Plasmodesmata at the crossroads between development, dormancy, and defense

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Abstract
Plants are frequently exposed to environmental stress and organisms that seek to benefit from their autotrophic nature. To cope with these challenges plants have developed stress-resistance mechanisms, which involve sensing, activation of signal transduction cascades, changes in gene expression, and physiological adjustment. Exposure to one kind of stress often leads to cross-tolerance, that is, resistance to different kinds of stresses. The search for a common underlying mechanism concentrates mostly on changes in cellular physiology and gene expression. We focus on the cross-protective measures that are taken at the level above the single cell. We argue that the controlled alterations in symplasmic permeability that underlie development also play a role in survival and defense strategies. In development, most of the alterations are transient and dynamic, whereas the more persistent alterations function predominantly in dormancy and defense and are under the control of two key enzymes: 1,3-β-D-glucan synthase and 1,3-β-D-glucanase. 1,3-β-D-Glucan synthase functions in the narrowing or closing of plasmodesmata, whereas 1,3-β-D-glucanase counteracts this process. We propose that the closing of symplasmic paths constitutes an unspecific but effective early measure in adaptation and defense, which is accompanied by specific strategies tailored to the various challenges plants face.Key words: cross-adaptation, dormancy sphincter, 1,3-β-D-glucanase, 1,3-β-D-glucan synthase, meristem, overwintering, plasmodesmata, virus movement.