Abstract
BackgroundThe cell wall component callose is mainly synthesized at certain developmental stages and after wounding or pathogen attack. Callose synthases are membrane-bound enzymes that have been relatively well characterized in vitro using isolated membrane fractions or purified enzyme. However, little is known about their functional properties in situ, under conditions when the cell wall is intact. To allow in situ investigations of the regulation of callose synthesis, cell suspensions of Arabidopsis thaliana (Col-0), and tobacco (BY-2), were permeabilized with the channel-forming peptide alamethicin.ResultsNucleic acid-binding dyes and marker enzymes demonstrated alamethicin permeabilization of plasma membrane, mitochondria and plastids, also allowing callose synthase measurements. In the presence of alamethicin, Ca2+ addition was required for callose synthase activity, and the activity was further stimulated by Mg2+ Cells pretreated with oryzalin to destabilize the microtubules prior to alamethicin permeabilization showed significantly lower callose synthase activity as compared to non-treated cells. As judged by aniline blue staining, the callose formed was deposited both at the cell walls joining adjacent cells and at discrete punctate locations earlier described as half plasmodesmata on the outer walls. This pattern was unaffected by oryzalin pretreatment, showing a quantitative rather than a qualitative effect of polymerized tubulin on callose synthase activity. No callose was deposited unless alamethicin, Ca2+ and UDP-glucose were present. Tubulin and callose synthase were furthermore part of the same plasma membrane protein complex, as judged by two-dimensional blue native SDS-PAGE.ConclusionAlamethicin permeabilization allowed determination of callose synthase regulation and tubulin interaction in the natural crowded cellular environment and under conditions where contacts between the cell wall, the plasma membrane and cytoskeletal macromolecules remained. The results also suggest that alamethicin permeabilization induces a defense response mimicking the natural physical separation of cells (for example when intercellulars are formed), during which plasmodesmata are transiently left open.
Highlights
The cell wall component callose is mainly synthesized at certain developmental stages and after wounding or pathogen attack
Alamethicin permeabilization of Arabidopsis thaliana Columbia-0 (Col-0) and Bright Yellow 2 (BY-2) cells It was previously shown that alamethicin could be used to permeabilize BY-2 cells [47]
To enable the use of A. thaliana cells in addition to BY-2 and to investigate the regulation of callose synthesis, we wished to establish if Col-0 suspension cultured cells were permeabilized by alamethicin
Summary
The cell wall component callose is mainly synthesized at certain developmental stages and after wounding or pathogen attack. Callose is deposited at plasmodesmata [4,5] and at sieve plates [6] to limit intercellular transport, often as a response to developmental cues or environmental signals, e.g., wounding and pathogen attack [7,8,9]. Genes encoding callose synthases (GSL) [17,18,19] have been identified in several plant species. In A. thaliana as much as 12 callose synthase genes have been identified [18]. Biochemical studies have indicated that at least some GSL genes can produce proteins capable of synthesizing callose [20]
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