Amyloplast displacement is necessary for gravisensing in Arabidopsis shoots as revealed by a centrifuge microscope

Abstract

SUMMARYThe starch-statolith hypothesis proposes that starch-filled amyloplasts act as statoliths in plant gravisensing, moving in response to the gravity vector and signaling its direction. However, recent studies suggest that amyloplasts show continuous, complex movements in Arabidopsis shoots, contradicting the idea of a so-called 'static' or 'settled' statolith. Here, we show that amyloplast movement underlies shoot gravisensing by using a custom-designed centrifuge microscope in combination with analysis of gravitropic mutants. The centrifuge microscope revealed that sedimentary movements of amyloplasts under hypergravity conditions are linearly correlated with gravitropic curvature in wild-type stems. We next analyzed the hypergravity response in the shoot gravitropism 2 (sgr2) mutant, which exhibits neither a shoot gravitropic response nor amyloplast sedimentation at 1 g. sgr2 mutants were able to sense and respond to gravity under 30 g conditions, during which the amyloplasts sedimented. These findings are consistent with amyloplast redistribution resulting from gravity-driven movements triggering shoot gravisensing. To further support this idea, we examined two additional gravitropic mutants, phosphoglucomutase (pgm) and sgr9, which show abnormal amyloplast distribution and reduced gravitropism at 1 g. We found that the correlation between hypergravity-induced amyloplast sedimentation and gravitropic curvature of these mutants was identical to that of wild-type plants. These observations suggest that Arabidopsis shoots have a gravisensing mechanism that linearly converts the number of amyloplasts that settle to the 'bottom' of the cell into gravitropic signals. Further, the restoration of the gravitropic response by hypergravity in the gravitropic mutants that we tested indicates that these lines probably have a functional gravisensing mechanism that is not triggered at 1 g.

The Arabidopsis LAZY1 Family Plays a Key Role in Gravity Signaling within Statocytes and in Branch Angle Control of Roots and Shoots

“…Y values were measured to indicate vertical displacement of plastids. Y was the distance in micrometers of the position of the center of brightness of a plastid from its original position at any given time point (Toyota et al, 2013). Measurements were performed using one or two S2 cells from at least three individuals of each genotype.…”

Molecular mechanisms of gravity perception and signal transduction in plants

“…Abnormal amyloplast sedimentation in graviperceptive endodermal cells and dramatic reduction in the gravitropism of inflorescence stems, hypocotyls, and petioles were observed in shoot gravitropism (sgr) 2 plants Mano et al 2006;Morita et al 2002;Toyota et al 2013b). The protein sequence of SGR2 is homologous to PA-hydrolyzing phospholipase А 1 .…”

“…The so-called Bmodel of hydrostatic pressure^was proposed to explain several observations. Starchless mutants have some residual gravitropism, and starchdeficient mutants have a normal graviresponse to hypergravity (Toyota et al 2013b;Wolverton et al 2011). Cells in the root distal elongation zone of higher plants and characean algae internodal cells contain no sedimentable amyloplasts, and lateral root gravitropism is initiated before their amyloplasts emerge.…”

Mechano-biological Coupling of Cellular Responses to Microgravity

“…One example is to validate the statolith hypothesis of plant gravisensing from the viewpoint of microrheological feature of amyloplast movement in the plant root gravity-sensing cells. While it is well known that the sedimentation movement of amyloplasts primarily triggers the asymmetric distribution of auxin and then leads to the differential growth of the plant root (Vanneste and Friml 2009;Leitz et al 2009;Morita 2010;Sato et al 2015), the inhomogeneous structures in statocytes are also found to significantly affect the movements of amyloplasts and the resulted gravimorphogenesis (Weise et al 2000;Toyota et al 2013a). Microrheological analysis, frequently used to detect micro-mechanical properties of cytoskeletal network, is employed for the first time and indicates that the intracellular environment of columella cells exhibits the spatial heterogeneity and the cage-confinement on amyloplasts.…”