【作者】 张娜；

【导师】 郭仰东；

【作者基本信息】 中国农业大学 ， 蔬菜学， 2014， 博士

【摘要】 褪黑素是吲哚类衍生物,化学名称为N-乙酰基-5-甲氧基色胺,是一种经典的动物激素。在脊椎动物体内由松果体合成并分泌到血液里,参与调节昼夜节律、改善睡眠、治疗神经衰弱。褪黑素是一种天然的抗氧化剂,能够高效的清除多种活性氧,并能提高细胞内抗氧化系统酶的活性。褪黑素在植物中也是广泛存在的,植物不仅能够吸收环境中的褪黑素还可以自行合成褪黑素。褪黑素的合成前体为色氨酸,通过四步酶促反应合成褪黑素。褪黑素具有调节生长的作用,能够促进生根和离体叶片膨大。植物在逆境条件下会积累更多的褪黑素,外源褪黑素处理植物能够提高植物对逆境胁迫的抵御能力。黄瓜是一种需水性很强的蔬菜作物,水分不足会严重影响黄瓜的生长,降低黄瓜的产量和品质。本试验以黄瓜为实验材料,PEG诱导的渗透胁迫会导致黄瓜的种子发芽率显著降低,褪黑素处理过的黄瓜种子在相同的水份胁迫下发芽率明显提高。渗透胁迫严重限制了黄瓜根系发育,养分向地上部分配比例增加,表现为根体积减小,根冠比下降。褪黑素处理提高了胁迫下的黄瓜根系侧根数、根粗和根体积,根冠比相比未处理过的植株有显著提高。褪黑素处理过的根系活力也明显高于未处理过的黄瓜。褪黑素处理促进了有机物质向根部的流动,有助于保持黄瓜植株在逆境胁迫下保持较强壮的根系。渗透胁迫导致的吸水障碍严重影响了黄瓜叶片的光合作用。净光合效率在渗透胁迫下显著下降,但是胞间CO：浓度没有发生显著变化,表明渗透胁迫导致的光合效率下降是非气孔因素引起的。水分胁迫导致了叶绿素的大量降解,褪黑素处理缓解了这一现象,使叶绿素含量维持在较高的水平。胁迫导致了一系列叶绿素荧光参数的变化,渗透胁迫下PSII光合电子传递量子效率(ΦPSⅡ)、PSⅡ天线转化效率(Fv’/Fm’)、光化学猝灭系数(qP)在渗透胁迫下均下降而非光化学猝灭系数(NPQ)升高。叶绿体是光合作用的主要器官,部分叶绿体在渗透胁迫下发生变形、破裂,褪黑素处理过的叶片中的叶绿体变形的数量较少。褪黑素处理有效的保持了叶绿体的完整和叶绿素的水平,缓解了渗透胁迫对光合作用的阻碍作用。正常条件下生长的植株能够保持体内活性氧合成与清除的动态平衡。逆境会加剧活性氧的合成,从而破坏活性氧的平衡。过多的活性氧会造成细胞内生物大分子的过氧化,如膜脂过氧化,同时会破坏细胞膜的选择透过性。渗透胁迫下黄瓜的羟自由基和过氧化氢大量产生,膜脂过氧化程度加剧,细胞膜透性被破坏,大量电解质外渗,电导率升高。褪黑素自身具备清除活性氧的能力,是一种高效的抗氧化剂。另外褪黑素还能够提高植株本身的抗氧化酶系统的活性,进一步帮助植物清除多余的活性氧。褪黑素处理黄瓜植株抗氧化酶系统活性显著提高,羟自由基、过氧化氢水平相比未处理的植株显著降低,膜脂过氧化水平和电解质外渗率显著降低。褪黑素处理提高了黄瓜对渗透胁迫的耐受力。褪黑素处理对黄瓜的根系发育尤其是侧根的发生有显著影响。利用RNA-seq技术进行深度测序,探讨褪黑素促进侧根发育的分子机制。褪黑素处理过的根系过氧化物酶活性较高,过氧化物酶基因表达水平较高。褪黑素处理过的样本多个与根发育相关的转录因子基因表达水平发生了变化。褪黑素通过调节转录因子的表达和过氧化物酶基因的表达间接的影响根系的发育。另外,褪黑素促进黄瓜侧根的发育是独立于生长素的。更多还原

【Abstract】 Melatonin (N-acetyl-5-methoxytryptamine) is an indole amine which was long thought to be a classic hormone in animals. It is an endogenously generated molecule in pineal gland of vertebrates that can synchronize circadian and circannual rhythm. Melatonin is a natural antioxidant that can detoxify, neutralize, and metabolize reactive species. What’s more, melatonin can regulate antioxidant system enzymes to further eliminate ROS. Melatonin has now been found to exist in plants as well. Plants can take up melatonin from the culture medium. Plants have the mechanisms to synthesize melatonin from tryptophan. Melatonin biosynthesis in plants occurs via four consecutive enzymatic steps. Melatonin is a plant growth regulator which can make the in vitro leaves expand and promote root regeneration.Cucumber is a vegetable crop which needs a lot of water along growth. Water deficiency can inhibit the plant growth, decrease the yield and quality. Melatonin alleviated the seed germination rate which was decreased by osmotic stress. Osmotic stress seriously limits the cucumber root development. Under osmotic stress, root viability, the volume and the diameter of root were decreased. Melatonin treatment increased lateral root formation. The root:shoot ratio represents the nutrient allocation of the plants. PEG treatment restricted the ability of seedlings to take up water and slightly increased the root:shoot ratio. Melatonin treatments increased the volume of root and whole seedlings and inverted the proportion of root to shoot volume.Water deficiency induced a reduction in photosynthesis. No significant drought-induced change was detected at the level of the intercellular concentration of carbon dioxide. The improvement of photosynthetic rates was likely due to melatonin-induced metabolic changes. Polyethylene glycol severely inhibited the ability of cucumber plants to take up water, causing significant degradation of chlorophyll. Melatonin treatment significantly inhibited chlorophyll degradation in a concentration dependent manner. The photochemical efficiency of open PSⅡ reaction centers (Fv’/Fm’ratio), the quantum yield of PSII electron transport (ΦPSⅡ), and the electron transport rate (ETR) decreased significantly under water stress. In contrast, the nonphotochemical quenching (NPQ) was enriched; indicating enhanced thermal energy dissipation at PSII. A transmission electron microscope revealed that chloroplasts were normal and exhibited long ellipsoidal shapes in the control groups. Meanwhile, water stress resulted in swollen chloroplasts. The number of abnormal chloroplasts was lower in melatonin treated samples.Oxidative stress is considered a major damaging factor in plants exposed to drought. A significant increase in H2O2and·OH content was induced by PEG treatment; however, melatonin-treated plants showed relatively low levels of H2O2and OH. Excess ROS can cause foliar lipid peroxidation which can damage the membrane system. Water stress induced high levels of EL and MDA which were reduced by melatonin treatment. Melatonin and several of its metabolites are known endogenous free radical scavengers. SOD, POD, and CAT showed significant rises in activity with the melatonin treatment. Melatonin in plants functions as the first line of defense against internal and environmental oxidative stressors.Melatonin modulates root system architecture especially stimulates lateral root formation. In this study, RNA sequencing was employed to explore the mechanism of melatonin-induced lateral root formation in cucumber. Based on their expression pattern, peroxidase-related genes were selected as the candidates to be involved in the melatonin response. Several transcription factor families might play important roles in lateral root formation processes. Melatonin affected the root pattern in an auxin-independent manner. Melatonin modulates root development in an indirect way, mostly by regulating the reactive oxygen species generation and transcription factors.更多还原