Editors' ChoicePlant biology

Follow the Water

Science Signaling  17 Feb 2009:
Vol. 2, Issue 58, pp. ec64
DOI: 10.1126/scisignal.258ec64

Because plants are stationary, they have evolved mechanisms to direct growth to follow the locations of nutrients and water, as well as to respond to gravity. It is known that the response to gravity (gravitropism) involves auxin signaling and requires proper vesicular delivery of the auxin efflux carrier to the root tips. Arabidopsis loss-of-function alleles of GNOM, as well as a weak allele, gnomB/E, disrupt auxin signaling, disrupting embryonic development and root gravitropism, respectively. GNOM encodes a guanine nucleotide exchange factor for ADP ribosylation factor–type guanosine trisphosphatases (ARF-GEF), which are involved in regulating vesicular trafficking. Miyazawa et al. have identified another GNOM allele, which they call miz2, that selectively disrupts hydrotropism without altering gravitropism. The roots of miz2 plants failed to display hydrotropic bending but exhibited normal gravitropism. The growth and morphology of the roots and shoots of miz2 plants were the same as those of wild-type plants. The localization of the auxin efflux carrier in roots of the miz2 plants was the same as in wild-type plant roots, and the response to auxin was normal. The miz2 mutation involved a change of the amino acid Gly951Glu in the C-terminal region following the conserved Sec7 domain. Although this residue was not conserved among all members of the larger protein family to which GNOM belongs (the Gea/GBF/GNOM family), it was conserved in GNOM homologs of several other plant species. Although the exact mechanism by which GNOM regulates hydrotropic responses remains unknown, it is clear that vesicle trafficking plays distinct roles in hydrotropic and gravitropic responses and that auxin does not appear to mediate hydrotropism.

Y. Miyazawa, A. Takahashi, A. Kobayashi, T. Kaneyasu, N. Fujii, H. Takahashi, GNOM-mediated vesicular trafficking plays an essential role in hydrotropism of Arabidopsis roots. Plant Physiol. 149, 835–840 (2009). [Abstract] [Full Text]