📚 A-Level Biology: Plant Hormones Key Points | A-Level 生物：植物激素 考点精讲
Plant hormones, also known as plant growth regulators, are chemical messengers that control and coordinate growth, development and responses to environmental stimuli. They are produced in one part of the plant, often at extremely low concentrations, and transported to target tissues where they trigger specific physiological changes. The five classic groups tested at A-Level are auxins, gibberellins, cytokinins, abscisic acid (ABA) and ethene. Understanding their interactions, mechanisms of action and commercial applications is essential for success in plant biology questions.
植物激素（又称植物生长调节剂）是控制和协调生长、发育和对环境刺激作出反应的化学信使。它们在植物的某个部位产生，通常浓度极低，然后被转运到靶组织，引发特定的生理变化。A-Level 考试涉及的五大经典植物激素是生长素、赤霉素、细胞分裂素、脱落酸和乙烯。理解它们之间的相互作用、作用机制和商业应用，是成功应对植物生物学考题的关键。
1. Overview of Plant Hormones | 植物激素概述
Unlike animal hormones, plant hormones are not produced in dedicated endocrine organs but in a variety of tissues, often at the site of action. They can act locally or be transported over long distances in the xylem or phloem. Instead of having a single enzyme‑linked signal pathway, plant hormones commonly interact with each other in an additive, synergistic or antagonistic manner to fine‑tune responses.
与动物激素不同，植物激素不是在专门的内分泌器官中产生，而是在多种组织中（常常在作用部位）产生。它们可以在局部起作用，也可以通过木质部或韧皮部长距离运输。植物激素通常不以单一的酶联信号通路起作用，而是以加成、协同或拮抗的方式相互作用，以精细调节反应。
A plant's response to a hormone depends on the concentration of the hormone, the sensitivity of the target tissue and the presence of other hormones. This creates flexible signalling networks that allow plants to adapt to changing environments without a nervous system.
植物对激素的反应取决于激素的浓度、靶组织的敏感性以及其他激素的存在。这创造了灵活的信号网络，使植物能够在没有神经系统的情况下适应变化的环境。
2. Auxins: Discovery and Action | 生长素的发现与作用
Auxins were the first plant hormones discovered. The most abundant natural auxin is indole‑3‑acetic acid (IAA). The classical experiments by Charles and Francis Darwin, followed by Boysen‑Jensen and Frits Went, demonstrated that a chemical signal produced in coleoptile tips diffuses downwards and causes bending towards light. Went isolated the substance that caused coleoptiles to curve when placed on one side of an agar block, and he named it 'auxin'.
生长素是最早被发现的植物激素。含量最丰富的天然生长素是吲哚‑3‑乙酸（IAA）。查尔斯·达尔文和弗朗西斯·达尔文、博伊森‑詹森以及弗里茨·温特的经典实验表明，胚芽鞘尖端产生的化学信号向下扩散，导致向光弯曲。温特分离出了当放置在琼脂块一侧时导致胚芽鞘弯曲的物质，并将其命名为“生长素”。
Auxins promote cell elongation, regulate apical dominance, stimulate root initiation and are involved in tropic responses. They are produced mainly in shoot and root apical meristems and young leaves. Synthetic auxins such as 2,4‑D and NAA are widely used in agriculture as herbicides and rooting powders.
生长素促进细胞伸长，调节顶端优势，刺激根的发生并参与向性反应。它们主要在茎尖和根尖分生组织以及幼叶中产生。合成生长素如 2,4‑D 和 NAA 在农业中广泛用作除草剂和生根粉。
3. Phototropism and Gravitropism | 向光性与向地性
In phototropism, unilateral light causes auxin to redistribute to the shaded side of a shoot. The higher auxin concentration stimulates cell elongation on the shaded side, causing the shoot to bend towards the light. This occurs because auxin is transported laterally away from the illuminated side, and the shaded cells are more responsive to auxin due to the acid‑growth mechanism.
在向光性中，单侧光照使生长素重新分布到茎的背光侧。较高的生长素浓度刺激背光侧的细胞伸长，导致茎向光弯曲。这是因为生长素从向光侧横向运输出去，而由于酸生长机制，背光侧细胞对生长素更敏感。
Gravitropism in roots works differently. In a horizontally placed root, auxin accumulates on the lower side due to gravity, but here a high auxin concentration inhibits cell elongation in roots, while a lower concentration promotes it. Thus, the upper side elongates more, causing the root to bend downwards.
根的向地性作用方式不同。在水平放置的根中，由于重力作用生长素在下侧积累，但在根中高浓度生长素抑制细胞伸长，而低浓度促进伸长。因此，上侧伸长更多，导致根向下弯曲。
4. Auxin Mechanism of Action | 生长素的作用机制
The acid‑growth hypothesis explains how auxin stimulates cell elongation. Auxin binds to a receptor protein and activates proton pumps (H⁺‑ATPases) in the plasma membrane. This pumps H⁺ ions into the cell wall, lowering the apoplastic pH to about 4.5. The acidic environment activates expansins, proteins that break hydrogen bonds between cellulose microfibrils and hemicellulose, loosening the cell wall. As the cell wall becomes more extensible, water uptake into the vacuole drives turgor‑driven expansion.
酸生长假说解释了生长素如何刺激细胞伸长。生长素与受体蛋白结合并激活质膜上的质子泵（H⁺‑ATP酶）。这将 H⁺ 离子泵入细胞壁，使质外体 pH 降至约 4.5。酸性环境激活扩展蛋白，这些蛋白破坏纤维素微纤丝与半纤维素之间的氢键，使细胞壁松弛。随着细胞壁变得更具延展性，水分进入液泡，驱动膨压驱动的扩张。
Auxin also alters gene expression. After binding to receptors such as TIR1, auxin promotes the degradation of Aux/IAA repressor proteins, allowing auxin response factors (ARFs) to activate transcription of specific genes. This leads to sustained growth responses and differentiation.
生长素也改变基因表达。与 TIR1 等受体结合后，生长素促进 Aux/IAA 抑制蛋白的降解，使生长素响应因子（ARFs）激活特定基因的转录。这导致持续的生长反应和分化。
5. Apical Dominance | 顶端优势
Apical dominance is the phenomenon whereby the growing shoot tip inhibits the growth of lateral buds, maintaining a single main shoot axis. Auxin produced by the apical bud is transported downwards and suppresses lateral bud outgrowth. If the apical bud is removed (decapitation), auxin levels decrease and lateral buds are released from inhibition, causing branching.
顶端优势是指生长的茎尖抑制侧芽生长，维持单一主茎轴的现象。顶芽产生的生长素向下运输，抑制侧芽的萌发。如果去除顶芽（去顶），生长素水平降低，侧芽从抑制中释放出来，导致分枝。
Cytokinins act antagonistically to auxin in this process. Applying cytokinin to lateral buds can overcome apical dominance, promoting branching. The ratio of auxin to cytokinin therefore determines the fate of axillary buds. This interplay is important in shaping plant architecture in horticulture and crop management.
在这一过程中，细胞分裂素与生长素起拮抗作用。对侧芽施用细胞分裂素可以克服顶端优势，促进分枝。因此，生长素与细胞分裂素的比例决定了腋芽的命运。这种相互作用在园艺和作物管理中塑造植物结构至关重要。
6. Gibberellins: Stem Elongation and Seed Germination | 赤霉素：茎伸长与种子萌发
Gibberellins (GAs) are a large family of diterpenoid compounds. The most studied bioactive form is gibberellic acid (GA₃). They promote stem elongation by stimulating cell division and cell elongation in the intercalary meristems of stems. Dwarf varieties of plants often lack functional GA biosynthesis genes and can be rescued by exogenous GA application, confirming their role.
赤霉素是一大类二萜化合物。研究最多的生物活性形式是赤霉酸（GA₃）。它们通过刺激茎的居间分生组织中的细胞分裂和伸长来促进茎伸长。植物的矮化品种通常缺乏功能性 GA 生物合成基因，外源施加赤霉素可使其恢复，证实了它们的作用。
Gibberellins also break seed dormancy. In cereal grains such as barley, water uptake by the embryo triggers GA synthesis. GA diffuses to the aleurone layer, where it induces the transcription of α‑amylase genes. This enzyme hydrolyses starch stored in the endosperm into soluble sugars, providing energy for germination. This process is exploited in malting to produce fermentable sugars for brewing.
赤霉素还能打破种子休眠。在大麦等谷物中，胚吸水启动 GA 合成。GA 扩散到糊粉层，诱导 α‑淀粉酶基因的转录。这种酶将胚乳中储存的淀粉水解为可溶性糖，为萌发提供能量。这一过程被利用于制麦芽，为酿造生产可发酵糖。
7. Cytokinins and Cell Division | 细胞分裂素与细胞分裂
Cytokinins are adenine‑derived hormones that promote cell division (cytokinesis) in shoots and roots. They are synthesised mainly in root apices and transported upwards in the xylem. Together with auxin, cytokinins regulate the cell cycle by controlling the transition from G2 to mitosis and from G1 to S phase. They also delay leaf senescence by promoting nutrient mobilisation and chlorophyll retention.
细胞分裂素是腺嘌呤衍生的激素，促进茎和根中的细胞分裂。它们主要在根尖合成，通过木质部向上运输。细胞分裂素与生长素一起，通过控制从 G2 到有丝分裂以及从 G1 到 S 期的转变来调节细胞周期。它们还通过促进营养动员和保留叶绿素来延缓叶片衰老。
In tissue culture, a high cytokinin‑to‑auxin ratio promotes shoot formation, whereas a high auxin‑to‑cytokinin ratio favours root development. This principle is used extensively in micropropagation. Cytokinins also interact with gibberellins and ABA to coordinate whole‑plant growth in response to nitrogen availability.
在组织培养中，高细胞分裂素与生长素比值促进茎的形成，而高生长素与细胞分裂素比值有利于根发育。这一原理在微繁殖中广泛应用。细胞分裂素还与赤霉素和脱落酸相互作用，协调整个植物对氮供应量的生长反应。
8. Abscisic Acid: Stress Response and Stomatal Closure | 脱落酸：胁迫响应与气孔关闭
Abscisic acid (ABA) is often called the 'stress hormone' because its levels rise during drought, salinity and cold. It plays a key role in stomatal closure. When water is scarce, roots produce more ABA and send it to the leaves. In guard cells, ABA binds to receptors, triggering a signalling cascade that releases Ca²⁺ and promotes the efflux of K⁺ and anions. Water follows by osmosis, guard cells become flaccid and stomata close, reducing transpiration.
脱落酸（ABA）常被称为“胁迫激素”，因为其水平在干旱、盐碱和寒冷时升高。它在气孔关闭中起关键作用。当缺水时，根产生更多的 ABA 并将其输送到叶片。在保卫细胞中，ABA 与受体结合，触发信号级联，释放 Ca²⁺ 并促进 K⁺ 和阴离子外流。水分通过渗透作用流出，保卫细胞变得松弛，气孔关闭，减少蒸腾作用。
ABA also promotes seed dormancy and inhibits premature germination. In many seeds, high ABA levels maintain the dormant state, and a drop in ABA or an increase in GA breaks dormancy. ABA acts antagonistically to gibberellins and auxins in these developmental switches. Furthermore, it induces the synthesis of protective proteins and osmolytes that help cells tolerate dehydration.
ABA 还促进种子休眠并抑制过早萌发。在许多种子中，高水平的 ABA 维持休眠状态，ABA 的下降或 GA 的升高打破休眠。ABA 在这些发育开关中与赤霉素和生长素起拮抗作用。此外，它诱导保护蛋白和渗透调节物质的合成，帮助细胞耐受脱水。
9. Ethene: Fruit Ripening and Senescence | 乙烯：果实成熟与衰老
ion and in response to wounding or stress. The biosynthesis of ethene follows the pathway: methionine → S‑adenosylmethionine (SAM) → ACC (1‑aminocyclopropane‑1‑carboxylic acid) → ethene, catalysed by ACC synthase and ACC oxidase.
乙烯（C₂H₄）是一种气态烃，可充当激素。它在许多植物组织中产生，特别是在果实成熟、叶片脱落以及响应创伤或胁迫时。乙烯的生物合成遵循以下途径：蛋氨酸 → S‑腺苷蛋氨酸（SAM）→ ACC（1‑氨基环丙烷‑1‑羧酸）→ 乙烯，由 ACC 合酶和 ACC 氧化酶催化。
Ethene triggers climacteric fruit ripening, a burst of respiration that converts starch to sugars, softens cell walls and changes colour and aroma. It acts through a receptor‑mediated signalling pathway that derepresses transcription factors such as EIN3. Ethene also promotes leaf and flower abscission by stimulating the production of cell wall‑degrading enzymes in the abscission zone. In agriculture, ethene or ethephon (an ethene‑releasing compound) is used to synchronise ripening of fruits like bananas and tomatoes.
乙烯触发跃变型果实的成熟过程，这是一次呼吸爆发，将淀粉转化为糖，软化细胞壁并改变颜色和香气。它通过受体介导的信号通路，解除对 EIN3 等转录因子的抑制。乙烯还通过刺激离区内细胞壁降解酶的产生来促进叶片和花朵脱落。在农业中，乙烯或乙烯利（一种乙烯释放化合物）用于使香蕉和西红柿等水果同步成熟。
10. Commercial Applications | 商业应用
Plant hormones and synthetic analogues are widely used in agriculture and horticulture. Auxins such as IBA and NAA are used as rooting powders to promote adventitious root formation in cuttings. High concentrations of synthetic auxins like 2,4‑D act as selective herbicides against broad‑leaved weeds because dicots are more sensitive than monocots (grasses).
植物激素及其合成类似物在农业和园艺中广泛应用。如 IBA 和 NAA 等生长素用作生根粉，促进插穗中不定根的形成。高浓度的合成生长素如 2,4‑D 作为选择性除草剂对抗阔叶杂草，因为双子叶植物比单子叶植物（禾本科）更敏感。
Gibberellins are used to increase fruit size in grapes ('Thompson Seedless' grape), to improve germination and to synchronise flowering. Cytokinins are used in tissue culture and to extend the shelf life of cut flowers and leafy vegetables. Ethephon is applied to accelerate fruit ripening and to facilitate mechanical harvesting by promoting fruit drop. ABA and its agonists are being explored as anti‑transpirants to conserve water in crops.
赤霉素用于增加葡萄（“汤普森无核”葡萄）的果实大小、改善萌发并同步花期。细胞分裂素用于组织培养和延长切花及叶菜的保质期。乙烯利用于加速果实成熟，并通过促进落果来方便机械采收。ABA 及其激动剂正作为抗蒸腾剂探索用于节省作物的水分。
11. Experimental Evidence | 实验证据
Key experiments underpin our understanding of plant hormones. The Darwins' coleoptile experiments showed that a tip‑produced signal mediates phototropism. Boysen‑Jensen inserted a mica barrier on either the illuminated or shaded side, proving that the signal travels on the shaded side. Went's agar block experiment quantified auxin by measuring the angle of curvature of decapitated coleoptiles. These classic techniques remain common in exam scenarios.
关键实验支撑了我们对植物激素的理解。达尔文父子的胚芽鞘实验表明，尖端产生的信号介导向光性。博伊森‑詹森在向光侧或背光侧插入云母屏障，证明信号沿背光侧传导。温特的琼脂块实验通过测量去尖胚芽鞘的弯曲角度来定量生长素。这些经典技术仍然是考试中的常见情景。
More recent experiments involve using mutants defective in hormone biosynthesis or signalling. For example, gibberellin‑deficient dwarf mutants remain short unless treated with GA. ABA‑insensitive mutants fail to close stomata under drought. These genetic approaches complement physiological studies and are frequently tested in A‑Level data‑analysis questions.
更近期的实验涉及使用激素生物合成或信号传导缺陷的突变体。例如，赤霉素缺乏的矮化突变体在未用 GA 处理时保持矮小。ABA 不敏感突变体在干旱条件下无法关闭气孔。这些遗传方法补充了生理学研究，并常在 A‑Level 数据分析题中进行考查。
12. Key Terminology and Common Misconceptions | 关键术语与常见误区
Students often confuse tropism (directional growth response) with nastic movements (non‑directional, e.g. Mimosa leaf folding). Remember that tropism involves differential growth caused by auxin redistribution. Also note that in roots high auxin inhibits, not promotes, growth, a concept opposite to shoots. This can be explained by differing sensitivities of root and shoot cells to auxin concentration.
学生经常将向性（方向性生长反应）与感性运动（非方向性，如含羞草叶折叠）混淆。记住向性涉及由生长素重新分布引起的差异性生长。还要注意，在根中高浓度生长素抑制而非促进生长，这与茎相反。这可以用根细胞和茎细胞对生长素浓度的敏感性不同来解释。
Another misconception is that ABA directly causes abscission; in fact, ethene is the primary hormone promoting leaf and fruit drop, while ABA is more involved in dormancy and stress. Additionally, gibberellins do not always promote growth in every context: in some dwarf cultivars, extra GA can cause excessive stem elongation that weakens the plant. Finally, do not treat plant hormones as entirely independent signals; their effects depend heavily on concentration ratios with other hormones.
另一个误区是认为 ABA 直接导致脱落；事实上，乙烯是促进叶片和果实脱落的主要激素，而 ABA 更多地参与休眠和胁迫。此外，赤霉素并非在任何情况下都促进生长：在一些矮化品种中，过量的 GA 会导致茎过度伸长，使植物变弱。最后，不要将植物激素视为完全独立的信号；它们的作用在很大程度上取决于与其他激素的浓度比。
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