Effects of de- and rehydration on food-conducting cells in the moss Polytrichum formosum: a cytological study

BACKGROUND AND AIMS: Moss food-conducting cells (leptoids and specialized parenchyma cells) have a highly distinctive cytology characterized by a polarized cytoplasmic organization and longitudinal alignment of plastids, mitochondria, endoplasmic reticulum and vesicles along endoplasmic microtubules. Previous studies on the desiccation biology of mosses have focused almost exclusively on photosynthetic tissues; the effects of desiccation on food-conducting cells are unknown. Reported here is a cytological study of the effects of de- and rehydration on food-conducting cells in the desiccation-tolerant moss Polytrichum formosum aimed at exploring whether the remarkable subcellular organization of these cells is related to the ability of mosses to survive desiccation. METHODS: Shoots of Polytrichum formosum were dehydrated under natural conditions and prepared for transmission and scanning electron microscopy using both standard and anhydrous chemical fixation protocols. Replicate samples were then fixed at intervals over a 24-h period following rehydration in either water or in a 10 microM solution of the microtubule-disrupting drug oryzalin. KEY RESULTS: Desiccation causes dramatic changes; the endoplasmic microtubules disappear; the nucleus, mitochondria and plastids become rounded and the longitudinal alignment of the organelles is lost, though cytoplasmic polarity is in part retained. Prominent stacks of endoplasmic reticulum, typical of the hydrated condition, are replaced with membranous tubules arranged at right angles to the main cellular axis. The internal cytoplasm becomes filled with small vacuoles and the plasmalemma forms labyrinthine tubular extensions outlining newly deposited ingrowths of cell wall material. Whereas plasmodesmata in meristematic cells at the shoot apex and in stem parenchyma cells appear to be unaffected by dehydration, those in leptoids become plugged with electron-opaque material. Starch deposits in parenchyma cells adjoining leptoids are depleted in desiccated plants. Rehydration sees complete reestablishment over a 12- to 24-h period of the cytology seen in the control plants. Oryzalin effectively prevents leptoid recovery. CONCLUSIONS: The results point to a key role of the microtubular cytoskeleton in the rapid re-establishment of the elaborate cytoplasmic architecture of leptoids during rehydration. The reassembly of the endoplasmic microtubule system appears to dictate the time frame for the recovery process. The failure of leptoids to recover normal cytology in the presence of oryzalin further underlines the key role of the microtubules in the control of leptoid cytological organization.     

Response of Chinese Wampee Axes and Maize Embryos to Dehydration at Different Rates

Survival of wampee （Clausena lansium Skeels） axes and maize （Zea mays L.） embryos decreased with rapid and slow dehydration. Damage of wampee axes by rapid dehydration was much less than by slow dehydration, and that was contrary to maize embryos. The malondialdehyde contents of wampee axes and maize embryos rapidly increased with dehydration, those of wampee axes were lower during rapid dehydration than during slow dehydration, and those of maize embryos were higher during rapid dehydration than during slow dehydration. Activities of superoxide dismutase （SOD）, ascorbate peroxidase （APX） and catalase （CAT） of wampee axes markedly increased during the early phase of dehydration, and then rapidly decreased, and those of rapidly dehydrated axes were higher than those of slow dehydrated axes when they were dehydrated to low water contents. Activities of SOD and APX of maize embryos notable decreased with dehydration. There were higher SOD activities and lower APX activities of slowly dehydrated maize embryos compared with rapidly dehydrated maize embryos. CAT activities of maize embryos markedly increased during the early phase of dehydration, and then decreased, and those of slowly dehydrated embryos were higher than those of rapidly dehydrated embryos during the late phase of dehydration.     

茸毛赤瓟种子不同发育时期脱水耐性和发芽力的初步研究

茸毛赤瓟种子自花后30 d发育至55 d,发芽率、发芽指数和活力指数由0升至最大;含水量逐渐下降,但下降速率不等,发育后期存在显著的成熟脱水期。花后45 d果实干重接近最大,种子干重在45 d达到最大,种子和果实的发育基本同步。自然风干1d后,花后40~50 d的种子含水量下降2%~4%。花后40 d的种子发芽力显著提高,花后45~50 d的种子无明显变化,继续干燥,发芽率、发芽指数和活力指数均有不同程度的降低,而花后50 d的种子直到含水量低至4%后才明显下降;花后35 d和55 d的种子经过不同天数干燥后,发芽力均下降。不同发育时期茸毛赤瓟种子耐脱水力有差别,由强至弱依次为花后50、45、55、40、35 d。用半致死含水量可准确地反映不同发育时期茸毛赤瓟种子的脱水敏感性的强弱。     

The Size of Cells and Organisms in Relation to the Evolution of Embryophytes

Abstract: The earliest O₂-_-evolvers were marine cyanobacteria (3.5 billion years ago) with marine eukaryotic phototrophs from 2.0 billion years ago. These organisms were, and are, poikilo-hydric, i.e., cannot remain hydrated when exposed to a desiccating atmosphere (as can occur for intertidal benthic algae and cy anobacteria at low tide). The smallest marine primarily poikilo-hydric O₂-_-evolvers are close to the lower size limit imposed by non-scaleable components such as minimum genome size and constant membrane thickness, with cyanobacterial unicells 0.65 μn in diameter and eukaryotic unicells 0.95 μm in diameter. The largest (multicellular) marine primarily aquatic poikilohydric O₂-_-evolvers are brown algae at least 60 m long and over 100 kg fresh mass; there are no obvious constraints on the max imum size of such organisms. In freshwaters the size range for primarily poikilohydric O₂-_-evolving organisms is smaller, due to the absence of very large organisms. An even smaller size range characterizes terrestrial algae and cyanobacteria which have occurred for about 1 billion years. Desiccation-tolerant cyanobacterium and algae (intertidal, freshwater, terrestrial) are at the lower end of the size ranges. Embryophytic terrestrial O₂-_-evolvers arose some 450 million years ago and were than all poikilohydric and (probably) desiccation-tolerant. Embryophytic defining structural features re quire organisms of at least 100 μm equivalent spherical diameter for both gametophyte and sporophyte phases. Primarily poi kilohydric embryophytes are not more than 1 m tall as a result of a mechanistically mysterious size limit for desiccation-tolerant organisms. Homoiohydric embryophytes evolved some 420 mil lion years ago in the sporophyte phase (later to become the dominant terrestrial vegetation) and possibly in the gameto phyte phase (although no such homoiohydric gametophytes are known today). The homoiohydric features of gas spaces, stomata, cuticle, endohydric water conducting system and water and nutrient uptake structures require an organism at least 5 mm high; this has implications for the minimum size of mega-spores and seeds. The tallest homoiohydric plants are (or were within historic times) 130 m high, with height constrained by re source costs of the synthesis and maintenance of the mechanical and water conduction systems, andbr of xylem water trans port. Secondarily poikilohydric embryophytes in aquatic, or very damp terrestrial, habitats are derived from homoiohydric plants; they retain most homoiohydric features but are not functionally homoiohydric. The smaller secondarily poikilohydric plants are less than one tenth of the size of the smallest functionally homoiohydric plants.     

Reactive Oxygen Species Scavenging Enzymes and Down-Adjustment of Metabolism Level in Mitochondria Associated with Desiccation-Tolerance Acquisition of Maize Embryo

It is a well-known fact that a mature seed can survive losing most of its water, yet how seeds acquire desiccation-tolerance is not well understood. Through sampling maize embryos of different developmental stages and comparatively studying the integrity, oxygen consumption rate and activities of antioxidant enzymes in the mitochondria, the main origin site of reactive oxygen species (ROS) production in seed cells, we found that before an embryo achieves desiccation-tolerance, its mitochondria shows a more active metabolism, and might produce more ROS and therefore need a more effective ROS scavenging system. However, embryo dehydration in this developmental stage declined the activities of most main antioxidant enzymes and accumulated thiobarbituric acid-reactive products in mitochondria, and then destroyed the structure and functional integrity of mitochondria. In physiologically-matured embryos (dehydration-tolerant), mitochondria showed lower metabolism levels, and no decline in ROS scavenging enzyme activities and less accumulation of thiobarbituric acid-reactive products after embryo dehydration. These data indicate that seed desiccation-tolerance acquisition might be associated with down-adjustment of the metabolism level in the late development stage, resulting in less ROS production, and ROS scavenging enzymes becoming desiccation-tolerant and then ensuring the structure and functional integrity of mitochondria.     

Resurrection Plants: The Puzzle of Surviving Extreme Vegetative Desiccation

Tolerance to near complete desiccation of vegetative organs is a widespread capability in bryophytes and is also shared by a small group of vascular plants known as resurrection plants. To date more than 300 species, belonging to pteridophytes and angiosperms, have been identified that possess this kind of desiccation-tolerance. The vegetative desiccation-tolerance of resurrection plants is an inductive process displayed only under environmental stress with or without the involvement of abscisic acid as molecular signal. The different problems associated with desiccation encountered by resurrection plants render the employment of many interacting mechanisms necessary. Preservation of cell order and correct structure of membranes and macromolecules is underpinned by the synthesis of large amounts of sugars, amino acids, and small polypeptides such as late embryogenesis abundant (LEA) proteins and dehydrins. Some of these compatible solutes, such as sucrose and LEA proteins, are also involved in cytoplasm vitrification, which occurs during the last phase of desiccation. Mechanical damage due to vacuole shrinkage in dehydrating cells is avoided by cell wall folding or by replacing the water in vacuoles with nonaqueous substances. Oxidative stress, due to enhanced production of reactive oxygen species (ROS) especially by chloroplasts, is minimized through two different strategies. The homoiochlorophyllous resurrection plants, which conserve chloroplasts with chlorophylls and thylakoids upon drying, fold leaf blades and synthesize anthocyanins, as both sunscreens and free radical scavengers, and additionally increase the activity of antioxidant systems in cells. In contrast, the chloroplasts in poikilochlorophyllous species degrade chlorophylls and thylakoid membranes yielding desiccoplasts that are devoid of any internal structures. These adaptive mechanisms preserve cells from damage by desiccation and allow them to resume vital functions once rehydrated. Even if based mainly on cell protection during drying, the vegetative desiccation-tolerance of resurrection plants also relies on systems of cell recovery and repair upon rehydration. However, most of these systems are prepared during cell dehydration.     

Morphological and ultrastructural aspects of dehydration and rehydration in leaves of Sporobolus stapfianus

The resurrection species Sporobolus stapfianus Gandoger has been studied by LM, TEM and SEM in order to define the leaf morphology and fine structure and to analyse the cellular changes occurring during the processes of dehydration and rehydration of the plant. Some characteristics of the fully hydrated leaf and some ultrastructural and physiological events which take place during leaf wilting are discussed in relation to their possible role in plant desiccation-tolerance.The leaves of S. stapfianus show several characteristics common among xerophytic species. In the resurrection leaf they could play a role in slowing down the drying rate, thus leaving time to activate the mechanisms protecting the cell structures against drought damage. Actually, the S. stapfianus leaves do not undergo important cellular alterations during dehydration. The chloroplasts, in particular, retain part of their photosynthetic pigments and thylakoid membranes. Upon rewatering leaf recovery is rather fast and the tissue structure and cell organization of the fully hydrated state are already regained after two days.     

Response of Chinese Wampee Axes and Maize Embryos to Dehydration at Different Rates

Survival of wampee (Clausena lansium Sksels) axes and maize (Zea mays L.) embryos decreased with rapid and slow dehydration. Damage of wampee axes by rapid dehydration was much less than by slow dehydration, and that was contrary to maize embryos. The malondialdehyde contents of wampee axes and maize embryos rapidly increased with dehydration, those of wampee axes were lower during rapid dehydration than during slow dehydration, and those of maize embryos were higher during rapid dehydration than during slow dehydration. Activities of superoxide dismutsse (SOD), ascorbate peroxidase (APX) and catalase (CAT) of wampee axes markedly increased during the sady phase of dehydration, and then rapidly decreased, and those of rapidly dehydrated axes were higher than those of slow dehydrated axes when they were dehydrated to low water contents. Activities of SOD and APX of maize embryos notable decreased with dehydration. There were higher SOD activities and lower APX activities of slowly dehydrated maize embryos compared with rapidly dehydrated maize embryos. CAT activities of maize embryos markedly increased during the eady phase of dehydration, and then decreased, and those of slowly dehydrated embryos were higher than those of rapidly dehydrated embryos during the late phase of dehydration.