Editors' ChoicePlant biology

Gravity-Defying Peptides

Sci. Signal.  27 Mar 2012:
Vol. 5, Issue 217, pp. ec96
DOI: 10.1126/scisignal.2003075

Many developmental and physiological processes in plants are regulated by both chemical and peptide hormone signals, and the plant growth response to gravity and light is one of these. Gravitropism is the process by which plants respond to light and gravity so that roots grow down and shoots grow upward. Whitford et al. reported that secreted GOLVEN (GLV) peptides, which are related to the RGF (root growth factor) peptides, were involved in gravitropism through their effects on auxin distribution. Gravitropic responses in plants with altered GLV expression were assayed by measuring downward root and upward shoot bending (gravicurvature) in seedlings grown on an inclined substrate or after seedling reorientation. The roots of wild-type seedlings grown on an inclined substrate grow in a regular, undulating wave pattern and do not form loops, but overexpression of GLV1, GLV2, or GLV3 caused roots to grow in irregular patterns and to form loops, an agravitropic phenotype. Phenotypes of plants grown in the presence of synthetic truncated GLV peptides were consistent with a model for GLV maturation that involved proteolytic processing. For the reoriented seedlings, GLV1, GLV2, or GLV3 overexpression affected both shoot and root gravicurvature, whereas knocking down expression of GLV1 and GLV2 by RNA interference (RNAi) affected only shoot gravicurvature, and knocking down GLV3 by RNAi affected only root gravicurvature. These results correlated with the observed expression of GLV1 and GLV2 in shoots and GLV3 in roots. Reorientation induces the accumulation of auxin on the lower sides of roots and shoots, which stimulates the cell elongation that drives bending. After reorientation, GLV1 and GLV2 transcripts were enriched on the lower side of reoriented shoots, and auxin application increased the expression of GLV1 and GLV2. GLV3 was expressed uniformly throughout the root apical meristem whether plants were reoriented or not. Treating seedlings with synthetic GLV1 or GLV3 peptides before reorientation reduced asymmetric expression of an auxin reporter in shoots (GLV1-treated) and roots (GLV3-treated). Application of synthetic GLV peptides or overexpression of GLV3 specifically increased the abundance of the auxin efflux carrier PIN2, and not that of other protein involved in auxin signaling, in the plasma membranes of root epidermal cells, and GLV3-directed RNAi decreased the abundance of PIN2 in root epidermis plasma membranes. Reporter assays and immunolocalization studies indicated that treatment with synthetic GLV peptides enhanced PIN2 accumulation in endosomes and at the plasma membrane of root epidermal cells. The authors propose that GLVs augment PIN2-mediated polar auxin transport by enhancing delivery of PIN2 to the plasma membrane. Remaining questions include the nature of the GLV receptor and the mechanisms by which GLV affects PIN2 trafficking.

R. Whitford, A. Fernandez, R. Tejos, A. C. Pérez, J. Kleine-Vehn, S. Vanneste, A. Drozdzecki, J. Leitner, L. Abas, M. Aerts, K. Hoogewijs, P. Baster, R. De Groodt, Y.-C. Lin, V. Storme, Y. Van de Peer, T. Beeckman, A. Madder, B. Devreese, C. Luschnig, J. Friml, P. Hilson, GOLVEN secretory peptides regulate auxin carrier turnover during plant gravitropic responses. Dev. Cell 22, 678–685 (2012). [PubMed]