Higher plants spend their sedentary lives at the site of their germination. Antithetically, the movement of plant organs has attracted a lot of attention. Tropism is a form of plant movement that has been studied for nearly two centuries. Conceptually, the tropic response in higher plants can be regarded as a sequential process comprising stimulus reception, conversion of the environmental signal to a biochemical one (signal transduction), transmission of the signal to the responsive tissues and organ bending. These steps are not necessarily distinct and can be continuous and overlapping. Our understanding of the genetic and molecular aspects of the mechanisms underlying this process has gradually grown during the past two decades, largely based on studies on the model plant, Arabidopsis. The mechanisms of tropism, with a particular focus on phototropism and gravitropism, are introduced.

Key Concepts:

  • Plant organs can show several types of movement, although plants have sedentary lives.

  • Tropism is a directional movement that is strongly related to the direction of stimulus.

  • Plant organs can sense various vectorial stimuli such as light, gravity, moisture and touch.

  • In the case of gravitropism and hydrotropism, sensing cells has been clarified.

  • In the case of phototropism, it is unlikely whether specific sensing cells exist but likely that a lot of cells sense the light signal and exhibit coordinated growth response in the organ.

  • The photoreceptors for phototropism have been identified.

  • Auxin, one of plant hormones, is vital not only for regulation of photo‐ and gravitropism, but also for general plant growth and development.

  • In the case of gravitropism and phototropism, auxin is unevenly distributed within the organ after perception of directional stimulus, resulting in organ bending.

Keywords: phototropism; gravitropism; hydrotropism; thigmotropism; nasty; photoreceptor; auxin; statolith; amyloplast

Figure 1.

Phototropic response and photoreceptor. (a) Schematic illustration indicates phototropic response of a dicot seedling. Unilateral blue‐light irradiation induces positive phototropism of its hypocotyl. Orange colour indicates differential auxin accumulation within the hypocotyl. Auxin is accumulated at the shaded side. In shoot including the hypocotyl, accumulated auxin in the response of blue light promotes cell elongation leading to asymmetric organ growth towards the direction of blue light. (b) Schematic illustration for the regulation mechanism of kinase by LOV domains. A phot protein has two LOV domains (LOV1 and LOV2) at the N‐terminal side and a (PKD) at the C‐terminal region. Diamonds attached to LOV domains represent chromophores. In the dark, each LOV domain binds noncovalently to a chromophore for reception of blue light. LOV2 domain inhibits the phosphorylation activity of PKD by binding (upper panel). Once blue light is irradiated, chromophores covalently bind to LOV domains followed by the release of the LOV2 domain from PKD (lower panel). Then, phosphorylation activity of PKD is induced, resulting in autophosphorylation. P with a red lollipop indicates phosphate group.

Figure 2.

Schematic model for auxin signalling. (a) ARFs bind to AuxRE in the promoters of auxin‐responsive genes. AuxRE is a cis‐element conserved in regulatory regions of a series of genes, expression of which is induced by auxin. ARFs are transcriptional activator in most cases. At low concentration of auxin, Aux/IAAs are stabilised and bind ARFs to inhibit ARF‐mediated gene transcription. (b) When auxin concentration is increased, ubiquitin ligase complex SCFTIR binds to auxin molecule. And then Aux/IAA proteins are trapped by the SCFTIR containing auxin molecule and are polyubiquitinylated. Finally, polyubiquitinylated Aux/IAA proteins become targets of poteasome‐mediated degradation. ARFs become free from Aux/IAAs and can induce the transcription of auxin‐responsive genes.

Figure 3.

Gravitropic response. Schematic illustration indicates gravitropic response of a root. Roots change its growth direction towards the direction of gravity (positive gravitropism). The columella cells in the root cap contain amyloplasts sedimenting to the direction of gravity. Brown dots indicate amyloplasts. Orange colour indicates differential auxin accumulation within the root. Auxin is accumulated at the lower flank of the root. In root, accumulated auxin upon gravi‐stimulation inhibits cell elongation leading to asymmetric organ growth towards the direction of gravity.

Figure 4.

PIN localisation and auxin flow in Arabidopsis root tip. Intracellular localisation of PIN proteins is schematically indicated. To simplify the explanation, several suggested auxin flows and PIN localisation involved in the flow are not indicated in the cartoon and the text. Red; PIN1, green; PIN2, orange; PIN3 or PIN7, purple; PIN4. Blue arrows indicate auxin flow.


Further Reading

Esmon CA, Pedmale UV and Liscum E (2005) Plant tropisms: providing the power of movement to a sessile organism. The International Journal of Developmental Biology 49: 665–674.

Gilroy S and Masson PH (eds) (2008) Plant Tropisms. Oxford: Blackwell Publishing Ltd.

Hart JW (1990) Plant Tropisms and Other Growth Movements. London: Chapman & Hall.

Morita MT (2010) Directional gravity sensing in gravitropism. Annual Reviews in Plant Biology 61: 705–720.

Takahashi H, Miyazawa Y and Fujii N (2009) Hormonal interactions during root tropic growth: hydrotropism versus gravitropism. Plant Molecular Biology 69: 489–502.

Teale WD, Paponov IA and Palme K (2006) Auxin in action: signalling, transport and the control of plant growth and development. Nature Reviews Molecular Cell Biology 11: 847–859.

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Morita, Miyo Terao, and Tasaka, Masao(Jun 2010) Tropism. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0022335]