Arabidopsis nanodomain-delimited ABA signaling pathway regulates the anion channel SLAH3.

The phytohormone abscisic acid (ABA) plays a key role in the plant response to drought stress. Hence, ABA-dependent gene transcription and ion transport is regulated by a variety of protein kinases and phosphatases. However, the nature of the membrane-delimited ABA signal transduction steps remains largely unknown. To gain insight into plasma membrane-bound ABA signaling, we identified sterol-dependent proteins associated with detergent resistant membranes from Arabidopsis thaliana mesophyll cells. Among those, we detected the central ABA signaling phosphatase ABI1 (abscisic-acid insensitive 1) and the calcium-dependent protein kinase 21 (CPK21). Using fluorescence microscopy, we found these proteins to localize in membrane nanodomains, as observed by colocalization with the nanodomain marker remorin Arabidopsis thaliana remorin 1.3 (AtRem 1.3). After transient coexpression, CPK21 interacted with SLAH3 [slow anion channel 1 (SLAC1) homolog 3] and activated this anion channel. Upon CPK21 stimulation, SLAH3 exhibited the hallmark properties of S-type anion channels. Coexpression of SLAH3/CPK21 with ABI1, however, prevented proper nanodomain localization of the SLAH3/CPK21 protein complex, and as a result anion channel activation failed. FRET studies revealed enhanced interaction of SLAH3 and CPK21 within the plasma membrane in response to ABA and thus confirmed our initial observations. Interestingly, the ABA-induced SLAH3/CPK21 interaction was modulated by ABI1 and the ABA receptor RCAR1/PYL9 [regulatory components of ABA receptor 1/PYR1 (pyrabactin resistance 1)-like protein 9]. We therefore propose that ABA signaling via inhibition of ABI1 modulates the apparent association of a signaling and transport complex within membrane domains that is necessary for phosphorylation and activation of the S-type anion channel SLAH3 by CPK21.

Methods of staining and visualization of sphingolipid enriched and non-enriched plasma membrane regions of Arabidopsis thaliana with fluorescent dyes and lipid analogues.

BACKGROUND: Sterols and Sphingolipids form lipid clusters in the plasma membranes of cell types throughout the animal and plant kingdoms. These lipid domains provide a medium for protein signaling complexes at the plasma membrane and are also observed to be principal regions of membrane contact at the inception of infection. We visualized different specific fluorescent lipophilic stains of the both sphingolipid enriched and non-sphingolipid enriched regions in the plasma membranes of live protoplasts of Arabidopsis thaliana. RESULTS: Lipid staining protocols for several fluorescent lipid analogues in plants are presented. The most emphasis was placed on successful protocols for the single and dual staining of sphingolipid enriched regions and exclusion of sphingolipid enriched regions on the plasma membrane of Arabidopsis thaliana protoplasts. A secondary focus was placed to ensure that these staining protocols presented still maintain cell viability. Furthermore, the protocols were successfully tested with the spectrally sensitive dye Laurdan. CONCLUSION: Almost all existing staining procedures of the plasma membrane with fluorescent lipid analogues are specified for animal cells and tissues. In order to develop lipid staining protocols for plants, procedures were established with critical steps for the plasma membrane staining of Arabidopsis leaf tissue and protoplasts. The success of the plasma membrane staining protocols was additionally verified by measurements of lipid dynamics by the fluorescence recovery after photobleaching technique and by the observation of new phenomena such as time dependent lipid polarization events in living protoplasts, for which a putative physiological relevance is suggested.