Päivi Rinne

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

Abstract. 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 α-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.

Networks for shoot design.

The intrinsic capacity of the shoot apical meristem for self-regulation and the positional specification of its cells implies the existence of an elaborate and versatile communication network. We propose a model that pictures this network as a system of overlapping signal circuits, which support local tasks as well as coordinating indeterminate shoot development.

Initiation, structure and sprouting of dormant basal buds in Betula pubescens

Summary The morphological basis for sprouting in Betula pubescens E hrh ., is studied with special reference to the initiation, structure and number of buds at the base of the tree and their development into sprouts. The material consists of seedlings of varying ages and stumps of mature trees. The dormant basal buds begin life as axillary buds at the seedling stage, positioned in accordance with the leaf arrangement. These primary basal buds are initiated in the axils of retarded leaves, and it is perhaps partly for this reason that they remain in a protracted state of dormancy. This dormancy is no more than a superficial feature, however, as, unlike the axillary buds higher up the stem, they are engaged in constant growth. They also differ structurally from ordinary axillary buds, possessing a growth point and a few scales after the first growing season, but no foliage leaf primordia, and gaining more scales as they age, normally one whorl per year. Similarly the vascular connection of a dormant basal bud grows year by year in accordance with the radial growth of the tree, thus ensuring that the bud does not become buried within the wood. The basal buds normally increase in number as the seedling grows, the primary buds branching to form clusters of secondary buds located in the axils of their scales. This branching requires a some degree of bursting of the buds. The resulting clusters are also found to vary in structure. Felling causes the majority of the dormant basal buds to burst, but only a certain number develop into sprouts. These sprouts differ morphologically from seedlings, especially in their pattern of ramification.

Sprouting ability and significance for coppicing of dormant buds on Betula pubescens Ehrh. Stumps

The numbers of basal dormant buds and resulting sprouts and their distribution on the stumps of the trees were studied in specimens of Betula pubescens Ehrh. of vegetative and seedling origin of different ages and growing on peatland or mineral soil. The results suggest that the number of buds at the base of the tree varies markedly from one tree to another. The old and young trees had quite similar numbers of buds, but those on mineral soil had less than those growing on peatland. The highest numbers of basal buds were found on sprout‐origin trees. Although about 90% of the basal buds were found below ground level, 1/3 of the sprouts originated from above the ground. This discrepancy can be explained by the formation of bud clusters, since the majority of the sprouts were derived from buds which occurred singly or in very small clusters. The total number of basal buds explains only part of the observed variation in sprouting potential. More important is the formation of bud clusters during the maturation...

Effects of various stress treatments on growth and ethylene evolution in seedlings and sprouts of betula pendula Roth and B. pubescens Ehrh.

Experiments were conducted to determine growth and ethylene evolution in seedlings and coppiced plants of Betula pendula Roth and B. pubescens Ehrh. when subjected to stresses relevant to the maintenance of natural forests, and especially in fuelwood plantations, i.e. cutting of the stem, thinning, bending, flooding and various combinations of these. Most of the experiments were carried out in the laboratory using 1‐year‐old seedlings or 1‐month‐old sprouts. Height and diameter growth, biomass production, morphology and ethylene evolution were studied for 1–2 months. Material for comparison was obtained from young sprouts on the stumps of fully developed trees growing under natural conditions and natural seedlings of a comparable age. Exposure of seedlings and coppiced plants to stress factors usually altered growth and increased ethylene evolution. Cutting of the stem and thinning had similar effects in that they stimulated transient ethylene evolution by both roots and stems. Bending retarded the height...

The symplasmic organization of the shoot apical meristem

The buildup of symplasmic organization has been a driving force in the evolution of multicellular plants. A symplasmic organization could arise when walled unicellular ancestors commenced subdividing their protoplasm by means of pore-bearing walls. By restricting the functional size of the pores, cells could develop physiological individuality without giving up cytoplasmic continuity. Further refinement of the pores, leading to the formation of true plasmodesmata, subsequently set the stage for the development of increasingly complex symplasmic organizations. Particularly the appearance of regulatory devices inside the pores, permitting a selective exchange of morphogenetic signals, might have been crucial in the evolution of higher plants (Carr1976; Gunning and Robards1976; Robards1976; Lucas et al.1993; Franceschi et al.1994). Since all higher plants arise from the activity of morphogenetic units, the root and shoot meristems, the symplasmic organization of these meristems might play an important role in primary morphogenesis. The symplasmic organization of the shoot apical meristem (AM) channels the flow of signals dependent on the patterns in which cells divide and establish de novo contacts with neighbours. In the following, we will examine how symplasmic networking in the AM might have played a role in the emergence of higher plants, and how it functions in the coordination of morphogenesis.