Melatonin improves the germination rate of cotton seeds under drought stress by opening pores in the seed coat

Abstract

<jats:p>The germination of cotton (<jats:italic>Gossypium hirsutum</jats:italic> L.) seeds is affected by drought stress; however, little is known about the physiological mechanism affecting germination and the effect of melatonin (MT) on cotton seed germination under drought stress. Therefore, we studied the effects of exogenous MT on the antioxidant capacity and epidermal microstructure of cotton under drought stress. The results demonstrated a retarded water absorption capacity of testa under drought stress, significantly inhibiting germination and growth in cotton seeds. Drought stress led to the accumulation of reactive oxygen species (ROS), malondialdehyde (MDA), and osmoregulatory substances (e.g., proline, soluble protein, and soluble sugars); it also decreased the activity of antioxidant enzymes and α-amylase. Drought stress inhibited gibberellin acid (GA<jats:sub>3</jats:sub>) synthesis and increased abscisic acid (ABA) content, seriously affecting seed germination. However, seeds pre-soaked with MT (100 µM) showed a positive regulation in the number and opening of stomata in cotton testa. The exogenous application of MT increased the germination rate, germination potential, radical length, and fresh weight, as well as the activities of superoxide dismutase (SOD), peroxidase (POD), and α-amylase. In addition, MT application increased the contents of organic osmotic substances by decreasing the hydrogen peroxide (H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub>), superoxide anion (O<jats:sub>2</jats:sub><jats:sup>-</jats:sup>), and MDA levels under drought stress. Further analysis demonstrated that seeds pre-soaked with MT alleviated drought stress by affecting the ABA and GA<jats:sub>3</jats:sub> contents. Our findings show that MT plays a positive role in protecting cotton seeds from drought stress.</jats:p>

Melatonin-Induced Water Stress Tolerance in Plants: Recent Advances

“…The key physiological aspects that are much regulated by phytohormones in response to drought stress include antioxidant metabolism, carbohydrate production (carbon metabolism), stomatal movement, and leaf senescence [ 82 ]. Drought stress upregulates ABA, BRs, and JA [ 59 , 82 ] and downregulates CKs and GAs [ 51 , 59 ], while melatonin enhances the levels of BRs, GAs, JA, and CKs [ 59 ] and decreases the ABA level [ 59 ] ( Figure 2 ).…”

“…Moreover, the overaccumulation of abscisic acid upregulates the ROS generation pathways and causes oxidative damage [ 13 ]. However, melatonin maintains the abscisic acid homeostasis (low to moderate concentrations) by positive regulation of its biosynthetic genes and negative regulation of the catabolic genes [ 49 , 51 , 53 , 59 ]. Li et al [ 53 ] clarified that melatonin effectively downregulates MdNCED3 , an abscisic acid synthesis gene, and upregulates its catabolic genes, MdCYP707A2 and MdCYP707A1 , causing abscisic acid reduction.…”

Melatonin-induced Water Stress Tolerance in Plants: Recent Advances

“…The key physiological aspects that are much regulated by phytohormones in response to drought stress include antioxidants metabolism, carbohydrate production (carbon metabolism), stomatal movement, and leaf senescence [84]. Drought stress up-regulates ABA, BRs, and JA [59,84] and down-regulates CKs and GAs [52,59], while melatonin enhances the levels of BRs, GAs, JA and CKs [59] and decreases ABA level [59] (Figure 2).…”

“…Moreover, the over-accumulation of ABA up-regulates the ROS generation pathways and causes oxidative damage [14]. However, melatonin maintains the ABA homeostasis (low to moderate ABA concentrations) by positive regulation of its biosynthetic genes and negative regulation of catabolic genes [52,54,59,72]. Li, et al [54] clarified that melatonin effectively down-regulates MdNCED3, ABA synthesis genes, and up-regulates its catabolic genes, MdCYP707A2 and MdCYP707A1, causing ABA reduction.…”

“…Gibberellins (GAs) are regulators of stomatal movement [101,102], photosynthesis [103], seed germination [104], and leaf senescence [4]. Drought stress inhibits gibberellins (GAs) biosynthesis [52,105], which are much enhanced in response to melatonin treatment causing drought tolerance [59]. Salicylic acid (SA) accumulation plays a vital role in stomatal movement, photosynthesis, and antioxidant defence system [4].…”