25 Modulation of Root Growth by Physical Stimuli

Abstract

Roots and shoots of higher plants change their growth patterns in response to physical stimuli such as gravity, light, humidity, and obstacles. One of the earliest studies of the stimulus-response interactions in young seedlings was done by Charles Darwin, who was interested in the circular movement of hypocotyls and roots, and who examined effects of light, gravity, and touching stimulation using experimental devices designed by himself (1880). Since the pioneering studies of Darwin, much information on the mechanism of root behavior triggered by several kinds of physical stimuli has been accumulated from physiological as well as anatomical analyses of roots (for review, see Wilkins 1966; Juniper 1976; Jackson and Barlow 1981; Evans 1991). Extensive studies on the genetic network of stimulus-response interactions in root systems have started recently with the isolation and analysis of mutants of aberrant morphology or of abnormal growth pattern. We have used Arabidopsis thaliana for the molecular genetic approach to elucidate the genetic network ruling the signaling pathways in roots, because roots of young seedlings show a variety of responses to several physical and chemical stimuli, and the structure of Arabidopsis roots is simpler than that of other plants (Schiefelbein and Benfey 1991; Dolan et al. 1993). To analyze the responses and to obtain mutant plants that show aberrant responses, we have developed a series of experimental procedures using agar plates (Fig. 1) (Okada and Shimura 1992a). The agar medium contains 1.5% agar and 0.5× Arabidopsis mineral nutrient solution. Young seedlings grow well on the surface...