Root cap angle and gravitropic response rate are uncoupled in the Arabidopsis pgm-1 mutant

• Published in: Physiologia plantarum Vol. 141; no. 4; pp. 373 - 382
• Main Authors: Wolverton, Chris, Paya, Alex M, Toska, Jonida
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.04.2011; Blackwell
• Subjects: Arabidopsis - anatomy & histology; Arabidopsis - growth & development; Arabidopsis - physiology; Arabidopsis Proteins - genetics; Arabidopsis Proteins - metabolism; Biological and medical sciences; Feedback, Physiological; Fundamental and applied biological sciences. Psychology; Gravitropism - physiology; Green Fluorescent Proteins - metabolism; Kinetics; Microscopy, Confocal; Movements; Mutation - genetics; Plant physiology and development; Plant Root Cap - anatomy & histology; Plant Root Cap - physiology; Plant Roots - growth & development; Plant Roots - physiology; Recombinant Fusion Proteins - metabolism; Arabidopsis thaliana; Response; Root; Plant physiology; Cruciferae; Dicotyledones; Angiospermae; Gravitropism; Spermatophyta; Mutation; Experimental plant
• ISSN: 0031-9317; 1399-3054
• DOI: 10.1111/j.1399-3054.2010.01439.x

The sedimentation of starch-filled plastids is thought to be the primary mechanism by which gravity is perceived in roots. Following gravity perception, auxin redistribution toward the lower flank of roots, initiated in the root cap, is believed to play a role in regulation of the gravity response. Amyloplast sedimentation and auxin flux, however, have never been directly linked. The overall aim of this study was to investigate the relationship among plastid sedimentation, gravitropism and auxin flux. Our data show that pgm-1 roots respond to gravity at one-third the rate of wild-type (WT) roots. Maintaining the root tip at a constant angle using image analysis coupled to a rotating stage resulted in a constant rate of response regardless of the angle of tip orientation in pgm-1 mutants, in contrast to the responses of WT and pin3-1 mutants, which showed increasing response rates as the tip was constrained at greater angles. To indirectly visualize auxin flux following reorientation, we generated a pgm-1 mutant line expressing the DR5::GFPm reporter gene. In WT roots a GFP gradient was observed with a maximum along the lower flank, whereas pgm-1 roots formed a GFP maximum in the central columella but lacked any observable gradient up to 6 h following reorientation. Our study suggests that the relationship between root cap angle and gravitropic response depends upon plastid sedimentation-based gravity sensing and supports the idea that there are multiple, overlapping sensory response networks involved in gravitropism.