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.