![]() Unfortunately, the nature and mechanism of that signal remain unresolved. Following downward migration and settling of amyloplasts in the columella cells, this physical stimulus is transduced into a physiological and biochemical signal ( Figure 1B). In roots, the amyloplasts’ sedimentation occurs in the columella cells. Notably, the sedimentation process of the amyloplasts is finely tuned by the actin microfilaments network ( Hou et al., 2003 Leitz et al., 2009 Blancaflor, 2013). Upon encountering gravitational pull, the amyloplasts can rapidly descend toward the new bottom of the cell, triggering the downstream gravitropism signal transduction via an as of yet unclear mechanism. These statocytes contain amyloplasts, which are filled with dense starch, thereby conferring to them a greater density than the surrounding cytoplasm ( Baldwin et al., 2013 Su et al., 2017). The starch-statolith theory posits that gravity is perceived by the statoliths within the statocyte cells. How exactly the plant perceives the gravitational pull is not yet fully understood. Plant gravitropism comprises four successive steps: gravity perception, signal transduction that converts the gravity stimulus into physiological signals, polar auxin transport (PAT) that asymmetrically distributes auxin to the responding organs, and differential elongation of cells leading to the curvature of organs. By contrast, the negative gravitropic response occurs in the aboveground shoot system, which grows up, against the gravity vector toward the sky to obtain space and sunlight for photosynthesis and reproduction. The positive gravitropic response occurs in the belowground root system, which re-orientates the root-tip growth angle toward the GSA and allows the roots to penetrate deep into the soil to anchor the plant and uptake nutrients and water. Plant gravitropism can either be a positive or a negative response ( Dong et al., 2013). We also discuss the evolutionary significance and conservation of the LAZY gene family in plants. Here we review current knowledge of the LAZY gene family and the mechanism modulated by LAZY proteins for controlling both roots and shoots gravitropism. The identification and characterization of the LAZY gene family have significantly advanced our understanding of plant gravitropism, and opened new frontiers of investigation into the novel molecular details of the early events of gravitropism. LAZY proteins appear to be participating in the early steps of gravity signaling, as the mutation of LAZY genes consistently leads to altered auxin redistribution in multiple plant species. In the past decade, the LAZY gene family has been identified as a crucial player that ensures the proper redistribution of auxin and a normal tropic response for both roots and shoots upon gravistimulation. Although this process has been studied for several 100 years, much remains unclear, particularly the early events that trigger the relocation of the auxin efflux carrier PIN-FORMED (PIN) proteins, which presumably leads to the asymmetrical redistribution of auxin. Triggered by gravistimulation, plant gravitropism is a highly complex, multistep process that requires many organelles and players to function in an intricate coordinated way. Plants have evolved a remarkable capability that not only allows them to live and develop within the Earth’s gravity field, but it also enables them to use the gravity vector to guide the growth of roots and shoots, in a process known as gravitropism. 5The Guangdong Subcenter of the National Center for Soybean Improvement, College of Agriculture, South China Agricultural University, Guangzhou, ChinaĪdapting to the omnipresent gravitational field was a fundamental basis driving the flourishing of terrestrial plants on the Earth.4Guangdong Provincial Key Laboratory of Protein Function and Regulation in Agricultural Organisms, College of Life Sciences, South China Agricultural University, Guangzhou, China.3State Key Laboratory for Conservation and Utilization of Subtropical Agro-bioresources, College of Life Sciences, South China Agricultural University, Guangzhou, China.2Guangdong Key Laboratory for Innovative Development and Utilization of Forest Plant Germplasm, South China Agricultural University, Guangzhou, China.1College of Forestry and Landscape Architecture, South China Agricultural University, Guangzhou, China.Zhicheng Jiao 1,2, Huan Du 1,2, Shu Chen 1,2, Wei Huang 3,4 and Liangfa Ge 1,2,5* ![]()
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