A7 Disturbance of Tropisms

Topic

Under normal conditions, plant tropisms and movements rely on a two-stage process: first, the plant detects an orienting physical signal—such as light, gravity, or mechanical contact—and subsequently, a response mechanism translates that detected signal into an actual reorientation of growth or position. For instance, polar auxin transport moves the hormone auxin to the side of the plant destined for greater growth, thereby generating tropisms; meanwhile, changes in pulvinus turgor alter internal pressure to fold or unfold leaves and petioles, thus producing movements. Plant tropisms and movements are disrupted if either of these stages fails: either the orienting physical signal is anomalous, or the response mechanism translating that signal into growth or movement is dysfunctional.

When the orienting physical signal itself is anomalous, the root of the problem lies in signal detection: a mutation in photoreceptors—such as phototropins or phytochromes—prevents the plant from correctly detecting light signals, while microgravity eliminates or distorts the gravity signal the plant normally uses to orient its growth. In both instances, the physical signal intended to guide tropisms arrives altered or fails to arrive at all, leaving the dependent response mechanism without a correct signal to translate into a reorientation of growth.

Conversely, when the response mechanism is dysfunctional, the problem arises even if the physical signal is detected correctly: hormonal imbalances—such as an excess of ethylene or an auxin deficiency—directly alter polar auxin transport; similarly, a mutation in PIN proteins (which transport auxin between cells within the plant) disrupts polar auxin transport, even when the physical signal remains intact. Similarly, mechanical damage to the pulvinus directly alters its turgor; consequently, even if the plant correctly detects the mechanical contact signal, the pulvinus can no longer translate that signal into the internal pressure change that normally produces movement.

Ultimately, plant tropisms and movements are disrupted via two distinct pathways that yield the same final result: when the physical signal—whether light, gravity, or mechanical contact—is anomalous, the response mechanism has nothing to translate; conversely, when polar auxin transport or pulvinus turgor becomes dysfunctional, the response mechanism fails to translate even a correct physical signal. In both instances, the plant loses the ability to reorient its growth through tropisms or to reposition its leaves and petioles through movements.

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