The plant hormone indole-3-acetic acid (IAA or auxin) mediates the elongation growth of shoot tissues by promoting cell expansion. According to the acid growth theory proposed in the 1970s, auxin activates plasma membrane H+-ATPases (PM H+- ATPases) to facilitate cell expansion by both loosening the cell wall through acidification and promoting solute uptake. Mechanistically, however, this process is poorly understood. Recent findings in Arabidopsis (Arabidopsis thaliana) have demonstrated that auxin-induced SMALL AUXIN UP RNA (SAUR) genes promote elongation growth and play a key role in PM H+-ATPase activation by inhibiting PP2C.D family protein phosphatases. Here, we extend these findings by demonstrating that SAUR proteins also inhibit tomato PP2C.D family phosphatases and that AtSAUR19 overexpression in tomato (Solanum lycopersicum) confers the same suite of phenotypes as previously reported for Arabidopsis. Furthermore, we employ a custom image-based method for measuring hypocotyl segment elongation with high resolution and a method for measuring cell wall mechanical properties, to add mechanistic details to the emerging description of auxin-mediated cell expansion. We find that constitutive expression of GFP-AtSAUR19 bypasses the normal requirement of auxin for elongation growth by increasing the mechanical extensibility of excised hypocotyl segments. In contrast, hypocotyl segments overexpressing a PP2C.D phosphatase are specifically impaired in auxin-mediated elongation. The time courses of auxin-induced SAUR expression and auxindependent elongation growth were closely correlated. These findings indicate that induction of SAUR expression is sufficient to elicit auxin-mediated expansion growth by activating PM H+-ATPases to facilitate apoplast acidification and mechanical wall loosening.
Bibliographical noteFunding Information:
This work was supported by the National Institutes of Health (GM067203 to W.M.G.), the U.S.–Israel Binational Agriculture Research and Development fund (4429-11 to N.E.O.), and the National Science Foundation (MCB-1613809 to W.M.G., MCB-1158089 to N.E.O., and IOS-1444456 to E.P.S.).
© 2017 American Society of Plant Biologists. All rights reserved.
PubMed: MeSH publication types
- Journal Article
- Research Support, Non-U.S. Gov't
- Research Support, U.S. Gov't, Non-P.H.S.
- Research Support, N.I.H., Extramural