Research advances in plant root geotropism

Plants cannot grow or develop properly without the support of their roots. Gravity plays an essential role in the formation of the root structure, but it is not clear how roots respond to gravity signals or how downward growth occurs. The two best-known models for root gravity sensing affirm the importance of starch. After the hyper-sensitive root crown perceives a gravity signal, starch granules within the rootlet cells settle to the endoplasmic reticulum in the direction of the signal, where they bind to specific receptors or open ion channels and release downstream signaling molecules. This triggers a series of signal transduction mechanisms, and this process involves signaling molecules such as indole-3?acetic acid (IAA), reactive oxygen species, and calcium signaling, which ultimately induce groundward root growth. This review summarizes the mechanism of action underlying, and a research overview of, how plant roots sense and respond to gravity. The role of key signals such as starch, IAA, and calcium ions in root gravitropism is analyzed by integrating available information. The results provide a more complete theoretical basis for how roots grow toward gravity, which will contribute to our understanding of gravitropism and lay the foundation for discovering new directions of scientific research.