📚 Plant Hormones in CCEA A-Level Biology | CCEA A-Level 生物：植物激素 考点精讲
Plant hormones are chemical messengers that regulate growth, development, and responses to environmental stimuli. Unlike animals, plants do not have a nervous system, so these signalling molecules coordinate everything from seed germination to leaf fall. For CCEA A-Level Biology, you need to understand the major plant hormone groups, their sites of production, modes of action, and the experimental evidence that underpins our knowledge. This revision guide walks you through each key hormone, linking molecular mechanisms to classic experiments such as those by Darwin and Went.
植物激素是调控生长、发育以及对环境刺激响应的化学信使。与动物不同，植物没有神经系统，因此这些信号分子协调着从种子萌发到叶片脱落的每一个过程。对于 CCEA A-Level 生物学，你需要掌握主要植物激素类别、它们的合成部位、作用方式以及支撑我们认知的实验证据。本复习指南将带你逐一梳理每种关键激素，将分子机制与达尔文、温特等经典实验联系起来。
1. Introduction to Plant Hormones | 植物激素简介
Plant hormones, also known as plant growth regulators, are organic compounds produced in one part of the plant and transported to another, where they elicit a response at very low concentrations. They can be stimulatory or inhibitory depending on the target tissue, hormone concentration, and developmental stage. The major groups examined by CCEA include auxins (e.g. indole-3-acetic acid, IAA), gibberellins, cytokinins, abscisic acid (ABA), and ethene. Each has distinct roles, but they often interact synergistically or antagonistically to fine-tune plant physiology.
植物激素，又称植物生长调节剂，是在植物某一部分合成并被运输到另一部分、在极低浓度下即可引发响应的有机化合物。根据靶组织、激素浓度和发育阶段的不同，它们既可以起促进作用也可以起抑制作用。CCEA 考察的主要类别包括生长素（例如吲哚-3-乙酸，IAA）、赤霉素、细胞分裂素、脱落酸（ABA）和乙烯。每种激素各有独特的功能，但它们常常协同或拮抗地相互作用，以精细调节植物生理过程。
2. Auxins (IAA) and Phototropism | 生长素（IAA）与向光性
Indole-3-acetic acid (IAA) is the most important naturally occurring auxin. It is synthesised in shoot apical meristems, young leaves, and developing seeds. IAA promotes cell elongation by increasing the plasticity of the cell wall. During phototropism, unilateral light causes IAA to redistribute to the shaded side of the coleoptile or shoot tip. This higher concentration stimulates more rapid elongation on the shaded side, causing the shoot to bend towards the light.
吲哚-3-乙酸（IAA）是最重要的天然生长素。它在茎顶端分生组织、幼叶和发育中的种子里合成。IAA 通过增加细胞壁的可塑性来促进细胞伸长。在向光性中，单侧光照导致 IAA 在胚芽鞘或茎尖的背光侧重新分配。较高浓度的 IAA 刺激背光侧细胞更快速地伸长，使茎朝光源弯曲。
Darwin’s experiments with grass coleoptiles provided the first evidence. When the tip was removed or covered with an opaque cap, no bending occurred. This showed that the tip perceives the light stimulus and transmits a chemical signal downwards. Boysen-Jensen later showed that inserting a permeable gelatin block between the tip and the growing zone allowed bending, whereas an impermeable mica sheet did not. Finally, Went isolated the growth-promoting chemical in agar blocks and demonstrated that placing agar blocks asymmetrically on decapitated coleoptiles caused curvature even in darkness.
达尔文的禾本科胚芽鞘实验提供了第一项证据。当切去尖端或用不透明帽遮盖尖端时，植物不发生弯曲。这表明尖端感知光刺激并向下传递化学信号。博伊森-延森后来证明，在尖端和生长区之间插入可渗透的明胶片允许弯曲，而不可渗透的云母片则不能。最终，温特从琼脂块中分离出促进生长的化学物质，并证明将琼脂块不对称地放在去顶胚芽鞘上，即使在黑暗中也能引起弯曲。
3. Auxin and Gravitropism | 生长素与向地性
Gravitropism (geotropism) is a growth response to gravity. In roots, high concentrations of IAA inhibit cell elongation, whereas lower concentrations promote elongation. When a root is placed horizontally, IAA accumulates on the lower side due to gravity. In roots, this high concentration suppresses growth on the lower side, causing the upper side to elongate more, and the root bends downwards. In shoots, the same accumulation on the lower side promotes elongation, so the shoot bends upwards.
向地性是对重力的生长响应。在根中，高浓度的 IAA 抑制细胞伸长，而较低浓度促进伸长。当根水平放置时，由于重力作用，IAA 在下侧积累。在根中，这种高浓度抑制下侧生长，导致上侧细胞伸长更多，根向下弯曲。在茎中，同样的 IAA 在下侧积累促进伸长，因此茎向上弯曲。
The mechanism involves the differential transport of IAA, mediated by PIN efflux carrier proteins. In roots, the PIN proteins are positioned differently, resulting in a higher IAA concentration in the lower epidermis. In shoots, the redistribution also leads to more IAA on the lower side. The contrasting sensitivity explains the opposite bending directions. Starch-filled amyloplasts called statoliths are believed to play a part in sensing gravity by settling on the lower side of root cap cells, influencing PIN protein distribution.
这一机制涉及由 PIN 外排载体蛋白介导的 IAA 不均等运输。在根中，PIN 蛋白的定位不同，导致下表皮中 IAA 浓度更高。在茎中，重新分配同样导致下侧 IAA 更多。敏感性的差异解释了相反的弯曲方向。充满淀粉的造粉体（称为平衡石）被认为在根冠细胞的下侧沉降，影响 PIN 蛋白分布，从而感知重力。
4. Auxin and Apical Dominance | 生长素与顶端优势
Apical dominance is the suppression of lateral bud growth by the actively growing shoot apex. The apex produces IAA, which is transported basipetally down the stem. High auxin concentration in the region of the lateral buds inhibits their outgrowth. If the shoot tip is removed (decapitated), auxin levels near the buds decline, and the lateral buds are released from inhibition, leading to branching.
顶端优势是活跃生长的茎尖抑制侧芽生长的现象。茎尖产生 IAA，并向基向下运输。侧芽区域的高浓度生长素抑制其萌发。如果去除茎尖（去顶），芽附近的生长素水平下降，侧芽的抑制被解除，从而产生分枝。
The interaction with cytokinins is crucial. Cytokinins produced in roots counteract auxin’s inhibitory effect. When the tip is removed, the ratio of cytokinin to auxin increases in favour of lateral bud growth. Applying exogenous cytokinins to axillary buds can override apical dominance. This shows that plant responses are rarely the result of a single hormone but depend on hormonal balance.
与细胞分裂素的相互作用至关重要。根中产生的细胞分裂素能抵消生长素的抑制作用。当茎尖去除后，细胞分裂素与生长素的比例上升，有利于侧芽生长。对外生侧芽施加外源细胞分裂素可以解除顶端优势。这表明植物反应很少是单一激素的作用，而是取决于激素间的平衡。
5. Gibberellins and Seed Germination | 赤霉素与种子萌发
Gibberellins are a large family of plant hormones, with gibberellic acid (GA3) being the most commonly studied. They are produced in young shoots, embryos, and roots. A major role in CCEA is the promotion of seed germination in species such as barley. Following water uptake (imbibition), the embryo releases gibberellins, which diffuse to the aleurone layer of the endosperm.
赤霉素是一个庞大的植物激素家族，赤霉酸（GA3）是最常被研究的一种。它们在幼茎、胚和根中合成。在 CCEA 考试中，一个主要的作用是促进大麦等种子的萌发。在吸水（吸胀）后，胚释放赤霉素，赤霉素扩散到胚乳的糊粉层。
Gibberellins trigger the transcription of the gene coding for α-amylase. This enzyme hydrolyses starch stored in the endosperm into maltose and then glucose, providing an energy source for the growing embryo. The classic experiment using de-embryonated barley grains demonstrated that only when gibberellin was supplied did α-amylase production occur; without the embryo or exogenous gibberellin, no enzyme was produced.
赤霉素触发编码 α-淀粉酶的基因转录。该酶将储存在胚乳中的淀粉水解为麦芽糖，进而转化为葡萄糖，为生长的胚提供能量来源。利用去胚大麦籽粒的经典实验表明，只有在提供赤霉素时，才会产生 α-淀粉酶；如果没有胚或外源赤霉素，便不会产生该酶。
6. Abscisic Acid (ABA) and Seed Dormancy | 脱落酸与种子休眠
Abscisic acid (ABA) is often called the stress hormone because it is involved in the response to drought, cold, and salinity. In CCEA, its main role concerns seed dormancy and inhibiting germination. ABA accumulates in developing seeds, preventing precocious germination. During seed maturation, a high ABA:gibberellin ratio maintains dormancy and promotes the synthesis of dehydration-tolerant proteins and protective LEA proteins.
脱落酸（ABA）常被称为胁迫激素，因为它参与对干旱、低温和盐分的响应。在 CCEA 中，其主要作用涉及种子休眠和抑制萌发。ABA 在发育中的种子中积累，防止过早萌发。在种子成熟过程中，高 ABA：赤霉素比率维持休眠，并促进耐脱水蛋白和保护性 LEA 蛋白的合成。
For germination to proceed, ABA levels must decline or the tissue must become less sensitive. This often occurs through chilling (stratification) or light (photoblastic seeds). The antagonistic interaction between ABA and gibberellins controls the dormancy–germination switch. In some species, gibberellin does not simply override ABA; rather, the balance between the two determines which developmental programme is activated.
要启动萌发，ABA 水平必须下降或组织必须变得不再敏感。这通常通过低温（层积处理）或光照（光敏种子）来实现。ABA 与赤霉素之间的拮抗作用控制着休眠-萌发的转换。在某些物种中，赤霉素并非简单地压倒 ABA；相反，两者之间的平衡决定了哪个发育程序被激活。
7. Cytokinins | 细胞分裂素
Cytokinins are plant hormones that primarily promote cell division (cytokinesis) and differentiation. They are synthesised in root tips and transported via the xylem to shoots. Cytokinins work in conjunction with auxins to regulate the growth of plant organs. In tissue culture, a high cytokinin:auxin ratio promotes shoot formation, whereas a low ratio favours root formation; an intermediate ratio produces an undifferentiated callus.
细胞分裂素是主要促进细胞分裂（胞质分裂）和分化的植物激素。它们在根尖合成，并通过木质部运输到地上部分。细胞分裂素与生长素协同作用，调控植物器官的生长。在组织培养中，高细胞分裂素：生长素比率促进芽形成，而低比率有利于根形成；中等比率则产生未分化的愈伤组织。
Cytokinins also delay senescence (ageing) in leaves. They reduce the breakdown of chlorophyll and proteins, and they can promote the movement of nutrients towards the treated area. This has applications in keeping cut flowers and leafy vegetables fresh. The effect is partly due to the activation of genes associated with nutrient mobilisation and chloroplast maintenance.
细胞分裂素还能延缓叶片的衰老。它们减少叶绿素和蛋白质的分解，并能促进营养物质向处理区域移动。这在切花和叶菜保鲜中有实际应用。这一效应部分归因于与营养动员和叶绿体维持相关的基因激活。
8. Ethene | 乙烯
Ethene (ethylene) is a gaseous plant hormone involved in fruit ripening, leaf abscission, and the response to mechanical stress. In climacteric fruits such as bananas, tomatoes, and apples, a burst of ethene production triggers a coordinated ripening process, including softening, starch breakdown, and colour change. Ethene stimulates its own biosynthesis in an autocatalytic loop.
乙烯是一种气态植物激素，参与果实成熟、叶片脱落以及对机械胁迫的响应。在呼吸跃变型果实如香蕉、番茄和苹果中，乙烯产生的高峰触发协调的成熟过程，包括软化、淀粉分解和颜色变化。乙烯通过自催化环刺激其自身的生物合成。
In leaf abscission, ethene stimulates the activity of enzymes such as cellulase and pectinase in the abscission zone, weakening cell walls and causing the leaf to detach. The delicate balance between auxin and ethene determines whether a leaf is retained or shed. High auxin levels from a healthy leaf suppress ethene production, but when auxin levels fall, ethene sensitivity increases, leading to senescence and abscission.
在叶片脱落中，乙烯刺激离层中纤维素酶和果胶酶等酶的活性，削弱细胞壁，导致叶片脱落。生长素与乙烯之间的微妙平衡决定叶片是保留还是脱落。来自健康叶片的高水平生长素抑制乙烯产生，但当生长素水平下降时，乙烯敏感性增加，导致衰老和脱落。
9. Synergism and Antagonism | 激素间的协同与拮抗
Rarely does a single hormone operate in isolation. CCEA expects you to appreciate the interactions. For instance, the balance between auxin and cytokinin determines apical dominance and organogenesis in tissue culture. The ABA:gibberellin ratio governs seed dormancy and germination. Auxin and gibberellin can act synergistically on stem elongation, but gibberellin does not cause bending in phototropism—that is specific to auxin redistribution.
植物激素极少单独起作用。CCEA 要求你理解它们之间的相互作用。例如，生长素和细胞分裂素之间的平衡决定顶端优势和组织培养中的器官发生。ABA 与赤霉素的比率控制种子休眠和萌发。生长素和赤霉素可以协同促进茎的伸长，但赤霉素不会引起向光性弯曲——那是生长素重新分配的特有反应。
Another example is the antagonism between ABA and gibberellins during germination: ABA promotes dormancy by activating transcription factors that repress gibberellin-responsive genes. When environmental conditions are favourable, gibberellin synthesis increases, tipping the balance towards germination. Understanding these cross-talks is vital for answering extended questions on hormone mechanism integration.
另一个例子是萌发过程中 ABA 与赤霉素的拮抗：ABA 通过激活抑制赤霉素响应基因的转录因子来促进休眠。当环境条件适宜时，赤霉素合成增加，将平衡导向萌发。理解这些交互作用对于回答有关激素机制整合的延伸题至关重要。
10. Experimental Investigations | 实验探究
CCEA often tests understanding of the design and interpretation of classical plant hormone experiments. Key examples include Darwin’s coleoptile tip removal, Boysen-Jensen’s mica/gelatin insertions, and Went’s agar block curvature test. Other important experiments are the split-root system for cytokinin transport and the use of GA-deficient mutants in barley germination studies.
CCEA 常考察对经典植物激素实验设计与解释的理解。关键示例包括达尔文的胚芽鞘去尖实验、博伊森-延森的云母/明胶插入试验以及温特的琼脂块弯曲实验。其他重要实验还有细胞分裂素运输的分根系统和大麦萌发研究中赤霉素缺陷突变体的使用。
Modern techniques include using reporter genes such as the DR5 promoter fused to GUS or fluorescent proteins to visualise auxin distribution in real time. Mutants defective in hormone synthesis or signalling (e.g. the gibberellin-insensitive gai mutant) further confirm hormone roles. These methods provide quantitative data that support the historical conclusions.
现代技术包括使用报告基因，如与 GUS 或荧光蛋白融合的 DR5 启动子，实时可视化生长素分布。激素合成或信号传导缺陷的突变体（如赤霉素不敏感 gai 突变体）进一步确证了激素功能。这些方法提供了支持历史结论的定量数据。
11. Agricultural and Horticultural Applications | 农业和园艺应用
Knowledge of plant hormones has direct practical uses. Auxins are employed as rooting powders for cuttings, often as synthetic analogues such as IBA (indole-3-butyric acid) or NAA (naphthaleneacetic acid). They are also used to produce seedless fruits (parthenocarpy) and as selective herbicides; 2,4-D is a synthetic auxin that selectively kills broad-leaved dicotyledons in cereal crops.
植物激素知识具有直接的实际用途。生长素被用作插条的促根粉，通常是合成类似物如 IBA（吲哚丁酸）或 NAA（萘乙酸）。它们还用于生产无籽果实（单性结实）和作为选择性除草剂；2,4-D 是一种合成生长素，可选择性地杀灭谷类作物中的阔叶双子叶杂草。
Gibberellins are sprayed on seedless grape varieties to increase berry size, used to delay ripening and senescence in citrus, and in the malting industry to speed up α-amylase production. Ethene-releasing compounds such as ethephon are applied to promote uniform ripening in tomatoes and facilitate fruit harvesting by loosening fruit stalks. Anti-ethene agents (e.g. 1-MCP) are used to increase the shelf life of cut flowers and stored fruit.
赤霉素被喷洒到无籽葡萄品种上以增大果粒，用于延缓柑橘的成熟和衰老，并在麦芽制造中加速 α-淀粉酶的产生。释放乙烯的化合物如乙烯利被用于促进番茄均匀成熟，并通过松动果梗便于采收。抗乙烯制剂（如 1-MCP）用于延长切花和储藏水果的货架期。
12. Summary and Key Points | 考点总结
Plant hormones are low-molecular-weight signalling molecules coordinating growth and responses. IAA governs phototropism, gravitropism, and apical dominance via differential distribution and tissue sensitivity. Gibberellins trigger germination by inducing α-amylase, while ABA antagonises this to maintain dormancy. Cytokinins promote cell division, delay senescence, and interact with auxin to determine organogenesis. Ethene drives ripening and abscission. CCEA questions frequently interlink these hormones, so never consider a process in isolation. Be ready to cite experimental evidence and apply your knowledge to novel scenarios.
植物激素是协调生长和响应的低分子量信号分子。IAA 通过区别分布和组织敏感性调控向光性、向地性和顶端优势。赤霉素通过诱导 α-淀粉酶触发萌发，而 ABA 则与之拮抗以维持休眠。细胞分裂素促进细胞分裂、延缓衰老，并与生长素相互作用决定器官分化。乙烯驱动成熟和脱落。CCEA 考题常将这些激素交叉联系，因此绝不要孤立地考虑某个过程。准备好引用实验证据，并将知识应用到新的情境中。
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