Effects of Cadmium on metabolism of photosynthetic pigment and photosynthetic system in Lactuca sativa L. revealed by physiological and proteomics analysis

Cadmium stress stunted lettuce growth, resulting in phenotypic changes such as leaf yellowing and shrinkage, as well as physiological changes such as reduced chlorophyll and photosynthesis. However, the molecular mechanism by which this metal inhibits lettuce development and plant stress response remains unclear. The effects of 20 μmol/L Cd treatment on chlorophyll and carotenoids contents as well as photosynthetic function of lettuce leaves was investigated to explore the mechanism of action of Cd on photosynthetic function. Using an IBT-based proteomics technique, the key proteins in these physiological processes were quantified. It was found that the expression of important enzymes in the chlorophyll synthesis pathway in leaves, such as HemB, POR and DVR, was shown to be hindered by Cd stress, resulting in a reduction in chlorophyll content. Meanwhile, Cd promoted the degradation of carotenoids and up-regulated the expression of degradation enzymes NCED4 and CCD8, resulting in a reduction in carotenoids content. Furthermore, the expression of the xanthophyll cycle-regulating enzymes ZEP and VDE was dramatically reduced. This means that lettuce's inherent antioxidant defensive mechanisms, such as the xanthophyll cycle and the NPQ-dependent energy dissipation process, are impaired. Moreover, the decrease in chlorophyll and carotenoids content reduced the stability of LHC, where the protein expression of Lhca4, Lhcb1, Lhcb3, Lhcb4 and Lhcb6 was significantly reduced. Additionally, Cd not only reduced the expression of PSII receptor-side proteins (PsbL, PsbP, and PsbR) and core of PSI response center (psaC, psaD, psaE, psaL and psaN), but also electron transport-related proteins (Cyt b6/f complex, Fd and H+-ATPase), carbon fixation-related ATPase subunits and Rubisco subunits expression were all significantly reduced, then the supply of carbon assimilation power was inhibited. Therefore, on one hand, Cd stress inhibited the synthesis of photosynthetic pigments and photosynthesis-related proteins or subunits, eventually affected the fixation of CO2, as manifested by the decreased Pn and plant biomass. On the other hand, Cd stress directly affected the protective mechanism of the photosynthetic apparatus, such as xanthophyll cycle and antioxidant capacity, which ultimately leads to the photodamaged of the photosynthetic apparatus. This study delves into the target sites of Cd attack on lettuce and provides an accurate reference for our research on Cd mitigants.