Describe the various types of tropic movements in plants. Discuss their mechanism.

Types of Tropic Movements and Their Mechanisms

Tropisms are classified based on the nature of the external stimulus causing the directional growth. They can be broadly categorized as positive (growth towards the stimulus) or negative (growth away from the stimulus).

1. Phototropism (Response to Light)

Phototropism is the growth of a plant or its parts in response to a light stimulus. Shoots typically exhibit positive phototropism, growing towards light to maximize photosynthesis, while roots often show negative phototropism, growing away from light.

• Mechanism: The primary mechanism involves photoreceptors, specifically phototropins (proteins sensitive to blue light), located in the tip of the plant (coleoptile or young shoot) [1, 14, 26].
• When light strikes a shoot unevenly, phototropins perceive the blue light, triggering a redistribution of the plant hormone auxin [14, 26].
• Auxin moves from the illuminated side to the shaded side of the shoot [15, 23].
• A higher concentration of auxin on the shaded side promotes cell elongation in that region, causing the shoot to bend towards the light source [9, 21, 23, 27].
• In roots, auxin generally inhibits cell elongation; thus, an accumulation on the shaded side would cause them to bend away from light, exhibiting negative phototropism.

2. Gravitropism (Response to Gravity)

Gravitropism, also known as geotropism, is the growth response of a plant to gravity. Roots typically show positive gravitropism, growing downwards into the soil for anchorage and water absorption, while shoots exhibit negative gravitropism, growing upwards to access sunlight [2, 3, 6].

• Mechanism: The perception of gravity occurs in specialized cells called statocytes, located in the root cap (columella cells) and shoot tips [3, 6, 25, 28].
• These cells contain dense starch-filled organelles called amyloplasts (statoliths) [3, 6, 22, 28].
• According to the Starch-Statolith Hypothesis, gravity causes these amyloplasts to sediment to the lowest side of the statocyte cells [6, 25, 28].
• This sedimentation triggers a signal transduction pathway involving calcium ions and the polar transport of auxin [6, 11, 25].
• In roots, auxin accumulates on the lower side, but at high concentrations, auxin inhibits cell elongation in roots, causing the cells on the upper side to elongate faster, bending the root downwards [3, 6, 24].
• In shoots, conversely, auxin accumulation on the lower side promotes cell elongation, leading to faster growth on the underside and bending the shoot upwards [3, 6].

3. Thigmotropism (Response to Touch/Contact)

Thigmotropism is the directional growth movement of a plant in response to touch or physical contact with a solid object. This is commonly observed in climbing plants and tendrils, which coil around support structures [5, 10, 18]. Roots also exhibit thigmotropism to navigate around obstacles in the soil [5, 18].

• Mechanism: When a plant part, such as a tendril, touches an object, mechanoreceptors in its cells detect the mechanical stimulus [10].
• This perception initiates a signal transduction pathway, often involving calcium ion influx into the cells, followed by the generation of electrical signals and secondary messengers [5, 10].
• Auxins and ethylene are plant hormones implicated in thigmotropic responses [12, 17].
• In tendrils, auxin often accumulates on the side away from the contact point, promoting differential growth. The cells on the untouched side elongate faster than those on the touched side, causing the tendril to coil around the object [5, 10, 18].
• Roots generally show negative thigmotropism, growing away from obstacles, which is also mediated by auxin redistribution [5].

4. Hydrotropism (Response to Water)

Hydrotropism is the growth response of plant roots towards a source of water or a moisture gradient [7, 16, 20]. This positive hydrotropic response is crucial for plants to find and absorb water, especially during drought conditions.

• Mechanism: The root cap senses water potential gradients in the soil [7, 24].
• While the precise molecular mechanism is still being fully elucidated, it is understood to involve signal perception in the root cap and subsequent signaling to the elongation zone of the root [7, 24].
• Plant hormones, including abscisic acid (ABA), auxins, and cytokinins, are involved in mediating hydrotropic responses [24].
• In some species, a lateral gradient of ABA or changes in ABA sensitivity across the root may play a role [24].
• Differential growth in the root's elongation zone causes it to bend towards higher water concentrations [7, 20].
• Hydrotropism can sometimes override gravitropism, directing roots horizontally or even upwards if a water source is present in that direction [7, 34].

5. Chemotropism (Response to Chemicals)

Chemotropism is the growth or movement of a plant or plant part in response to a chemical stimulus [4, 8, 29]. This allows plants to navigate towards beneficial chemicals (e.g., nutrients) and away from harmful ones.

• Mechanism: Plant cells possess receptors that detect specific chemical gradients in their environment [8].
• A prime example is pollen tube growth during fertilization in angiosperms [4, 38]. The ovule secretes chemical attractants (e.g., defensin-like peptides LURE1 and LURE2 in Arabidopsis thaliana) that guide the pollen tube towards it [8].
• This guidance involves a calcium gradient localized at the tip of the pollen tube, promoting directional elongation [4, 8].
• In roots, chemotropism directs growth towards nutrient-rich zones (positive chemotropism, e.g., nitrates, phosphates) or away from toxic substances (negative chemotropism, e.g., harmful acids) [4, 8, 29].
• Auxins and other hormones can mediate these responses by influencing differential cell growth [8, 29].