Plants adapt their development in response to environmental signals in order to optimize their chances of survival. These environmental cues include directional signals such as gravity and light. Plant organs adjust their position relative to such signals by employing tropic responses to modify their direction of growth. For example, roots employ directional signals like gravity to explore the soil environment and acquire anchorage and resources.
Tropic responses represent key adaptive traits throughout root development. During germination of some species, primary roots develop the ability to respond to gravity prior to emergence from the seed coat (Ma and Hasenstein 2006), thereby aiding seedling establishment. When secondary roots emerge from the primary root, they initially grow out horizontally (Mullen and Hangarter 2003), while higher orders of lateral roots often exhibit randomized directions. These diagravitropic and plagiotropic patterns of root growth greatly facilitate the acquisition of water and nutrients (Basu et al. 2007), particularly in the topsoil where phosphorus availability is the greatest (Lynch and Brown 2001). While root angle is primarily regulated by the gravitropic response, other directional signals like touch and water/oxygen gradients also modify root angle by inducing thigmo-, hydro-, and oxytropic responses when roots encounter compacted, drying, and waterlogged soil, respectively (Porterfield 2002).
Tropic responses have fascinated scientists for over 300 years. Hydrotropism was initially proposed by Dodart (1700) as amechanism for roots to grow downward in response to soil water content. Root gravitropism was first demonstrated by Knight (1806) after observing that young roots reorient downward irrespective of the angle the germinating seed was positioned. Darwin (1881) first described root thigmotropism in his book “The power of movement in plants,” while oxytropism was initially described by Molisch (1884) after observing that submerged roots grew toward the water–air interface. Despite the large intervening period of time, our understanding about the mechanisms regulating many of these root tropisms remains patchy. In contrast, the last 15 years have seen several important breakthroughs in our understanding about the molecular components, signals, and tissues that function during root gravitropism.