Melatonin Improves Waterlogging Tolerance of Malus baccata (Linn.) Borkh. Seedlings by Maintaining Aerobic Respiration, Photosynthesis and ROS Migration

Abstract

Waterlogging, one of the notorious abiotic stressors, retards the growth of apple plants and reduces their production. Thus, it is an urgent agenda for scientists to identify the suitable remedies for this problem. In the current study, we found that melatonin significantly improved the tolerance of apple seedlings against waterlogging stress. This was indicated by the reduced chlorosis and wilting of the seedlings after melatonin applications either by leaf spray or root irrigation. The mechanisms involve in that melatonin functions to maintain aerobic respiration, preserves photosynthesis and reduces oxidative damage of the plants which are under waterlogging stress. Melatonin application also enhances the gene expression of its synthetic enzymes (MbT5H1, MbAANAT3, MbASMT9) and increases melatonin production. This is the first report of a positive feedback that exogenous melatonin application promotes the melatonin synthesis in plants. A post-transcriptional regulation apparently participated in this regulation. When exogenous melatonin meets the requirement of the plants it is found that the protein synthesis of MbASMT9 was suppressed. Taken together, the results showed that melatonin was an effective molecule to protect plant, particularly apple plant, against waterlogging stress.

Keywords: ROS; aerobic respiration; melatonin; oxidative stress; photosynthesis; waterlogging.

FIGURE 1

The effects of melatonin on the M. baccata seedlings which were suffered from waterlogging stress. (A) Before and after waterlogging stress. (B): (a) The phenotype of M. baccata seedlings with melatonin leaf spray at different concentrations before and after waterlogging stress. (b) The phenotype of M. baccata seedlings with melatonin irrigation at different concentrations before and after waterlogging stress.

FIGURE 2

Effects of waterlogging and exogenous melatonin application on the melatonin synthesis in M. baccata seedlings 9 days after waterlogging stress. (A) Endogenous melatonin levels in leaves and in roots of the M. baccata seedlings. (B) The relative expression of melatonin synthetic genes (MbASMT9, MbAANAT3 and MbT5H1) in the leaves. (C) The protein level of MbASMT9 detected by Western blot analysis in the leaves. The data are means ± SD of triplicate experiments. Asterisks (^∗) and different letters indicate significant differences from the control (P < 0.05).

FIGURE 3

Effects of melatonin on activities of antioxidant enzymes. (A) Superoxide dismutase (SOD). (B) Peroxidase (POD). (C) Catalase (CAT) in both leaves and roots. The waterlogging stress was lasted 9 days. The data are means ± SD of triplicate experiments. Different letters indicate significant differences from the control (P < 0.05).

FIGURE 4

Effects of melatonin on aerobic and anaerobic respirations of M. baccata seedlings under waterlogging stress. (A) Anaerobic respiration which is indicated by ADH activity. (B) Aerobic respiration which is indicated by SDH activity. The waterlogging stress was last for 9 days. The data are means ± SD of triplicate experiments. Different letters indicate significant differences from the control (P < 0.05).

FIGURE 5

Effects of melatonin on ROS levels and oxidative damage in M. baccata seedlings which were under waterlogging stress. (A) ROS level in roots. (B) MDA production in roots. (C) The levels of superoxide anion and hydrogen peroxide in leaves. (D) MDA production in leaves. The waterlogging stress lasted for 9 days. Scale bars in (A) represent 0.5 cm. Scale bars in (C) represent 1cm. The data are means ± SD of triplicate experiments. Different letters indicate significant differences from the control (P < 0.05).

FIGURE 6

Effects of melatonin on the chlorophyll content and photosynthetic rate of the M. baccata seedlings before and after waterlogging stress. (A) Chlorophyll content. (B) Photosynthetic rate. The data are means ± SD of triplicate experiments. Different letters indicate significant differences from the control (P < 0.05).

FIGURE 7

The diagraph of the speculated mechanisms regarding the oxidative damage induced by waterlogging stress and the protective effects of melatonin application. Arrow heads indicated the stimulation and the even bars indicated the inhibition.