Minisymposium 4: Signaling

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Abs # M0401: Ca²⁺ regulation of transmembrane proton fluxes and ROS production: a common mechanism in tip growth and mechanosensing?

Presenter:	Monshausen, Gabriele       Contact Presenter
Authors	Monshausen, Gabriele  (A)   Bibikova, Tatiana  (A)   Gilroy, Simon  (A)
Affiliations:	(A): Penn State University

Plants are exquisitely sensitive to mechanical stress. These stresses may be generated internally via turgor-driven growth or imposed externally e.g. by a touch stimulus. Mechanical stimulation is well characterized as triggering a transient elevation in cytosolic Ca²⁺ and apical growth of root hairs is also associated with a tip-focused Ca²⁺ gradient. Here we show that touch- and tip growth-associated Ca²⁺ increases likely activate a common signal transduction pathway that involves changes in pH and the production of extracellular ROS. Thus, touch stimulation of the root surface caused a rapid (within 1 s) and transient apoplastic alkalinization and cytosolic acidification. Similarly, tip growth of root hairs involves oscillatory changes in extra- and intracellular pH. In addition, both touch stimulation and tip growth trigger apoplastic ROS production. Apoplastic ROS then induce changes in plasma membrane characteristics as revealed by the rapid disappearance of fluorescence of the membrane probe FM4-64. All of these responses were shown to be dependent on Ca²⁺. In the Arabidopsis mutant rhd2, which lacks a functional NADPH oxidase C (ATRBOH C), root hairs fail to sustain tip growth. In this background, touch stimulation still triggered pH changes but not the increase in ROS production seen in wild type plants. Thus, touch responses likely elicit Ca²⁺-dependent activation of ATRBOH C yielding ROS production to the cell wall. These localized changes in wall pH and ROS may help rapidly cross-link the cell wall as part of a defense response to environmental stress and as a means to stabilize the wall against the intrinsic mechanical stress generated during growth. (This work is supported by the NSF.)