The shoot apical meristem restores its symplasmic organization during chilling-induced release from dormancy

The shoot apex of overwintering perennials ceases its morphogenetic activity at the end of the growing season and transforms into a bud which is dormant and freezing-tolerant. In birch (Betula pubescens) these events are triggered by short photoperiod, and involve the production of 1,3-β-D-glucan containing sphincters on the plasmodesmata. As a result, all symplasmic pathways shut down. Here we show that breakage of bud dormancy by chilling involves restoration of the symplasmic organization of the meristem. This restoration is likely to be mediated by 1,3-β-D-glucanase, which was present in small spherosome-like vacuoles that arose de novo during dormancy induction. During chilling these vacuoles were displaced from the bulk cytoplasm to the cortical cytoplasm where they became aligned with the plasma membrane, often associated with plasmodesmata. At this stage the enzyme also appeared outside the vacuoles. During chilling, 1,3-β-D-glucan disappeared from the plasmodesmal channels and wall sleeves, and the plasmodesmata regained the capacity for cell-cell transport, as demonstrated by microinjection of Lucifer Yellow CH and Fluorescein-tagged gibberellic acid. Collectively, the present experiments demonstrate that restoration of the symplasmic organization of the meristem is indispensable for the release of buds from dormancy and the assumption of a proliferation-competent state, and implicate 1,3-β-D-glucanase action at the plasmodesmata. Based on these findings we propose a model for 'dormancy cycling' which depicts the meristem as passing through three sequential states of cellular communication with characteristic sensitivities to distinct environmental cues

The shoot apical meristem restores its symplasmic organization during chilling-induced release from dormancy

The shoot apex of overwintering perennials ceases its morphogenetic activity at the end of the growing season and transforms into a bud which is dormant and freezing-tolerant. In birch (Betula pubescens) these events are triggered by short photoperiod, and involve the production of 1,3-β-D-glucan containing sphincters on the plasmodesmata. As a result, all symplasmic pathways shut down. Here we show that breakage of bud dormancy by chilling involves restoration of the symplasmic organization of the meristem. This restoration is likely to be mediated by 1,3-β-D-glucanase, which was present in small spherosome-like vacuoles that arose de novo during dormancy induction. During chilling these vacuoles were displaced from the bulk cytoplasm to the cortical cytoplasm where they became aligned with the plasma membrane, often associated with plasmodesmata. At this stage the enzyme also appeared outside the vacuoles. During chilling, 1,3-β-D-glucan disappeared from the plasmodesmal channels and wall sleeves, and the plasmodesmata regained the capacity for cell-cell transport, as demonstrated by microinjection of Lucifer Yellow CH and Fluorescein-tagged gibberellic acid. Collectively, the present experiments demonstrate that restoration of the symplasmic organization of the meristem is indispensable for the release of buds from dormancy and the assumption of a proliferation-competent state, and implicate 1,3-β-D-glucanase action at the plasmodesmata. Based on these findings we propose a model for 'dormancy cycling' which depicts the meristem as passing through three sequential states of cellular communication with characteristic sensitivities to distinct environmental cues

Dehydrins in cold-acclimated apices of birch (Betula pubescens Ehr.) : production, localization and potential role in rescuing enzyme function during dehydration

Dehydrins accumulate in various plant tissues during dehydration. Their physiological role is not well understood, but it is commonly assumed that they assist cells in tolerating dehydration. Since in perennials the ability of the shoot apex to withstand dehydration is pivotal for survival through winter, we investigated if and how dehydrins may be involved. A first step in assessing such a role is the identification of their subcellular location. We therefore mapped the location of dehydrin homologues, abscisic acid-responsive (RAB 16-like) polypeptides, in the apex of birch (Betula pubescens Ehrh.). In non-cold-acclimated plants a single low-abundant RAB 16-member (a 33-kDa polypeptide) was produced, and localized in the cytoplasm only. During cold acclimation two additional members were produced (24 and 30 kDa) and accumulated in nuclei, storage protein bodies and starch-rich amyloplasts. Western blots of proteins isolated from purified starch granules and from protein bodies revealed the presence of the 24-kDa dehydrin. Since starch and protein reserves are gradually consumed during winter, serving cell maintenance, starch- and protein-degrading enzymes must remain locally active. We therefore investigated the hypothesis that dehydrins might create local pools of water in otherwise dehydrated cells, thereby maintaining enzyme function. In agreement with our hypothesis, enzyme assays showed that under conditions of low water activity a partially purified dehydrin fraction was able to improve the activity of !-amylase (EC 3.2.1.1.) relative to fractions from which dehydrin was removed by immunoprecipitation. The results confirm the general belief that dehydrins serve desiccation tolerance, and suggest that a major function is to rescue the metabolic processes that are required for survival and re-growth

Dehydrins in cold-acclimated apices of birch (Betula pubescens Ehr.) : production, localization and potential role in rescuing enzyme function during dehydration

Dehydrins accumulate in various plant tissues during dehydration. Their physiological role is not well understood, but it is commonly assumed that they assist cells in tolerating dehydration. Since in perennials the ability of the shoot apex to withstand dehydration is pivotal for survival through winter, we investigated if and how dehydrins may be involved. A first step in assessing such a role is the identification of their subcellular location. We therefore mapped the location of dehydrin homologues, abscisic acid-responsive (RAB 16-like) polypeptides, in the apex of birch (Betula pubescens Ehrh.). In non-cold-acclimated plants a single low-abundant RAB 16-member (a 33-kDa polypeptide) was produced, and localized in the cytoplasm only. During cold acclimation two additional members were produced (24 and 30 kDa) and accumulated in nuclei, storage protein bodies and starch-rich amyloplasts. Western blots of proteins isolated from purified starch granules and from protein bodies revealed the presence of the 24-kDa dehydrin. Since starch and protein reserves are gradually consumed during winter, serving cell maintenance, starch- and protein-degrading enzymes must remain locally active. We therefore investigated the hypothesis that dehydrins might create local pools of water in otherwise dehydrated cells, thereby maintaining enzyme function. In agreement with our hypothesis, enzyme assays showed that under conditions of low water activity a partially purified dehydrin fraction was able to improve the activity of !-amylase (EC 3.2.1.1.) relative to fractions from which dehydrin was removed by immunoprecipitation. The results confirm the general belief that dehydrins serve desiccation tolerance, and suggest that a major function is to rescue the metabolic processes that are required for survival and re-growth