Photosynthetic lesions can trigger accelerated senescence in Arabidopsis thaliana

Abstract

Senescence is a highly regulated process characterized by the active breakdown of cells, which ultimately leads to the death of plant organs or whole plants. In annual plants such as Arabidopsis thaliana senescence can be observed in each individual leaf. Whether deficiencies in photosynthesis promote the induction of senescence was investigated by monitoring chlorophyll degradation, photosynthetic parameters, and reactive oxygen species accumulation in photosynthetic mutants. Several mutations affecting components of the photosynthetic apparatus, including psal-2, psan-2, and psbs, were found to lead to premature or faster senescence, as did simultaneous inactivation of the STN7 and STN8 kinases. Premature senescence is apparently not directly linked to an overall reduction in photosynthesis but to perturbations in specific aspects of the process. Dark-induced senescence is accelerated in mutants affected in linear electron flow, especially psad2-1, psan-2, and pete2-1, as well as in stn7 and stn8 mutants and STN7 and STN8 overexpressor lines. Interestingly, no direct link with ROS production could be observed.

Keywords: Arabidopsis; ROS; STN7; STN8.; photosynthesis; photosystem; senescence.

Figure 1.

Time course of age-dependent senescence of Col-0 and mutants adjusted to the day on which the chlorophyll content reached its maximum. The chlorophyll content of leaf No. 6 was measured. The highest chlorophyll content was set to 100% and the day on which the chlorophyll content peaked was aligned with that for Col-0 or WS as appropriate. Days after germination (dag) refers to the respective WT. Error bars (for visual clarity only in one direction) represent the SE (n = 4–6 independent experiments with three plants each). Significant deviation from WT (P < 0.05) is marked with an asterisk.

Figure 2.

Effects of age-dependent senescence on photosynthetic efficiency in WT and mutant strains of Arabidopsis thaliana between 28 and 40 dag. The plots show the difference between the parameter values on 40 and 28 dags. Measured parameters were ϕ_II, 1-qP, qN, and F_v/F_m. Significant differences from Col-0 are marked with an asterisk.

Figure 3.

Detection of ROS by quantification of DCF in chloroplasts. The rise in fluorescence during the first 30 s of excitation was calculated. Significant deviation from WT (P < 0.05) is marked with an asterisk. Representative protoplasts imaged by fluorescence microscopy via the GFP filter at the beginning of the experiment and 1min later are shown (left: Col-0; right: stn8-1). Bar = 20 µm.

Figure 4.

Dark-induced senescence in Col-0. Col-0 plants were kept in the dark for 3, 7, and 10 days and returned to light after the indicated time. Arrows indicate the respective endpoint of the dark period. Error bars represent the SE (n = 12 measurements from four different experiments) and in some cases are only indicated in one direction to avoid overlaps.

Figure 5.

Levels of chlorophyll in leaf No. 6 of Col-0 and mutant plants kept for 3 (left), 7 (middle), and 10 (right) days in the dark. Shaded areas indicate the respective dark period. Error bars represent the SE (n = 12 measurements from four different experiments) and are only indicated in WT to avoid overlaps. Significant deviation from WT (P < 0.05) is marked with an asterisk.

Figure 6.

Detection of H₂O₂ using DAB staining in leaves of Col-0 and mutants at 40 dag. The intensity of the brown stain reflects the level of ROS accumulation.