Altered gravitropic response, amyloplast sedimentation and circumnutation in the Arabidopsis shoot gravitropism 5 mutant are associated with reduced starch levels

Plants have developed sophisticated gravity sensing mechanisms to interpret environmental signals that are vital for optimum plant growth. Loss of SHOOT GRAVITROPISM 5 (SGR5) gene function has been shown to affect the gravitropic response of Arabidopsis inflorescence stems. SGR5 is a member of the INDETERMINATE DOMAIN (IDD) zinc finger protein family of putative transcription factors. As part of an ongoing functional analysis of Arabidopsis IDD genes (AtIDD) we have extended the characterisation of SGR5, and show that gravity sensing amyloplasts in the shoot endodermis of sgr5 mutants sediment more slowly than wild type, suggesting a defect in gravity perception. This is correlated with lower amyloplast starch levels, which may account for the reduced gravitropic sensitivity in sgr5. Further, we find that sgr5 mutants have a severely attenuated stem circumnutation movement typified by a reduced amplitude and an decreased periodicity. adg1-1 and sex1-1 mutants, which contain no starch or increased starch, respectively, also show alterations in the amplitude and period of circumnutation. Together these results suggest that plant growth movement may depend on starch levels and/or gravity sensing. Overall, we propose that loss of SGR5 regulatory activity affects starch accumulation in Arabidopsis shoot tissues and causes decreased sensitivity to gravity and diminished circumnutational movements.