Scientists have been trying to understand gravitropism for more than 120 years (Darwin and Darwin 1880). Gravitropism is the orientation of growth in response to gravity, which is necessary for roots to grow into soil, to acquire water and nutrients and to anchor plants, providing stability and preventing lodging.
The starch–statolith hypothesis and the Cholodny–Went theory attempt to explain some aspects of gravitropism (Haberlandt 1900; Němec 1900; Went 1926; Cholodny N 1927). The starch–statolith hypothesis proposes that the starch-filled amyloplasts of gravity-sensing cells act as statoliths, signalling the direction of gravity by their sedimentation. The Cholodny–Went theory indicated, gravi-bending is the result of differential accumulation of auxin on opposite sides of the elongation zone, resulting in differential growth and tip curvature. In addition, mechanosensitive ion channel hypothesis also could explain some parts of the gravitropism(Ding and Pickard 1993).
A lot of hormones and genes have been found to be involved in gravitropism previously (Blancaflor and Masson 2003; Baldwin et al. 2013; Mai et al. 2014; Ge and Chen 2016). Auxin and its related transporters(e.g. AUX1 and AtPIN2)have been observed to regulate gravitropism (Bennett et al. 1996; Müller et al. 1998; Rigó et al. 2013). Cytokinin functions as an anti-gravitropic signaller in lateral roots(Waidmann et al. 2019). Additionally, brassinosteroid also play important role in the root gravitropic response(Kim et al. 2000; Chang et al. 2004; Amzallag and Vaisman 2006). AGR1 gene involved in root gravitropism could increase root-growth sensitivity to auxin and decrease sensitivity to ethylene in Arabidopsis (Chen et al. 1998). NPY genes play an essential role in root gravitropic responses in Arabidopsis(Li et al. 2011). Although the vast majority of research to date has been conducted in Arabidopsis thaliana, few examples of genes affecting gravitropism have been identified in crop species. The organization of the actin cytoskeleton influence the gravitropic response of primary roots of maize (Blancaflor and Hasenstein 1997). Maize LAZY1 mediates shoot gravitropism through regulating auxin transport(Dong et al. 2013).
Rice is a staple food for nearly half of the world’s population. It has a typical fibrous root system. In rice, some mutants and genes related to gravitropism have been identified. LAZY1 gene controls rice shoot gravitropism through regulating polar auxin transport(Li et al. 2007), but the primary roots of lazy1 mutants show normal gravitropism and circumnutation (Yoshihara et al. 2013). Aem1 mutant causes defects in root development and gravity response (Debi et al. 2005a). Overexpression of OsRAA1 effects root development and root response to gravity(Ge et al. 2004).
The mechanism of gravity sensing in plants is one of the most fascinating questions in molecular biology, and because of the new availability of high-throughput sequencing and phenotyping technology, we can expand our knowledge of this trait through association analysis. For example, in the common bean, Liao et al mapped the QTLs controlling basal root gravitropism (Liao et al. 2004). Using a mapping population derived from a Bala × Azucena, two main QTLs for gravitropic response have been mapped to chromosome 6 and 11(Norton and Price 2009). Measurement of gravitropism related traits with a throughput lending itself to the sample size required for association analysis is now more feasible with tools such as the ROTATO with an automated camera, that could help researchers to dissect the gravi-response(Mullen et al. 2000).
Despite progress made in the past decades, processes involved in positive root gravitropic response in the root tip still remains largely unclear in rice. Because root gravitropism is widely believed to be regulated by a tipping-point mechanism (Band et al. 2012), the gravitropic response speed could be represented by the bending angle of the seminal in agar-filled Perspex chambers after rotation as Uga and Price et al did (Norton and Price 2009; Uga et al. 2013). Here, we measured gravitropic response speed in an association mapping population (Lou et al. 2015), and identified several QTLs related to gravitropic response that can be deployed into marker assisted selection programmes.