Melatonin Improves the Photosynthetic Apparatus in Pea Leaves Stressed by Paraquat via Chlorophyll Breakdown Regulation and Its Accelerated de novo Synthesis
Streszczenie
The positive effect of melatonin on the function of the photosynthetic apparatus is
known, but little is known about the specific mechanisms of melatonin’s action in plants.
The influence of melatonin on chlorophyll metabolism of 24-day-old Pisum sativum
L. seedlings during paraquat (PQ)-induced oxidative stress was investigated in this study.
Seeds were hydro-primed with water (H), 50 and 200 µM melatonin/water solutions
(H-MEL50, H-MEL200), while non-primed seeds were used as controls (C). Increases
in chlorophyllase activity (key enzyme in chlorophyll degradation) and 5-aminolevulinic
acid contents (the first compound in the porphyrin synthesis pathway) were observed
in H-MEL50 and H-MEL200 leaf disks. This suggests that melatonin may accelerate
damaged chlorophyll breakdown and its de novo synthesis during the first hours of
PQ treatment. Elevated level of pheophytin in control leaf disks following 24 h of PQ
incubation probably was associated with an enhanced rate of chlorophyll degradation
through formation of pheophytin as a chlorophyll derivative. This validates the hypothesis
that chlorophyllide, considered for many years, as a first intermediate of chlorophyll
breakdown is not. This is indicated by the almost unchanged chlorophyll to chlorophyllide
ratio after 24 h of PQ treatment. However, prolonged effects of PQ-induced stress
(48 h) revealed extensive discolouration of control and water-treated leaf disks, while
melatonin treatment alleviated PQ-induced photobleaching. Also the ratio of chlorophyll
to chlorophyllide and porphyrin contents were significantly higher in plants treated with
melatonin, which may indicate that this indoleamine both retards chlorophyll breakdown
and stimulates its de novo synthesis during extended stress. We concluded that
melatonin added into the seeds enhances the ability of pea seedlings to accelerate
chlorophyll breakdown and its de novo synthesis before stress appeared and for
several hours after, while during prolonged PQ incubation melatonin delays chlorophyll
degradation.
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