Graviperception in maize plants: is amyloplast sedimentation a red herring?

Abstract

Land plants perceive gravity and respond to it in an organ-specific way; shoots typically direct growth upwards, roots typically downwards. Historically, at least with respect to maize plants, this phenomenon is attributed to three sequential processes, namely graviperception, the transduction of the perceived signal, and the graviresponse, resulting in a typical (re)positioning of the organ or entire plant body relative to the gravivector. For decades, sedimentation of starch-containing plastids within the cells of special tissues has been regarded as the primary and initiating process fundamental for gravitropic growth (starch-statolith hypothesis). Based on Popper's falsification principle, uncompromising experiments were executed. The results indicate that the model of graviperception based on amyloplast sedimentation does not apply to maize seedlings.

Keywords: Ethylene; Gravi-model; Graviperception; Gravitropic growth; Gravitropism; Plant signal transduction; Starch-statolith hypothesis; Tensegrity model.

Fig. 1

a Typical 3-day-old maize seedling as germinated in the dark at room temperature, exhibiting the coleoptile, kernel, and the primary root (vertical bar represents 1-cm length). b Coleoptile basis after removal of the enclosed primary leaf and surgical removal of the vascular bundle-containing tissues of the coleoptile illustrating the basis of the remaining longitudinal tissue arches without vascular bundle tissue. c Root tip with (+) and without (−) root cap

Fig. 2

Typical images of a, b horizontally positioned maize root coleoptiles without vascular bundle sheets at time zero; inserts indicate intact coleoptiles (i.e., containing vascular bundle sheets); c, d the same coleoptiles after 9 h of gravistimulation (inserts indicate adequately incubated intact coleoptiles). g indicating direction of the gravivector

Fig. 3

a, b Different magnification images of decapped roots of maize seedlings horizontally gravistimulated for 24 h. c, d Root graviresponse of decapped roots incubated for 1 h in latrunculin previous to horizontal gravistimulation