with their functional differentiation in C4 photosynthesis. These results demonstrate that photosynthetic responses are spatially organized according to leaf anatomical structure. Although section-based measurements do not reproduce the native optical and gas environments of intact leaves, they enable the comparison of intrinsic tissue-specific photochemical properties under approximately equivalent illumination. The present approach enables tissue-resolved chlorophyll fluorescence analysis in living leaves, providing a new framework for investigating how leaf architecture shapes the spatial organization of photosynthetic activity.

plant biology↗

bioRxiv2026

The PSI-NDH supercomplex prevents chilling-induced PSI photoinhibition

Chilling stress induces photosystem I (PSI) photoinhibition in chilling-sensitive cucumber, in which insufficient activity of the chloroplast NADH dehydrogenase-like complex (NDH) leads to PSI over-reduction and damage. However, it is not yet clear whether these findings can be generalized to other species or what the molecular mechanism underlying impaired NDH function is. In this study, we first examined whether NDH is essential for PSI protection under chilling stress using an NDH-deficient rice mutant. Compared with wild-type plants, the NDH-deficient mutant exhibited enhanced PSI over-reduction and pronounced PSI photoinhibition under chilling stress. In contrast, rice plants expressing flavodiiron protein (FLV), which functions as an alternative electron acceptor downstream of PSI, did not exhibit PSI photoinhibition under chilling stress, demonstrating that electron sink capacity of NDH is important for PSI protection under chilling stress. Furthermore, analysis of the factors responsible for NDH dysfunction under chilling stress in cucumber revealed that chilling stress destabilizes the PSI-NDH supercomplex, leading to NDH monomerization and a consequent loss of NDH activity. This NDH monomerization is likely attributable to chilling-induced damage to the light-harvesting complex Lhca, which mediates the association between PSI and NDH. Together, these results indicate that NDH is essential for protecting PSI from photoinhibition under chilling stress in both rice and cucumber, and that chilling-induced destabilization of the PSI-NDH supercomplex represents a key molecular mechanism underlying PSI over-reduction and photoinhibition.

plant biology↗

bioRxiv2025

The protective role of chloroplast NADH dehydrogenase-like complex (NDH) against PSI photoinhibition under chilling stress

Chilling stress induces photosystem I (PSI) photoinhibition in various plants, severely impairing their growth. However, the mechanisms suppressing chilling-induced PSI photoinhibition remain unclear. This study aimed to identify factors preventing PSI photoinhibition by comparing two cucumber cultivars with different susceptibilities to PSI photoinhibition and chilling stress tolerance. In the chilling-sensitive cultivar, partial degradation of the CF1-{gamma} subunit of chloroplast ATPase led to uncoupling of the thylakoid membrane. In addition, electron efflux from Fe-S clusters downstream of PSI was restricted under chilling stress, resulting in highly reduced Fe-S clusters. Notably, this PSI over-reduction in the chilling-sensitive cultivar was observed not only under chilling stress but also under fluctuating light conditions, limited CO2 conditions, and during the transition from darkness to light, suggesting that cyclic electron flow contributes to cultivar differences in PSI photoinhibition. Indeed, the chilling-tolerant cultivar exhibited higher activity of the chloroplast NADH dehydrogenase-like complex (NDH) and suppressed reactive oxygen species (ROS) accumulation during the early stages of chilling stress. In contrast, in the chilling-sensitive cultivar, destabilization of PSI-NDH supercomplex under chilling stress led to the loss of NDH activity, resulting in severe PSI over-reduction. This study provides evidence that NDH acts as a crucial electron sink to prevent PSI photoinhibition and provides new insights into the mechanisms underlying low-temperature stress tolerance.

plant biology↗

bioRxiv2025

PSII photoinhibition as a protective strategy against PSI photoinhibition: Maintaining PSI in an oxidized state by suppressing PSII activity under environmental stress

Photosystem I (PSI) can be photoinhibited by excessive electron flow from Photosystem II (PSII), causing serious growth inhibition due to PSIs limited repair capacity. In contrast, PSII is more susceptible to photoinhibition under stress but has an efficient repair system. Consequently, PSII photoinhibition is considered a protective mechanism that mitigates PSI over-reduction. However, this photoprotective role under environmental stress remains unexplored in intact plants without using mutants or chemical treatments. To address this, we examined the relationship between PSII photoinhibition and susceptibility to PSI photoinhibition under two representative stresses that selectively induce PSI photoinhibition: chilling stress and fluctuating light, using A. thaliana and cucumber. Under chilling stress, A. thaliana exhibited marked PSII photoinhibition and maintained active PSI, whereas cucumber showed insufficient PSII downregulation and suffered PSI photoinhibition. In addition, when fluctuating light treatment was applied to plants with various Fv/Fm (the maximum quantum yield of PSII), plants with reduced Fv/Fm maintained oxidized PSI, and PSI photoinhibition progressed slowly. The susceptibility to PSI photoinhibition under fluctuating light strongly correlated with Fv/Fm, providing clear evidence that PSII photoinhibition protects PSI in vivo. Interestingly, even in plants where P700 remained oxidized during saturation pulses due to PSII photoinhibition, the iron-sulfur clusters (Fe-S clusters) remained reduced. However, the re-oxidation rate of reduced Fe-S clusters was enhanced in PSII-photoinhibited plants, suggesting that charge recombination to P700+ with reduced components on the acceptor side would suppress ROS generation downstream of PSI. This study clarifies how PSII photoinhibition suppresses PSI photoinhibition and prevents ROS-induced damage in wild-type plants under environmental stress.

plant biology↗