📚 Plant Hormones: Key Points for IB Biology | IB 生物：植物激素 考点精讲
Plant hormones are chemical messengers that regulate growth, development, and responses to environmental stimuli. They are produced in small amounts in one part of the plant and transported to target cells, where they trigger specific physiological responses. For IB Biology students, understanding the main types of plant hormones, their mechanisms of action, and their roles in classic experiments is essential for mastering the topic.
植物激素是调节生长、发育和对环境刺激反应的化学信使。它们在植物体的某个部位以极少量合成，并被运输至靶细胞，触发特定的生理反应。对于 IB 生物的学生而言，掌握主要植物激素的种类、作用机制及其在经典实验中的作用是学好这一主题的关键。
1. Introduction to Plant Hormones | 植物激素概述
Plant hormones, or phytohormones, share several key characteristics. They are active at very low concentrations, act locally or at a distance, and often interact synergistically or antagonistically. The major groups include auxins, gibberellins, cytokinins, abscisic acid (ABA), and ethylene.
植物激素具有几个关键特征。它们在极低浓度下即有活性，可在局部或远距离发挥作用，并且常常表现出协同或拮抗作用。主要类别包括生长素、赤霉素、细胞分裂素、脱落酸和乙烯。
These hormones regulate processes such as cell division, elongation, differentiation, apical dominance, fruit ripening, seed dormancy, and stress responses. Understanding each hormone’s specific effects is central to the IB syllabus.
这些激素调控细胞分裂、伸长、分化、顶端优势、果实成熟、种子休眠和胁迫反应等过程。理解每种激素的特定效应是 IB 大纲的核心要求。
| Hormone | Key Functions |
| Auxin (IAA) | Cell elongation, apical dominance, tropisms |
| Gibberellins | Stem elongation, seed germination |
| Cytokinins | Cell division, delay of senescence |
| ABA | Seed dormancy, stomatal closure |
| Ethylene | Fruit ripening, leaf abscission |
Table 1: Major plant hormones and their key functions.
表 1：主要植物激素及其关键功能。
2. Auxin and Cell Elongation | 生长素与细胞伸长
The most common natural auxin is indole-3-acetic acid (IAA). Auxin stimulates cell elongation in stems by loosening the cell wall. According to the acid growth hypothesis, auxin activates H⁺-ATPases in the plasma membrane, pumping protons (H⁺) into the cell wall space.
最常见的天然生长素是吲哚-3-乙酸 (IAA)。生长素通过松弛细胞壁来刺激茎中细胞的伸长。根据酸生长假说，生长素激活质膜上的 H⁺-ATP 酶，将质子 (H⁺) 泵入细胞壁间隙。
The resulting acidic pH activates expansins, proteins that break hydrogen bonds between cellulose microfibrils, allowing the cell to expand under turgor pressure. Auxin also promotes gene expression for new wall materials.
由此产生的酸性 pH 值激活了伸展蛋白，这些蛋白能断裂纤维素微纤丝之间的氢键，使细胞在膨压下得以扩展。生长素还促进新细胞壁材料相关基因的表达。
3. Phototropism: Role of Auxin | 向光性：生长素的作用
Phototropism is the growth of a plant shoot toward a light source. The Cholodny-Went hypothesis explains that unilateral light causes lateral redistribution of auxin to the shaded side of the coleoptile. A higher auxin concentration on the shaded side promotes greater cell elongation, causing the shoot to bend toward the light.
向光性是指植物茎向着光源生长。Cholodny-Went 假说认为，单侧光照导致生长素向胚芽鞘背光侧横向再分布。背光侧较高的生长素浓度促进更多的细胞伸长，使茎向光弯曲。
This was demonstrated by Went’s experiment, in which auxin collected on agar blocks from illuminated coleoptile tips caused curvature when placed asymmetrically on decapitated coleoptiles. The experiment confirmed that a diffusible chemical (auxin) promotes elongation.
温特实验证明了这一点：从光照下的胚芽鞘尖端收集在琼脂块上的生长素，当不对称地放置在去尖的胚芽鞘上时引起了弯曲。该实验证实了可扩散的化学物质（生长素）促进伸长。
4. Gravitropism: Roots and Shoots | 向地性：根与茎
Gravitropism is directional growth in response to gravity. In a horizontal root, auxin accumulates on the lower side, but root cells are more sensitive to auxin: high auxin inhibits cell elongation, so the lower side grows more slowly, and the root bends downward. In shoots, high auxin stimulates elongation, so the shoot bends upward.
向地性是对重力产生的定向生长反应。在水平放置的根中，生长素积累在下侧，但根细胞对生长素更敏感：高浓度生长素抑制细胞伸长，因此下侧生长较慢，根向下弯曲。在茎中，高浓度生长素促进伸长，茎向上弯曲。
Statoliths (dense amyloplasts) in root cap cells settle under gravity and trigger the redistribution of auxin transporters (PIN proteins), leading to asymmetric auxin distribution.
根冠细胞中的平衡石（致密的淀粉体）在重力作用下沉降，触发生长素转运蛋白 (PIN) 的重新分布，导致生长素不对称分布。
5. Auxin and Apical Dominance | 生长素与顶端优势
Apical dominance is the phenomenon whereby the shoot apex inhibits the growth of lateral buds. The terminal bud produces auxin, which travels down the stem and suppresses axillary bud outgrowth. Removing the apical bud (decapitation) eliminates this inhibition, allowing lateral branches to develop.
顶端优势是指茎端抑制侧芽生长的现象。顶芽产生生长素，沿茎向下运输并抑制腋芽的萌发。去除顶芽（打顶）可消除这种抑制，使侧枝得以发育。
Cytokinins, produced in roots, act antagonistically to promote bud growth. The balance between auxin (inhibitor) and cytokinin (promoter) determines branching pattern.
根中产生的细胞分裂素则起拮抗作用，促进芽生长。生长素（抑制剂）与细胞分裂素（促进剂）之间的平衡决定了分枝模式。
6. Gibberellins: Stem Elongation and Seed Germination | 赤霉素：茎伸长与种子萌发
Gibberellins (GA) are a group of hormones that promote stem elongation, particularly by stimulating cell division and elongation in internodes. Dwarf varieties of plants often lack functional GA synthesis, and applying gibberellins can restore normal height.
赤霉素是一类促进茎伸长的激素，特别是通过刺激节间的细胞分裂和伸长。矮生植物品种通常缺乏功能性 GA 合成，施用赤霉素可恢复其正常高度。
In seed germination, gibberellins are released from the embryo and diffuse into the aleurone layer, where they trigger the synthesis of α-amylase. This enzyme hydrolyzes stored starch into maltose, providing energy for the growing embryo.
在种子萌发过程中，赤霉素从胚中释放并扩散到糊粉层，在那里触发 α-淀粉酶的合成。该酶将储存的淀粉水解为麦芽糖，为生长的胚胎提供能量。
7. Cytokinins and Cell Division | 细胞分裂素与细胞分裂
Cytokinins are hormones that promote cell division (cytokinesis) and differentiation. They are produced in root tips and transported upward through the xylem. Cytokinins work in tandem with auxin; together they regulate organogenesis in tissue culture.
细胞分裂素是促进细胞分裂（胞质分裂）和分化的激素。它们在根尖产生，通过木质部向上运输。细胞分裂素与生长素协同作用，共同调控组织培养中的器官发生。
A high auxin:cytokinin ratio induces root formation, whereas a high cytokinin:auxin ratio promotes shoot formation. Cytokinins also delay leaf senescence by promoting protein synthesis and chlorophyll retention.
高生长素与细胞分裂素之比诱导根的形成，而高细胞分裂素与生长素之比促进芽的形成。细胞分裂素还通过促进蛋白质合成和保持叶绿素来延缓叶片衰老。
8. Abscisic Acid and Stress Responses | 脱落酸与胁迫反应
Abscisic acid (ABA) is often called the ‘stress hormone’ because it mediates responses to drought and other stresses. It inhibits growth and promotes seed dormancy. During water deficit, ABA accumulates in leaves and causes stomatal closure by triggering the loss of K⁺ from guard cells, reducing water loss by transpiration.
脱落酸常被称为“胁迫激素”，因为它介导对干旱和其他胁迫的反应。它抑制生长、促进种子休眠。在水分亏缺期间，ABA 在叶片中积累，通过触发保卫细胞排出 K⁺ 导致气孔关闭，减少蒸腾失水。
ABA also maintains seed dormancy by inhibiting germination until conditions are favorable. Its levels decline after cold treatment or imbibition, allowing gibberellins to promote germination.
ABA 还通过抑制萌发来维持种子休眠，直到条件适宜。在低温处理或吸胀后 ABA 水平下降，让赤霉素得以促进萌发。
9. Ethylene and Fruit Ripening | 乙烯与果实成熟
Ethylene is a gaseous plant hormone that promotes fruit ripening, leaf abscission, and senescence. Climacteric fruits, such as bananas and tomatoes, show a burst of ethylene production that coordinates ripening. Ethylene triggers the conversion of starch to sugar, softening of cell walls, and color changes.
乙烯是一种气体植物激素，促进果实成熟、叶片脱落和衰老。跃变型果实，如香蕉和番茄，会表现出乙烯释放高峰，协调成熟过程。乙烯促使淀粉转化为糖、细胞壁软化和颜色变化。
Ethylene also induces the ‘triple response’ in etiolated seedlings: reduced stem elongation, thickening of stem, and horizontal growth. This response helps seedlings overcome mechanical barriers in the soil.
乙烯还诱导黄化幼苗的“三重反应”：茎伸长减少、茎增粗和水平生长。这一反应有助于幼苗克服土壤中的机械障碍。
10. Commercial Applications and Micropropagation | 商业应用与微繁殖
Plant hormones are widely used in agriculture and horticulture. Auxin-based rooting powders promote adventitious root formation on stem cuttings. Gibberellins are sprayed on seedless grapes to increase size and cluster elongation. Ethylene is used to synchronise fruit ripening in storage.
植物激素广泛应用于农业和园艺。基于生长素的生根粉促进茎插条上不定根的形成。赤霉素喷施于无籽葡萄以增大果粒和拉长果穗。乙烯用于贮藏过程中的同步催熟。
In micropropagation (tissue culture), explants are grown on agar media containing specific ratios of auxin and cytokinin to generate shoots and roots. This technique allows rapid clonal propagation of disease-free plants.
在微繁殖（组织培养）中，外植体在含有特定比例生长素和细胞分裂素的琼脂培养基上生长，生成芽和根。该技术可实现无病植株的快速克隆繁殖。
11. Auxin’s Mechanism of Action: Gene Regulation | 生长素的作用机制：基因调控
Auxin acts by regulating gene expression. In the absence of auxin, Aux/IAA repressor proteins bind to auxin response factors (ARFs) and inhibit transcription of auxin-responsive genes. When auxin is present, it binds to the TIR1 receptor, part of an E3 ubiquitin ligase complex.
生长素通过调控基因表达发挥作用。无生长素时，Aux/IAA 抑制蛋白与生长素响应因子 (ARF) 结合，抑制生长素响应基因的转录。当生长素存在时，它与 TIR1 受体结合，后者是 E3 泛素连接酶复合物的组成部分。
This binding triggers ubiquitination and proteasomal degradation of Aux/IAA repressors, freeing ARFs to activate transcription. This de-repression mechanism allows rapid and specific responses to auxin.
这一结合触发了 Aux/IAA 抑制蛋白的泛素化及蛋白酶体降解，释放 ARF 激活转录。这种去抑制机制使得对生长素的反应快速而特异。
12. Classic Experiments on Phototropism | 向光性经典实验回顾
Darwin and his son Francis Darwin (1880) showed that the tip of the coleoptile perceives light, since decapitated coleoptiles or those with tips covered failed to bend, while those with bases covered still bent. Boysen-Jensen (1913) demonstrated that a chemical signal passes from the tip through gelatin but not mica.
达尔文父子 (1880) 证明胚芽鞘尖端感知光，因为去尖或尖端被遮盖的胚芽鞘不弯曲，而基部被遮盖的仍能弯曲。Boysen-Jensen (1913) 证明化学信号可通过明胶从尖端传递，但不能通过云母片。
Went (1926) isolated this chemical signal by collecting it on agar blocks and showing that placing blocks asymmetrically on decapitated coleoptiles induced curvature, even in darkness. These experiments form the foundation of our understanding of auxin-mediated tropisms and are frequently examined in IB Biology.
Went (1926) 通过琼脂块收集了这一化学信号，并证明即使在黑暗中，将琼脂块不对称地放置在去尖胚芽鞘上也能引起弯曲。这些实验奠定了我们对生长素介导向性作用理解的基础，也是 IB 生物中常考内容。
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