植物对水淹胁迫的响应与适应

水淹是植物遭受的主要的非生物胁迫之一.水淹胁迫使植物处于周期或长期的厌氧或缺氧状态,限制植物的需氧呼吸和维持生命活动所需的能量产生,导致土壤还原势的降低和有毒物质的积累,从而对植物的生存构成严重威胁.在长期的进化过程中,一些植物能够忍耐短期或长期的水淹生境而存活下来.目前分析植物感知水淹胁迫的主要途径为感知体内氧浓度的降低和感知体内乙烯浓度的增加.淹水胁迫下植株的适应策略主要包括:1)茎的伸长生长、不定根和通气组织的形成等形态学方面的适应;2)代谢途径的改变,淹水植物主要通过厌氧代谢获得维持生命的能量;3)通过体内乙烯、赤霉素和脱落酸等激素含量水平的改变来调节生理活动或形态、解剖等方面的变化;4)抗氧化酶系统对厌氧胁迫植株体内有毒的活性氧自由基的清除.运用分子生物学和生物信息学等手段找出由水淹胁迫诱导的相关基因并对其进行克隆,繁殖与培育具有耐水淹能力强的植物种类、品种和生态型,将是从事植物抗水淹胁迫研究的科研人员的目标和方向.     

植物根内通气组织形成的研究进展

植物能否在湿地或淹涝环境中生长,很大程度上取决于植物是否具有健全发达的通气组织。在结合形态学和分子生物学等方面研究的基础上,概述了植物根内通气组织的形成过程,主要涉及生理功能、诱导因子和相关酶等,推测细胞程序性死亡是溶生性通气组织形成的机理,乙烯在整体信号转导网络中起关键性中介作用。     

植物根内通气组织形成机理的研究进展

植物能否在湿地或淹涝环境中生长, 很大程度上取决于植物是否具有健全发达的通气组织。在结合形态学和分子生物学等方面研究的基础上, 概述了植物根内通气组织的形成过程, 主要涉及生理功能、诱导因子和相关酶等, 推测细胞程序性死亡是溶生性通气组织形成的机理, 乙烯在整体信号转导网络中起关键性中介作用。     

湿地植物根形态结构和泌氧与盐和重金属吸收、积累、耐性关系的研究进展

目前大面积湿地面临着重金属污染和盐渍化问题。利用湿地植物修复这些受损生态系统和提高海水稻的产量、减少毒性金属元素在稻米中的积累是当前面临的重要任务。湿地植物(包括水稻)已发展出各种策略和机制来耐受不同的环境胁迫,它们的根系发育具有可塑性,如根形态和解剖结构会随外界条件的变化而变化,这些变化直接影响其对环境胁迫的适应性能。近年来,对湿地植物根形态和结构、泌氧与其对盐、重金属的吸收、积累和耐性之间的关系方面进行了一些重要研究。本文分别对湿地植物根系形态、质外体屏障、通气组织和泌氧与其对盐和重金属吸收、积累和耐性的关系等方面的研究进展进行了综述,并对该领域未来的发展方向作了展望。     

水翁(Cleistocalyx operculatus)幼苗对淹水的反应初报

研究了水翁在生理和形态上对3个月淹水期的反应。在潮湿或淹水条件下水翁能存活并保持一定的净光合速率和生长速率,全淹条件下存活期为60d,水翁对淹水的适应包括：（1）淹和的茎部产生肥大皮孔和不定根,（2）不定根系的活力比正常根系的活力高,有不定根的植株的气孔传导率和蒸腾速率比没有不定根的植株高得多。水翁是一种耐淹植物,可在河岸、库岸等水位波动地区种植。     

独脚金内酯调控植物根系发育的分子机制研究的进展

独脚金内酯(strigolactones,SLs)是近年来发现的新型植物激素,参与调控植物生长发育过程,SLs在调控根系形态方面具有重要的作用。该文重点综述了SLs对植物主根、侧根、根毛及不定根的调节,特别是SLs与其他信号分子如生长素、乙烯、NO等的相互作用,以及SLs在氮磷胁迫条件下对根系调控的研究进展,为进一步深入了解SLs对植物生长和发育的调节奠定基础。     

植物的补偿性生长

植物有多种适应方式来保护自身生存和与外界环境协调共存.综述了植物适应动物采食伤害的一种策略--补偿性生长.主要介绍了植物的补偿性生长现象、补偿性生长过程中植物的形态与生理响应、补偿性生长产生的机理和条件以及目前对植物补偿性生长现象认识存在的争议等.植物的补偿性生长特性研究是建立在达尔文进化论基础之上的,对于进一步明确植物对环境的适应机理具有重要的意义.     

根毛的生长发育及其遗传基础

根毛是根系特异化表皮细胞外伸形成的管状凸起物,是植物吸收矿质养分和水分的重要器官。根毛的发育可分为根毛细胞命运决定、根毛起始、根毛顶端生长和根毛成熟等阶段。本文对根毛发育生长过程中的细胞形态及其生理生化变化进行了综述,并从根表皮细胞命运决定分子机理, EXPANSIN、bHLH和MYB等转录因子以及小G蛋白和生长素/乙烯等方面简要说明了根毛生长发育的遗传基础。     

湿地植物对泥沙淤积的适应

泥沙淤积是湿地常见自然现象,常造成土壤容重、水含量和金属元素含量增加,而使土壤有机质、通气性及温度降低等,深刻影响着湿地植物生存、生长及植被演化.在长期适应进化过程中,适应者演化出一套有效的适应策略,克服了泥沙淤积的负面影响甚至依赖泥沙淤积完成生命周期,而不适应者则被驱离出湿地环境.主要适应策略包括:1)生活史对策,如...     

不同耐渍基因型芝麻在厌氧胁迫下根系的生理与结构变化

利用4种耐渍性不同的芝麻基因型,在厌氧胁迫条件下检测了根部无氧呼吸酶活性、内源乙烯含量并调查了根形态和解剖结构,以比较研究旱生植物耐渍性的主要机制。同时设计了田间分期多次淹水试验,观察早期淹水训练对芝麻生长和产量的影响。结果表明：耐渍种质的乙烯释放量在根中增加了6．06倍,茎中1．76倍,不定根数量增加了4．0～5．0倍,在初生根和不定根皮层形成典型的通气组织。非耐渍种质中未检测到乙烯变化,不定根数量增加了0．79～1．8倍,根中无明显的通气组织发生,但是乙醇脱氢酶(ADH)活性可增加4．2～9．3倍,高于耐渍种质。分期淹水试验以四对真叶期和初花期处理产量较高,与对照无显著差异,而终花期一次性淹水产量损失最大。综合分析认为,不定根增生和根皮层通气组织的形成是芝麻耐渍性的重要机制,根中内源乙烯的增加与结构适应变异有关。淹水训练能够有效地改善芝麻品种耐渍能力为结构适应提供了进一步的证据。     

Enhanced formation of aerenchyma and induction of a barrier to radial oxygen loss in adventitious roots of Zea nicaraguensis contribute to its waterlogging tolerance as compared with maize (Zea mays ssp. mays)

Enhancement of oxygen transport from shoot to root tip by the formation of aerenchyma and also a barrier to radial oxygen loss (ROL) in roots is common in waterlogging‐tolerant plants. Zea nicaraguensis (teosinte), a wild relative of maize (Zea mays ssp. mays), grows in waterlogged soils. We investigated the formation of aerenchyma and ROL barrier induction in roots of Z. nicaraguensis, in comparison with roots of maize (inbred line Mi29), in a pot soil system and in hydroponics. Furthermore, depositions of suberin in the exodermis/hypodermis and lignin in the epidermis of adventitious roots of Z. nicaraguensis and maize grown in aerated or stagnant deoxygenated nutrient solution were studied. Growth of maize was more adversely affected by low oxygen in the root zone (waterlogged soil or stagnant deoxygenated nutrient solution) compared with Z. nicaraguensis. In stagnant deoxygenated solution, Z. nicaraguensis was superior to maize in transporting oxygen from shoot base to root tip due to formation of larger aerenchyma and a stronger barrier to ROL in adventitious roots. The relationships between the ROL barrier formation and suberin and lignin depositions in roots are discussed. The ROL barrier, in addition to aerenchyma, would contribute to the waterlogging tolerance of Z. nicaraguensis.     

Roots of willow (Salix viminalis L.) show marked tolerance to oxygen shortage in flooded soils and in solution culture

Responses to soil flooding and oxygen shortage were studied in field, glasshouse and controlled environment conditions. Established stools ofSalix viminalis L., were compared at five field sites in close proximity but with contrasting water table levels and flooding intensities during the preceding winter. There was no marked effect of site on shoot extension rate, time to half maximum length or final length attained. When rooted cuttings were waterlogged for 4 weeks in a glasshouse, soil redox potentials quickly decreased to below zero. Shoot extension was slowed after a delay of 20 d, while, in the upper 100 mm of soil, formation and outgrowth of unbranched adventitious roots with enhanced aerenchyma development was promoted after 7 d. At depths of 100–200 mm and 200–300 mm, extension by existing root axes was halted by soil flooding, while adventitious roots from above failed to penetrate these deeper zones. After 4 weeks waterlogging, all arrested root tips recommenced elongation when the soil was drained; their extension rates exceeding those of roots that were well-drained throughout. Growth in fresh mass was also stimulated. The additional aerenchyma found in adventitious roots in the upper 100 mm of soil may have been ethylene regulated since gas space development was inhibited by silver nitrate, an ethylene action inhibitor. The effectiveness of aerenchyma was tested by blocking the entry of atmospheric oxygen into plants with lanolin applied to lenticels of woody shoots of plants grown in solution culture. Root extension was halved, while shoot growth remained unaffected. H Lambers Section editor     

Formation and function of secondary aerenchyma in hypocotyl, roots and nodules of soybean (Glycine max) under flooded conditions

Flooding is a major problem in many areas of the world and soybean is susceptible to the stress. Understanding the morphological mechanisms of flooding tolerance is important for developing flood-tolerant genotypes. We investigated secondary aerenchyma formation and function in soybean (Glycine max) seedlings grown under flooded conditions. Secondary aerenchyma, a white and spongy tissue, was formed in the hypocotyl, tap root, adventitious roots and root nodules after 3 weeks of flooding. Under irrigated conditions aerenchyma development was either absent or rare and phellem was formed in the hypocotyl, tap root, adventitious roots and root nodules. Secondary meristem partially appeared at the outer parts of the interfascicular cambium and girdled the stele, and then cells differentiated to construct secondary aerenchyma in the flooded hypocotyl. These morphological changes proceeded for 4 days after the initiation of the flooding. After 14 days of treatment, porosity exceeded 30% in flooded hypocotyl with well-developed secondary aerenchyma, while it was below 10% in hypocotyl of irrigated plants that had no aerenchyma. When Vaseline was applied to the hypocotyl of plants from a flooded treatment to prevent the entry of atmospheric oxygen into secondary aerenchyma, plant growth, especially that of roots, was sharply inhibited. Thus secondary aerenchyma might be an adaptive response to flooding.     

G proteins as regulators in ethylene-mediated hypoxia signaling

Waterlogging or flooding are frequently or constitutively encountered by many plant species. The resulting reduction in endogenous O₂ concentration poses a severe threat. Numerous adaptations at the anatomical, morphological and metabolic level help plants to either escape low oxygen conditions or to endure them. Formation of aerenchyma or rapid shoot elongation are escape responses, as is the formation of adventitious roots. The metabolic shift from aerobic respiration to anaerobic fermentation contributes to a basal energy supply at low oxygen conditions. Ethylene plays a central role in hypoxic stress signaling, and G proteins have been recognized as crucial signal transducers in various hypoxic signaling pathways. The programmed death of parenchyma cells that results in hypoxia-induced aerenchyma formation is an ethylene response. In maize, aerenchyma are induced in the absence of ethylene when G proteins are constitutively activated. Similarly, ethylene induced death of epidermal cells that cover adventitious roots at the stem node of rice is strictly dependent on heterotrimeric G protein activity. Knock down of the unique Gα gene RGA1 in rice prevents epidermal cell death. Finally, in Arabidopsis, induction of alcohol dehydrogenase with resulting increased plant survival relies on the balanced activities of a small Rop G protein and its deactivating protein RopGAP4. Identifying the general mechanisms of G protein signaling in hypoxia adaptation of plants is one of the tasks ahead.Key words: submergence, hypoxia, ethylene, G protein, reactive oxygen species, H₂O₂     

Ethylene-induced activation of xylanase in adventitious roots of maize as a response to the stress effect of root submersion]

Submersion of roots of ten-day-old maize (Zea mays L.) seedlings was accompanied by a decrease in pO2 and an increase in pCO2 of the medium adjacent to roots. These changes stimulated ethylene evolution in intact plants. Enhanced biosynthesis of ethylene was accompanied by xylanase activation in adventitious roots. As a result, an enhanced formation of aerenchyma was observed in the cortex of adventitious roots. Therefore, these processes resulted in the development of a ventilation system by which O2 can reach the root system exposed to hypoxia. The volume of aerenchyma was assessed by the volume of gas cavities (porosity). In contrast to the main root, the growth of adventitious roots was not inhibited under these conditions. Enlargement of the stem base and increase in the number of aerenchymatous adventitious roots facilitated the oxygen supply to submerged organs of plants.     

Cortical Aerenchyma formation in hypocotyl and adventitious roots of Luffa cylindrica subjected to soil flooding

BACKGROUND AND AIMS: Aerenchyma formation is thought to be one of the important morphological adaptations to hypoxic stress. Although sponge gourd is an annual vegetable upland crop, in response to flooding the hypocotyl and newly formed adventitious roots create aerenchyma that is neither schizogenous nor lysigenous, but is produced by radial elongation of cortical cells. The aim of this study is to characterize the morphological changes in flooded tissues and the pattern of cortical aerenchyma formation, and to analyse the relative amount of aerenchyma formed. METHODS: Plants were harvested at 16 d after the flooding treatment was initiated. The root system was observed, and sections of fresh materials (hypocotyl, tap root and adventitious root) were viewed with a light or fluorescence microscope. Distributions of porosity along adventitious roots were estimated by a pycnometer method. KEY RESULTS: Under flooded conditions, a considerable part of the root system consisted of new adventitious roots which soon emerged and grew quickly over the soil surface. The outer cortical cells of these roots and those of the hypocotyl elongated radially and contributed to the development of large intercellular spaces. The elongated cortical cells of adventitious roots were clearly T-shaped, and occurred regularly in mesh-like lacunate structures. In these positions, slits were formed in the epidermis. In the roots, the enlargement of the gas space system began close to the apex in the cortical cell layers immediately beneath the epidermis. The porosity along these roots was 11-45 %. In non-flooded plants, adventitious roots were not formed and no aerenchyma developed in the hypocotyl or tap root. CONCLUSIONS: Sponge gourd aerenchyma is produced by the unique radial elongation of cells that make the expansigeny. These morphological changes seem to enhance flooding tolerance by promoting tissue gas exchange, and sponge gourd might thereby adapt to flooding stress.     

Aerenchyma (Gas-space) Formation in Adventitious Roots of Rice (Oryza sativa L.) is not Controlled by Ethylene or Small Partial Pressures of Oxygen

Jackson, M. B., Fenning, T. M., and Jenkins, W. 1985 Aerenchyma(gas-space) formation in adventitious roots of rice (Oryza sativaL.) is not controlled by ethylene or small partial pressuresof oxygen.—J. exp. Bot. 36: 1566–1572. The extent of gas-filled voids (aerenchyma) within the cortexof adventitious roots of vegetative rice plants (Oryza sativaL. cv. RB3) was estimated microscopically from transverse sectionswith the aid of a computer-linked digitizer drawing board. Gas-spacewas detectable in 1-d-old tissue and increased in extent withage. After 7 d, approximately 70% of the cortex had degeneratedto form aerenchyma. The extent of the voids in 1-4-d-old tissuewas not increased by stagnant, poorly-aerated external environmentscharacterized by sub-ambient oxygen partial pressures and accumulationsof carbon dioxide and ethylene. Treatment with small oxygenpartial pressures, or with carbon dioxide or ethylene appliedin vigorously stirred nutrient solution also failed to promotethe formation of cortical gas-space. Furthermore, ethylene productionby rice roots was slowed by small oxygen partial pressures typicalof stagnant conditions. Silver nitrate, an inhibitor of ethylene action, did not retardgas-space formation; similarly when endogenous ethylene productionwas inhibited by the application of aminoethoxyvinylglycine(A VG), aerenchyma development continued unabated. Cobalt chloride,another presumed inhibitor of ethylene biosynthesis, did notimpair formation of the gas in rice roots nor did it decreasethe extent of aerenchyma even if A VG was supplied simultaneously.These results contrast with those obtained earlier using rootsof Zea mays L. We conclude that in rice, aerenchyma forms speedily even inwell-aerated environments as an integral part of ordinary rootdevelopment There seems to be little or no requirement for ethyleneas a stimulus in stagnant root-environments where aerenchymais likely to increase the probability of survival. Key words: Rice (Oryza sativa L.), ethylene, flooding, aeration, aerenchyma, environmental stress     

Partial flooding enhances aeration in adventitious roots of black willow (Salix nigra) cuttings

Black willow (Salix nigra) cuttings are used for streambank stabilization where they are subjected to a range of soil moisture conditions including flooding. Flooding has been shown to adversely impact cutting performance, and improved understanding of natural adaptations to flooding might suggest handling and planting techniques to enhance success. However, data assessing the root aeration in adventitious roots that are developed on cuttings of woody species are scant. In addition, it appears that no data are available regarding aeration of the root system under partially flooded conditions. This experiment was designed to examine the effects of continuous flooding (CF) and partial flooding (PF) on aerenchyma formation and radial oxygen loss (ROL) in black willow cuttings. Photosynthetic and growth responses to these conditions were also investigated. Under laboratory condition, replicated potted cuttings were subjected to three treatments: no flooding (control, C), CF, and PF. Water was maintained above the soil surface in CF and at 10 cm depth in PF. Results indicated that after the 28-d treatments, root porosity ranged between 28.6% and 33.0% for the CF and C plants but was greater for the PF plants (39.2% for the drained and 37.2% for the flooded portions). A similar response pattern was found for ROL. In addition, CF treatment led to decreases in final root biomass and root/shoot ratio. Neither CF nor PF had any detectable adverse effects on plant gas exchange or photosystem II functioning. Our results indicated that S. nigra cuttings exhibited avoidance mechanisms in response to flooding, especially the partially flooded condition which is the most common occurrence in riparian systems.     

Physiological adaptation of waxapple to waterlogging

Waxapple (Syzygium samarangense Men. et. Perry) plants receiving up to 40d of continuous flooding treatment showed no symptoms of physiological disorder, but the treatment resulted in early flowering. In this report, several physiological parameters of flooded plants are compared with those of nonflooded plants. Both control plants and 9-d-flooding-treated plants exhibited aerenchyma formation in the cortex tissue beginning 5 cm from the root tip. After 7d flooding treatment, the oxygen consumption rate of the root section was only 20% of that of the controls. Following flooding treatment, the roots showed an increase in alcohol dehydrogenase activity as well as an increase in three isozymes. However, malate dehydrogenase activity was decreased, and no significant change of NADP-malic enzyme activity was observed. There were no significant differences in levels of ethylene, 1-aminocyclopropane-1-carboxylic acid and 1-(malonylamino) cyclopropane-1-carboxylic acid in petiole and roots of flooded and non-flooded plants during the stage examined. It is inferred that the presence of aerenchyma in the root cortex allows a higher level of internal gas exchange, and thus, makes waxapple surprisingly flood tolerant. However, reduced root oxygen consumption rate may have limited root respiration rate and vegetative growth.     

The barrier to radial oxygen loss from roots of rice (Oryza sativa L.) is induced by growth in stagnant solution

The present report describes experiments in which the effects of growth in aerated and stagnant nutrient solutions on adventitious root porosities and patterns of radial O2 loss (ROL) from the roots of four genotypes of rice (Oryza sativa L.) were evaluated. The genotypes studied are usually cultivated in farming systems which differ markedly in their degree of soil waterlogging and flooding. Rice genotypes were found to differ in the constitutive porosity (% gas space) of their adventitious roots when grown in aerated solutions (lowest was 16%, highest was 30%), and the roots grown in stagnant conditions had porosities between 28% and 38%. ROL from the adventitious roots raised in aerated solution increased with distance behind the tip in three of the four genotypes; whereas for roots raised in stagnant solution, ROL decreased with distance behind the tip which is indicative of a high resistance to diffusion between the aerenchyma and external medium. For example, at 35 mm behind the root tip the ROL from roots of the 'deepwater' cultivar grown in stagnant conditions was 0.7% of the rate of its aerated roots, for the 'lowland' cultivar it was 5.6%, and for one of the 'upland' cultivars it was 43.6%. Thus, the barrier to ROL from the adventitious roots in three of the four genotypes was induced by growth in stagnant nutrient solution. A low rate of ROL from the basal zones of roots in an O2-free environment is of adaptative value since longitudinal diffusion of O2 to the root apex would be enhanced which, in turn, enables greater penetration of roots into anaerobic soils.