Abscisic acid plays a key role in the mechanism of photosynthetic and physiological response effect of Tetrabromobisphenol A on tobacco

The toxicity of bromide to animals and microorganisms has been widely studied, but the mechanism by which bromide toxicity affects plants is rarely studied. This study used the bromophenol compound Tetrabromobisphenol A (TBBPA) as a representative of bromide to explore the physiological and molecular response mechanism of tobacco leaves to TBBPA. In addition, physiological determination, transcriptomics, weighted gene co-expression network analysis (WGCNA) analysis, and random forest prediction model were conducted. The findings from this study indicated that TBBPA limited the photoreaction process by destroying the light-catching antenna protein of tobacco leaves, the activity of the photosystem reaction centers (PSII and PSI), and the linear electron transport efficiency. TBBPA also reduced the rate of the Calvin-Benson cycle by inhibiting the activities of gene such as Rubisco, PGK, and TPI, and finally destroyed the photosynthesis process. Although cyclic electron transport was enhanced under stress conditions, it could not reverse the damage caused by TBBPA on photosynthesis. TBBPA exposure resulted in the accumulation of reactive oxygen species (ROS) in tobacco leaves, and the activities of Superoxide dismutase (SOD), Ascorbate peroxidase (APX), and Glutathione peroxidase (GPX) and their coding genes were significantly down-regulated. Although POD activity and proline (Pro) content were increased, they were insufficient to remove excess O₂^·- free radicals to relieve ROS stress. WCGNA and random forest models predicted that the damage of TBBPA to the above processes in tobacco was closely related to the increase in abscisic acid (ABA) content. TBBPA affects the Calvin cycle by inducing ABA signal transduction and stomatal closure, which leads to a series of chain reactions, such as electron transport chain obstruction, excess of ROS, decrease in chlorophyll synthesis, and photosystem reaction center damage.

Elevated air temperature damage to photosynthetic apparatus alleviated by enhanced cyclic electron flow around photosystem I in tobacco leaves

High temperature can lead to increased production of excess light energy, thus reducing photosynthetic capacity in plants. Photosynthetic cyclic electron flow (CEF) in photosystem I (PSI) can effectively protect photosystems, but its physiological mechanism under high temperature is poorly understood. In this study, antimycin A (AA) and thenoyltrifluoroacetone (TTFA) were used to inhibit PGR5-and NDH-dependent CEF pathways, respectively, to reveal the photoprotective functions of CEF for PSII in tobacco leaves under high temperature stress (37 °C, HT). High temperatures caused decreases in maximal photochemistry efficiency (Fv/Fm) and damaged photosystem II (PSII) in tobacco leaves. Under AA inhibition of PGR5-dependent CEF, high temperature increased the fluorescence intensity of point O (Fo) in OJIP curves, i.e., the energy absorption per active reaction center (ABS/RC), the trapping rate of the reaction center (TRo/RC), and the electron transport efficiency per reaction center (ETo/RC) in tobacco leaves. High temperature induced an increase in the hydrogen peroxide content and a decrease in pigment content in tobacco leaves. Under the high temperature treatment, inhibition of PGR5-dependent CEF reduced the activities of the PSII reaction center significantly, destroyed the oxygen-evolving complex (OEC), and impeded photosynthetic electron transfer from PSII to the plastoquinone (PQ) pool in tobacco leaves. The TTFA treatment inhibited the NDH-dependent pathway under high temperature conditions, with the relative fluorescence intensity of point I (VI) decreased significantly, and the content of hydrogen peroxide and superoxide anion increased significantly. Additionally, Fo and the redox degree of the PSII donor side (Wk) increased, and pigment content decreased compared to the control, but with little change compared to high temperature treatment, indicating that the inhibition of the NDH-dependent pathway directly weakened the capacity of the PQ pool to lead to the accumulation of reactive oxygen species (ROS) in tobacco leaves. In conclusion, CEF alleviated damage to the photosynthetic apparatus in tobacco leaves by increasing PSII heat dissipation, reducing ROS production, and maintaining the stability of the PQ pool to accommodate photosynthetic electron flow.