Despite growing underground, largely in darkness, roots emerge to be very sensitive to light. Recently, several important papers have been published which reveal that plant roots not only express all known light receptors but also that their growth, physiology and adaptive stress responses are light-sensitive. In Arabidopsis, illumination of roots speeds-up root growth via reactive oxygen species-mediated and F-actin dependent process. On the other hand, keeping Arabidopsis roots in darkness alters F-actin distribution, polar localization of PIN proteins as well as polar transport of auxin. Several signaling components activated by phytohormones are overlapping with light-related signaling cascade. We demonstrated that the sensitivity of roots to salinity is altered in the light-grown Arabidopsis roots. Particularly, light-exposed roots are less effective in their salt-avoidance behavior known as root halotropism. Here we discuss these new aspects of light-mediated root behavior from cellular, physiological and evolutionary perspectives.

Arabidopsis thaliana is a widely used model plant for plant biology research. Under traditional agar-plate culture system (TPG, traditional plant-growing), both plant shoots and roots are exposed to illumination, and roots are grown in sucrose-added medium. This is not a natural environment for the roots and may cause artifact responses. We have developed an improved agar-plate culture system (IPG, improved plant-growing) where shoots are illuminated but roots are grown in darkness without sucrose addition. Compared to TPG, IPG produced plants with significantly less total root length, lateral root length and root hair density, although their primary roots were longer. Root gravitropism, PIN2 (an auxin efflux carrier) abundance, H+ efflux or Ca2+ influx in root apexes, were weaker in IPG-grown roots than those in TPG-grown roots. We conclude that IPG offers a more natural way to study the root growth and response of Arabidopsis thaliana.

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Light can penetrate several centimeters below the soil surface. Growth, development and behavior of plant roots are markedly affected by light despite their underground lifestyle. Early studies provided contrasting information on the spatial and temporal distribution of light-sensing cells in the apical region of root apex and discussed the physiological roles of plant hormones in root responses to light. Recent biological and microscopic advances have improved our understanding of the processes involved in the sensing and transduction of light signals, resulting in subsequent physiological and behavioral responses in growing root apices. Here, we review current knowledge of cellular distributions of photoreceptors and their signal transduction pathways in diverse root tissues and root apex zones. We are discussing also the roles of auxin transporters in roots exposed to light, as well as interactions of light signal perceptions with sensing of other environmental factors relevant to plant roots.