The laterals of both shoots and roots often maintain a particular angle with respect to the gravity vector, and this angle can change during organ development and in response to environmental stimuli. However, the cellular and molecular mechanisms of the lateral organ gravitropic response are still poorly understood. Here it is demonstrated that the young siliques of Arabidopsis thalinana plants subjected to 3-D clinostat rotation exhibited automorphogenesis with increased growth angles between pedicels and the main stem. In addition, the 3-D clinostat rotation treatment significantly influenced the development of vascular bundles in the pedicel and caused an enlargement of gap cells at the branch point site together with a decrease in KNAT1 expression. Comparisons performed between normal and empty siliques revealed that only the pedicels of siliques with normally developing seeds could change their growth angle under the 3-D clinostat rotational condition, while the pedicels of the empty siliques lost the ability to respond to the altered gravity environment. These results indicate that the response of siliques to altered gravity depends on the normal development of seeds, and may be mediated by vascular bundle cells in the pedicel and gap cells at branch point sites.
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