Plant gravitropism. Unraveling the ups and downs of a complex process.

Wind and heavy rain can have a devastating effect on crop production if they occur late during the life cycle of the plants. They often flatten the crop on the ground, leaving seed and other harvestable products at the mercy of soil moisture and pathogens and inaccessible to mechanical harvesting machinery. However, farmers will not worry much if a storm of similar strength hits a crop earlier in its cycle. They know that younger plants are able to straighten up and resume upward growth. This example illustrates the significance of tropism, or the ability of plant organs to direct their growth along a path that is dictated by their surrounding environment. In nature, plant organs can use a variety of environmental cues to guide their growth, including gravity, touch, light, gradients in temperature, humidity, ions, chemicals, and oxygen. In the example illustrated above, the stems of younger crops were able to perceive a change in their orientation within the gravity field. The corresponding information was then translated into a complex growth response called gravitropism that allowed them to straighten up and to eventually resume growth at a defined angle from the gravity vector, the gravitational set point angle (GSPA). The GSPA associated with specific plant organs is a function of the identity of the species and organ under consideration, its developmental phase, the physiological status of the plant, and a variety of environmental parameters to which the plant has been exposed. A plant organ is capable of detecting any deviation from its assigned GSPA and responds to the corresponding stimulus (gravitropic stimulus) by developing differential cellular elongation on opposite flanks, resulting in tip curvature and subsequent realignment with the GSPA (Firn and Digby, 1997). Gravitropism has been observed and analyzed in a variety of plant organs, including roots, hypocotyls, and inflorescence stems of dicots; roots, coleoptiles, and pulvini of monocots; and rhizoids and protonemata of algae and moss. It also has been observed and studied in fungal fruiting bodies. In 1999, an Update discussing the status of our understanding of gravitropism in roots of higher plants was published (Chen et al., 1999). Since then, there have been several developments in the field that have provided new insights on the different issues related to gravitropism in multiple plant systems. These breakthroughs are discussed in this Update.