A-Level Biology Plant Hormones Tropisms Auxin

Introduction to Plant Hormones 植物激素简介

Plant hormones, also known as phytohormones, are chemical messengers that regulate growth, development, and responses to environmental stimuli in plants. Unlike animal hormones which are produced in specialised endocrine glands and transported via the bloodstream, plant hormones are synthesised in various tissues throughout the plant and act locally or at a distance through the vascular system. The five major classes of plant hormones are auxins, gibberellins, cytokinins, abscisic acid (ABA), and ethene. Each hormone can have multiple, sometimes opposing, effects depending on its concentration and the target tissue. A single hormone can promote growth in one context while inhibiting it in another： this context-dependent action is a key principle for A-Level exam questions.

植物激素，也称为植物生长物质，是调节植物生长、发育和对环境刺激反应的化学信使。与动物激素在专门的内分泌腺中产生并通过血液运输不同，植物激素在植物各组织中合成，并通过维管系统在局部或远距离发挥作用。五大类植物激素包括：生长素、赤霉素、细胞分裂素、脱落酸和乙烯。每种激素根据其浓度和靶组织，可能具有多种、有时甚至相反的作用。同一种激素在某一情境下促进生长，而在另一情境下则抑制生长：这种依赖于背景的作用是A-Level考试题目的一个关键原则。

Auxins and Cell Elongation 生长素与细胞伸长

Auxins are the most extensively studied group of plant hormones, with indole-3-acetic acid (IAA) being the most common naturally occurring auxin. Auxins are synthesised primarily in the shoot apical meristem and young leaves, then transported polarly from cell to cell in a unidirectional manner from the shoot tip downward. The primary mechanism of auxin action is the promotion of cell elongation through the acid growth hypothesis： auxin stimulates proton pumps (H+-ATPases) in the plasma membrane to pump hydrogen ions into the cell wall, lowering the pH. This acidic environment activates expansin proteins that loosen the cellulose-hemicellulose network, allowing turgor-driven cell expansion. Auxin also promotes gene transcription of proteins involved in cell wall synthesis and modification, ensuring that new wall material is added as the cell elongates.

生长素是研究最为深入的植物激素，吲哚-3-乙酸（IAA）是最常见的天然生长素。生长素主要在茎尖分生组织和幼叶中合成，然后以极性运输的方式从一个细胞单向传递到另一个细胞，从茎尖向下运输。生长素作用的主要机制是通过酸生长假说促进细胞伸长：生长素刺激质膜上的质子泵将氢离子泵入细胞壁，降低pH值。这种酸性环境激活了扩张蛋白，松解纤维素-半纤维素网络，使细胞在膨压驱动下扩张。生长素还促进了参与细胞壁合成和修饰的蛋白质的基因转录，确保在细胞伸长过程中添加新的细胞壁材料。

Phototropism： How Plants Grow Toward Light 向光性：植物如何向光生长

Phototropism is the directional growth of a plant in response to unilateral light, with shoots exhibiting positive phototropism (growing toward light) and roots typically showing negative phototropism (growing away from light). The mechanism was elucidated by the classic Went experiment and the Cholodny-Went hypothesis, which proposes that asymmetric auxin distribution drives differential growth. When light strikes one side of a coleoptile tip, auxin produced in the tip is redistributed laterally toward the shaded side. The shaded side experiences a higher concentration of auxin, which stimulates greater cell elongation compared to the illuminated side, causing the shoot to bend toward the light. Phototropins, which are blue-light photoreceptors, detect the light stimulus and trigger the redistribution of PIN auxin efflux carrier proteins to the shaded side of cells, facilitating lateral auxin transport.

向光性是植物对单侧光作出定向生长的反应，茎表现出正向光性（朝向光生长），而根通常表现出负向光性（背离光生长）。其机制由经典的Went实验和Cholodny-Went假说阐明，该假说认为不对称的生长素分布驱动了差异性生长。当光线照射到胚芽鞘尖端的一侧时，尖端产生的生长素会向背光侧重新分布。背光侧的生长素浓度较高，与向光侧相比刺激更大的细胞伸长，导致茎向光弯曲。向光蛋白是蓝光光感受器，它们检测到光刺激后触发PIN生长素外流载体蛋白向细胞的背光侧重新分布，促进生长素的侧向运输。

Gravitropism： Roots Down, Shoots Up 向地性：根向下，茎向上

Gravitropism, also known as geotropism, enables plants to orient their growth relative to gravity. Shoots exhibit negative gravitropism (growing upward against gravity) while roots show positive gravitropism (growing downward with gravity). The mechanism involves the sedimentation of starch-filled organelles called amyloplasts (statoliths) within specialised gravity-sensing cells called statocytes, located in the root cap and the endodermal starch sheath of shoots. When a root is placed horizontally, amyloplasts settle on the lower side of the statocytes, triggering a signal transduction cascade that leads to asymmetric auxin redistribution. In roots, the key difference from phototropism is the sensitivity to auxin： root cells are far more sensitive and are inhibited by auxin concentrations that would stimulate shoot cell elongation. Therefore, auxin accumulates on the lower side of the root tip, inhibiting cell elongation on that side and allowing the upper side to grow faster, bending the root downward. The PIN3 and PIN7 proteins in root statocytes relocalise to redistribute auxin laterally in response to gravity.

向地性，也称向重力性，使植物能够相对于重力定向生长。茎表现出负向地性（逆重力向上生长），而根表现出正向地性（顺重力向下生长）。其机制涉及充满淀粉的细胞器：淀粉体（平衡石）在称为平衡细胞的特化重力感应细胞中沉降，这些细胞位于根冠和茎的内胚层淀粉鞘中。当根水平放置时，淀粉体沉降到平衡细胞的下侧，触发信号转导级联反应，导致生长素的不对称重新分布。在根中，与向光性的关键区别在于对生长素的敏感度：根细胞远比茎细胞敏感，在茎中能刺激细胞伸长的生长素浓度反而会抑制根细胞的伸长。因此，生长素在根尖下侧积累，抑制该侧细胞伸长，使上侧生长更快，从而使根向下弯曲。根平衡细胞中的PIN3和PIN7蛋白在重力响应下重新定位，促进生长素的侧向运输。

Apical Dominance 顶端优势

Apical dominance is the phenomenon where the apical bud (shoot tip) suppresses the growth of lateral buds, ensuring that the plant grows upward rather than branching out. Auxin produced in the apical meristem is transported downward and inhibits the outgrowth of axillary buds positioned lower on the stem. When the apical bud is removed (decapitated), lateral buds are released from inhibition and begin to grow, producing a bushier plant. This is the basis of pruning and pinching techniques in horticulture. The mechanism of apical dominance involves not only auxin but also interactions with cytokinins and strigolactones. Auxin from the shoot tip suppresses cytokinin synthesis in the axillary buds (cytokinins promote branching), while simultaneously promoting strigolactone production, which further inhibits bud outgrowth. The auxin-cytokinin-strigolactone balance determines whether lateral buds remain dormant or begin growing.

顶端优势是指顶芽抑制侧芽生长的现象，确保植物向上生长而非分枝。在顶端分生组织产生的生长素向下运输，抑制位于茎下部的腋芽生长。当顶芽被摘除后，侧芽从抑制作用中释放出来并开始生长，使植物变得更加茂密。这就是园艺中修剪和摘心技术的原理。顶端优势的机制不仅涉及生长素，还涉及与细胞分裂素和独脚金内酯的相互作用。来自茎尖的生长素抑制腋芽中细胞分裂素的合成（细胞分裂素促进分枝），同时促进独脚金内酯的产生，进一步抑制芽的生长。生长素-细胞分裂素-独脚金内酯的平衡决定了侧芽是保持休眠还是开始生长。

Gibberellins： Stem Elongation and Seed Germination 赤霉素：茎伸长与种子萌发

Gibberellins are a large family of diterpenoid compounds that promote stem elongation, seed germination, and fruit development. The most biologically active form is gibberellic acid (GA3). Gibberellins promote stem elongation by stimulating both cell division and cell elongation： they activate the degradation of DELLA proteins, which are growth-repressing transcription regulators. Without DELLA proteins blocking gene expression, genes promoting cell wall loosening and cell cycle progression are transcribed. During seed germination, gibberellins play a critical role in mobilising stored nutrients. Following water uptake (imbibition), the embryo produces gibberellins that diffuse to the aleurone layer of the endosperm. There, gibberellins stimulate the synthesis of alpha-amylase and other hydrolytic enzymes, which break down stored starch into sugars and proteins into amino acids to fuel the growing embryo. The gibberellin-DELLA interaction also explains why dwarf varieties of crops like wheat and rice have mutations in GA biosynthesis or signalling： they were the basis of the Green Revolution.

赤霉素是一大类双萜类化合物，促进茎伸长、种子萌发和果实发育。最具生物活性的形式是赤霉酸（GA3）。赤霉素通过刺激细胞分裂和细胞伸长来促进茎伸长：它们激活DELLA蛋白的降解，DELLA蛋白是抑制生长的转录调控因子。当DELLA蛋白不再阻断基因表达时，促进细胞壁松解和细胞周期进程的基因被转录。在种子萌发过程中，赤霉素在调运储存养分方面发挥关键作用。吸水后，胚产生赤霉素，扩散到胚乳的糊粉层。在那里，赤霉素刺激α-淀粉酶和其他水解酶的合成，将储存的淀粉分解为糖，将蛋白质分解为氨基酸，为生长的胚提供能量。赤霉素-DELLA的相互作用也解释了为什么小麦和水稻等作物的矮化品种在赤霉素生物合成或信号传导中存在突变：它们是绿色革命的基础。

Cytokinins, Abscisic Acid, and Ethene 细胞分裂素、脱落酸与乙烯

Cytokinins promote cell division (cytokinesis) and are produced primarily in root tips, from where they travel upward through the xylem to shoots. They delay leaf senescence by maintaining protein and chlorophyll synthesis, which is why cytokinin sprays are used to keep cut flowers and leafy vegetables fresh. Cytokinins work antagonistically with auxins： a high auxin-to-cytokinin ratio promotes root formation in tissue culture, while a high cytokinin-to-auxin ratio promotes shoot formation. Abscisic acid (ABA) is often called the stress hormone because it mediates responses to drought, salinity, and cold. ABA triggers stomatal closure by causing guard cells to lose potassium ions and water, reducing transpiration. It also promotes seed dormancy by inhibiting germination until conditions are favourable, which is why ABA levels are high in dormant seeds and drop during stratification (cold treatment). Ethene is a gaseous hormone best known for promoting fruit ripening (climacteric fruits like bananas, apples, and tomatoes produce a burst of ethene that triggers ripening). Ethene also promotes leaf abscission (leaf fall in autumn) and the triple response in seedlings (reduced stem elongation, increased radial swelling, and horizontal growth), which helps seedlings navigate around obstacles in the soil.

细胞分裂素促进细胞分裂，主要在根尖产生，通过木质部向上运输到茎。它们通过维持蛋白质和叶绿素的合成延缓叶片衰老，这就是为什么细胞分裂素喷雾被用来保持切花和叶菜新鲜。细胞分裂素与生长素之间存在拮抗作用：在组织培养中，高生长素-细胞分裂素比促进根的形成，而高细胞分裂素-生长素比则促进芽的形成。脱落酸通常被称为胁迫激素，因为它介导对干旱、盐分和寒冷的反应。脱落酸通过使保卫细胞丧失钾离子和水分来触发气孔关闭，减少蒸腾作用。它还通过抑制萌发促进种子休眠，直到条件适宜为止，这就是为什么休眠种子中脱落酸水平高，而在层积处理（低温处理）期间下降。乙烯是一种气体激素，最为人所知的是促进果实成熟（像香蕉、苹果和番茄这类跃变型果实在成熟时会爆发乙烯）。乙烯还促进叶片脱落（秋季落叶）和幼苗的三重反应（茎伸长减少、径向膨胀增加和水平生长），这有助于幼苗绕过土壤中的障碍物。

Commercial Applications of Plant Hormones 植物激素的商业应用

Understanding plant hormones has led to numerous agricultural and horticultural applications. Synthetic auxins such as 2,4-D and NAA are widely used as selective herbicides because dicotyledonous weeds are far more sensitive to auxin overdose than monocotyledonous cereal crops, leading to uncontrolled growth and death of the weed while the crop remains unharmed. Rooting powders contain synthetic auxins (IBA or NAA) applied to the cut ends of stem cuttings to stimulate adventitious root formation, enabling vegetative propagation of ornamental and fruit plants. Gibberellin sprays are applied to seedless grape varieties to increase berry size and to sugarcane to boost stem elongation and sugar yield. Cytokinins are used in tissue culture (micropropagation) to produce large numbers of genetically identical plants, which is essential for orchid propagation and disease-free potato production. Ethene-releasing compounds such as ethephon are used to synchronise fruit ripening in commercial orchards, allowing growers to harvest an entire crop at once. Conversely, ethene inhibitors like 1-MCP (1-methylcyclopropene) are used post-harvest to block ethene receptors and extend the shelf life of fruits and vegetables during transport and storage.

对植物激素的了解已带来了大量的农业和园艺应用。合成生长素如2,4-D和萘乙酸被广泛用作选择性除草剂，因为双子叶杂草对生长素过量远比单子叶谷类作物敏感，导致杂草失控生长并死亡，而作物保持不受伤害。生根粉含有合成生长素（吲哚丁酸或萘乙酸），涂抹在茎插条的切口末端，刺激不定根的形成，使观赏植物和果树的营养繁殖成为可能。赤霉素喷雾用于无籽葡萄品种以增加果实大小，用于甘蔗以促进茎伸长和糖产量。细胞分裂素用于组织培养（微繁殖）中，大量生产基因相同的植株，这对兰花繁殖和无病马铃薯生产至关重要。释乙烯化合物如乙烯利在商业果园中用于同步果实成熟，使种植者能够一次性收获整片作物。相反，乙烯抑制剂如1-MCP（1-甲基环丙烯）在采后用于阻断乙烯受体，延长果蔬在运输和储存过程中的货架期。

Exam Tips for Plant Hormones 植物激素应试技巧

When answering exam questions on plant hormones, always distinguish between the direction of the stimulus and the direction of the growth response. For phototropism, the stimulus is unilateral light and the response is bending toward the light (positive phototropism in shoots). For gravitropism, the stimulus is gravity and the response is upward growth in shoots (negative) and downward growth in roots (positive). Be precise with terminology： IAA is a specific auxin, not synonymous with all auxins. In questions requiring explanation of mechanisms, link each step in the signalling pathway clearly： stimulus detection (phototropins for light, amyloplast sedimentation for gravity) → auxin redistribution (PIN protein relocalisation) → differential cell elongation → bending response. For practical-based questions, you should be able to describe the classic experiments： Went’s agar block experiment, Boysen-Jensen’s mica insertion experiment, and Darwin’s coleoptile experiments. Common pitfalls include confusing the effect of auxin on shoots (stimulates elongation) with its effect on roots (inhibits elongation at the same concentration) and failing to mention that auxin moves from cell to cell via polar transport, not through the xylem or phloem.

在回答关于植物激素的考试题目时，务必区分刺激的方向和生长反应的方向。对于向光性，刺激是单侧光，反应是向光弯曲（茎的正向光性）。对于向地性，刺激是重力，反应是茎的向上生长（负向地性）和根的向下生长（正向地性）。术语要准确：IAA是一种特定的生长素，并非所有生长素的同义词。在需要解释机制的题目中，清晰地连接信号通路中的每一步：刺激检测（光的光受体为向光蛋白，重力的受体为淀粉体沉降） → 生长素重新分布（PIN蛋白重新定位） → 差异性细胞伸长 → 弯曲反应。对于基于实验的题目，你应能够描述经典实验：Went的琼脂块实验、Boysen-Jensen的云母片插入实验以及Darwin的胚芽鞘实验。常见误区包括混淆生长素对茎的作用（刺激伸长）与对根的作用（同等浓度下抑制伸长），以及未能提到生长素通过极性运输在细胞间移动，而非通过木质部或韧皮部。

Key Bilingual Terms 关键双语术语

A-Level Biology Plant Hormones Tropisms Auxin