Abstract
Since melatonin was identified in plants decades ago, much attention has been devoted to discovering its role in plant science. There is still a great deal to learn about the functional importance of melatonin, as well as its functional mode. In this paper, we examine the role of melatonin treatment in the response of Malus hupehensis Rehd. to alkaline conditions. Stressed seedlings showed chlorosis and suppressed growth. However, this phenotype was ameliorated when 5 µM melatonin was added to the irrigation solution. This supplementation was also associated with a reduction in cell membrane damage and maintenance of a normal root system architecture. Fewer reactive oxygen species (ROS) were accumulated due to the enhanced scavenging activity of antioxidant enzymes superoxide dismutase, peroxidase, and catalase. In addition, alkaline-stressed seedlings that received the melatonin supplement accumulated more polyamines compared with untreated seedlings. Transcript levels of six genes involved in polyamine synthesis, including SAMDC1, -3, and -4, and SPDS1, -3, and -5, -6, were upregulated in response to melatonin application. All of these results demonstrate that melatonin has a positive function in plant tolerance to alkaline stress because it regulates enzyme activity and the biosynthesis of polyamines.
Highlights
Plants are frequently threatened by a variety of biotic and abiotic stresses during their life cycle
To determine whether supplemental melatonin can alleviate the adverse effects of alkaline stress, we added 2 mM Na2 CO3 /NaHCO3 to the hydroponics system used for growing seedlings of Malus hupehensis Redh
When alkaline stress was induced for 15 d, young leaves from seedlings that had not received the melatonin treatment (AL group) showed obvious yellowing, while those from seedlings exposed earlier to melatonin were only slightly yellowed (Figure 1, AL versus AL + MT)
Summary
Plants are frequently threatened by a variety of biotic and abiotic stresses during their life cycle. Salt-alkaline soils are widely distributed across arid and semi-arid regions of the world, and have detrimental effects on plant growth and development [1,2]. Soil alkalization is often associated with soil salinity, the former is considered much more hazardous to plants. This condition is generally linked with high pH stress and sodium toxicity caused by an excess of Na2 CO3 and NaHCO3 in the soil, as well as osmotic stress. The comprehensive stress caused by alkaline soils directly affects physiological homeostasis at the cellular and whole-plant levels [3,4,5].
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