How Auxins Help Plants Climb and Grow: The Science of Tendrils

Tendrils are slender, coiled structures that some plants use to climb and attach to supports. They can be modified stems, leaves, or leaflets, and they can be found in many plants, such as peas, grapes, cucumbers, and passionflowers. But how do tendrils sense and respond to the presence of a support? And what role do plant hormones play in this process? In this article, we will explore how auxins, a type of plant hormone and how do auxins promote the growth of a tendril around a support.

 Introduction

Plants are sessile organisms, meaning they cannot move from place to place. However, some plants have evolved ways to overcome this limitation and expand their range and access to resources. One of these ways is by climbing and twining around other plants or structures, such as trellises, fences, or walls. This allows them to reach higher levels of light, avoid competition, and disperse their seeds more widely.

To achieve this, some plants have developed tendrils, which are specialized organs that can sense and grasp a support. Tendrils are usually thin, flexible, and sensitive to touch. When they come in contact with a suitable support, they curl around it and hold on tightly. This helps the plant to anchor itself and grow upwards or sideways.

But how do tendrils know when and where to coil? And what makes them grow faster or slower on different sides? The answer lies in the action of auxins, a group of plant hormones that regulate growth and development.

 What are auxins and how do they work?

Auxins are a class of plant hormones that are mainly produced in the shoot tips, where they stimulate cell elongation and division. Auxins can also move from one part of the plant to another, creating a gradient of concentration. This gradient affects the direction and rate of growth, as well as the response to external stimuli, such as light, gravity, and touch.

Auxins work by influencing the activity of proton pumps, which are enzymes that transport hydrogen ions across the cell membrane. This creates an acidification of the cell wall, which loosens the bonds between the cellulose fibers and makes the cell wall more flexible and elastic. This allows the cell to expand and elongate under the pressure of the vacuole, which is a large sac filled with water and solutes inside the cell.

The effect of auxins on cell growth depends on the concentration and distribution of the hormone. In general, low concentrations of auxins stimulate cell elongation, while high concentrations inhibit it. Moreover, auxins tend to accumulate on the shaded or lower side of the plant, creating an asymmetrical distribution. This causes the cells on that side to grow faster than the cells on the opposite side, resulting in a bending or curvature of the plant. This is how plants respond to light (phototropism) and gravity (gravitropism).

 How do auxins promote the growth of a tendril around a support?

Tendrils are also sensitive to touch (thigmotropism), and they use auxins to adjust their growth and coiling around a support. When a tendril touches a support, the cells on the contact side produce more auxins, which are then transported to the opposite side. This creates a higher concentration of auxins on the far side of the tendril, which stimulates faster growth of those cells. This causes the tendril to bend and curl around the support, forming a helix.

The coiling of the tendril also reduces the tension on the stem, which allows the plant to grow faster and taller. Moreover, the coiling of the tendril increases the stability and strength of the attachment, as the helix can resist both pulling and twisting forces.

The coiling of the tendril is also influenced by the diameter and shape of the support. The smaller the diameter, the tighter the coil. The shape of the support also affects the angle and direction of the coil. For example, a tendril will coil clockwise around a square support, but counterclockwise around a triangular support.

 Conclusion

Tendrils are remarkable adaptations that allow some plants to climb and grow. They are controlled by auxins, a type of plant hormone that regulates growth and response to stimuli. Auxins promote the growth and coiling of tendrils around a support by creating an uneven distribution of the hormone, which affects the rate of cell elongation on different sides of the tendril. This helps the plant to anchor itself and reach higher levels of light and resources.