Abstract
Boron (B) toxicity is an important abiotic constraint that limits crop productivity mainly in arid and semi-arid areas of the world. High levels of B in soil disturbs several physiological and biochemical processes in plant. The aim of this study was to investigate the function of melatonin (Mel) in the regulation of carbohydrate and proline (Pro) metabolism, photosynthesis process and antioxidant system of wheat seedlings under B toxicity conditions. High levels of B inhibited net photosynthetic rate (PN), stomatal conductance (gs), content of chlorophyll (Chl) a, b, δ-aminolevulinic acid (δ-ALA), nitrogen (N) and phosphorus (P), and increased accumulation of B, Chl degradation and activity of chlorophyllase (Chlase; a Chl degrading enzyme), and downregulated the activity of enzymes (δ-ALAD; δ-aminolevulinic acid dehydratase) involved in the biosynthesis of photosynthesis pigments, photosynthesis (carbonic anhydrase and ribulose-1,5-bisphosphate carboxylase/oxygenase) and carbohydrate metabolism (cell wall invertase, CWI) in wheat seedlings. Also, high levels of B caused oxidative damage by increasing the content of malondialdehyde, superoxide anion and H2O2, and activity of glycolate oxidase (an H2O2-producing enzyme) in leaves of seedlings. However, foliar application of Mel significantly improved photosynthetic pigments concentration by increasing δ-ALA, δ-ALAD and decreasing Chl degradation and Chlase activity and led to an increase of plant growth attributes under both B toxicity and non-toxicity conditions. Under normal and B toxicity conditions, exogenous Mel also improved content of N, P, total soluble carbohydrates (TSCs) and Pro, and upregulated activity of CWI and Δ1-pyrroline-5-carboxylate synthetase. Mel significantly suppressed the adverse effects of excess B by alleviating cellular oxidative damage through enhanced reactive oxygen species scavenging by superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase and lipoxygenase, and content of total phenolic compounds (TPC), ascorbate and reduced glutathione. These results postulate that Mel induced plant defense mechanisms by enhancing Pro, TSCs, TPC, nutrients (N and P) uptake and enzymatic and non-enzymatic antioxidants.
Schematic diagram of melatonin induced resistance against B-toxicity by regulating antioxidant system, carbohydrate metabolism, proline and photosynthetic pigments biosynthesis in wheat seedlings. RuBisco—ribulose-1,5-bisphosphate carboxylase/oxygenase, CWI—cell wall invertase, ROS—reactive oxygen species, Chlase— Chlorophyllase, GOx—glycolate oxidase, CO2—carbon dioxide, Ch—chlorophyll, CA—carbonic anhydrase, and δ-ALAD—δ-aminolevulinic acid dehydratase, δ-ALA—δ-aminolevulinic acid, Pro—proline, P5CS—Δ1-pyrroline-5-carboxylate synthetase activity, LOX—lipoxygenase, TPC—content of total phenolic compounds, ASC—ascorbate, GSH—reduced glutathione, SOD—superoxide dismutase, GR— glutathione reductase, APX—ascorbate peroxidase, and GR—glutathione reductase. [Display omitted]
•Boron toxicity impairs growth, Chl biosynthesis, photosynthesis process, and metabolism of TSCs and Pro.•High levels of boron enhances ROS, MDA and glycolate oxidase activity.•Melatonin suppresses boron toxicity-induced oxidative damage.•Melatonin regulates antioxidant metabolism, and modulates TSCs and Pro metabolism.•Melatonin maintains nutrients homeostasis in of plant under boron toxicity.