Working model on the antagonistic interaction of signalling triggered by harpin and auxin.

Abstract

<p>To reduce complexity, only the earliest events are depicted, omitting ROS activation of calcium influx and effects of rac1-signalling on auxin transport. ① harpin activates the NADPH-dependent oxidase RboH leading to the production of superoxide that can spread in the apoplast. ② Superoxide can penetrate through the plasma membrane (probably by aquaporins) and glutathionylate actin in residue Cys374. This will sequester G-actin from being integrated into the growing end of actin filaments. ③ Alternatively, superoxide can be recruited to transduce the effect of auxin (perceived via the auxin-binding protein, ABP) upon the activation of phospholipase D (PLD) through the small G-protein Rac. ④ PLD will generate phosphatidic acids (PA) that can sequester actin capping proteins (cap) to the membrane, such that elongation of actin filaments is enabled. Alternatively, PA can be partitioned to recruit Rac for the activation of the RboH complex. In this case, the capping proteins will not be recruited to the membrane and constrain the elongation of actin filaments leading as secondary consequence to the formation of thick cables through the activity of severing proteins in combination with free G-actin the formation. ⑤ As third alternative, PA can be converted to PIP2, which will recruit actin-depolymerization factor (ADF) to the membrane. Since ADF is sustaining the monomer turnover at the minus end of actin filaments, this recruitment results in a higher stability of fine cortical actin filaments. The molecular targets for the inhibitors diphenyliodonium (DPI), and <i>n</i>-butanol are inserted in red. Hypothetical aspects of the model that have not been addressed experimentally in plant cells, are indicated by blue question marks: The interaction of Harpin with RboH (①) has not been addressed experimentally so far. Also the glutathionylation of actin in consequence of superoxide penetration (②), so far has been shown for animal systems, but not been addressed in plant cells.</p