边缘细胞对荞麦根尖铝毒的防护效应和对细胞壁多糖的影响

以2个荞麦(Fygopyrum esculentum Moench)基因型‘江西荞麦’(耐性)和‘内蒙荞麦’(敏感)为材料,采用悬空培养(保持边缘细胞附着于根尖和去除根尖边缘细胞),研究边缘细胞对根尖铝毒的防护效应以及对细胞壁多糖组分的影响。结果表明,铝毒抑制荞麦根系伸长,导致根尖Al积累。去除边缘细胞的根伸长抑制率和根尖Al含量高于保留边缘细胞的根。去除边缘细胞使江西荞麦和内蒙荞麦根尖的酸性磷酸酶(APA)活性显著升高,前者在铝毒下增幅更大。同时,铝毒胁迫下去除边缘细胞的根尖果胶甲酯酶(PME)活性和细胞壁果胶、半纤维素1、半纤维素2含量显著高于保留边缘细胞的酶活性和细胞壁多糖含量。表明边缘细胞对荞麦根尖的防护效应,与其阻止Al的吸收,降低根尖细胞壁多糖含量及提高酸性磷酸酶活性有关,以此缓解Al对根伸长的抑制。     

Contrasting below-ground views of an ectomycorrhizal fungal community

Ectomycorrhizal fungal communities have been characterized in a number of ways. Here we compare colonized root-tip and mycelia views of an ectomycorrhizal fungal community. Ectomycorrhizal fungi, both as mycelia and colonized root tips, were identified in soil samples taken from a pine plantation. We determined that for some ectomycorrhizal fungal species multiple root tips from a single soil sample were not independent. Therefore in the comparison of root-tip and mycelia views, we considered species to be present or absent from each soil sample irrespective of the number of root tips colonized by the species. We observed 39 ectomycorrhizal fungal species in total, but 12 were observed exclusively as mycelia and 11 exclusively colonizing root tips. The relative frequencies of 10 species occurring as both mycelia and root tips were not independent of the method of observation. Our results suggest that ectomycorrhizal fungal species differ in their spatial distributions on root tips, and that root-tip and mycelia views of the community are different.     

Influence of the root tip and the duration of washing on k retention by excised apical root segments of corn

Excised apical segments of corn root (Zea mays) (5-15 mm from the root cap), some with and some without the root tips (0-5 mm) attached, were washed for varying time periods up to 4 hours in 0.5 mm CaSO(4). After washing, tips were removed from those segments washed with tips attached, and then all segments and tips were analyzed for K(+) content. The root tips (0-5 mm) initially contained about twice the K(+) of the apical segments (5-15 mm). The loss of K(+) did not exceed 15% in the tips or 20% in the apical segments. Loss of K(+) was most pronounced during the 1st hour of washing. There was little difference in K(+) content of apical segments washed with tips attached compared with those washed tipless. Thus, the presence of the intact root tip had no consistent influence on the ability of the older root tissue to retain K(+).     

Sodium fluxes in corn roots: comparison to Cl and K fluxes, and to Na-fluxes in barley

Abstract. Carbonylcyanide, m -chlorophenyl hydra-zone (CCCP) decreased the ATP content of barley and corn roots by 80% within 5 min. The protonophore inhibited K and Cl absorption by largely unvacuolated root tips, and vacuolated root segments of barley and corn. The protonophore also inhibited Na absorption by root segments and Na extrusion by root tips of barley; it did not affect these Na fluxes in corn root tips and segments, and Na Influx in barley root tips. It was concluded that corn roots lack a metabolic mechanism for Na extrusion from the cytoplasm to the external solution or vacuole, which is functional in barley roots.     

A study of root distribution and growth in a Pinus sylvestris L. (Scots pine) plantation in east anglia

Summary The root systems of Scots pine in a plantation were studied by three methods; soil coring, soil monoliths, and a root trench with observation windows were used to estimate root length, root diameters and the initiatio of new root tips. The vertical and horizontal distribution of roots is described and root distribution has been related to distance from the tree and soil heterogeneity. It was found that the initiation of new root tips was not readily relatable to the soil environment and the usefulness of the root window technique for observing new root tips is questioned.     

Hypoxic Induction of Anoxia Tolerance in Roots of Adh1 Null Zea mays L

Seedlings of alcohol dehydrogenase 1 null mutants (Adh1-) of Zea mays L., which fail to synthesize alcohol dehydrogenase 1 (ADH1) isozymes, were hypoxically acclimated by 18 h of exposure to an atmosphere of 4% (v/v) O2 in N2 at 25[deg]C. Their ability to tolerate subsequent anoxia by exposure to anaerobic (O2-free) conditions was compared with that of unacclimated seedlings that were transferred immediately from an atmosphere of 40% (v/v) O2 to anaerobic conditions. Only 10% of the root tips of unacclimated seminal roots survived 6 h of anoxia, whereas 70% of the hypoxically acclimated root tips were viable at 24 h. During anoxia, acclimated root tips had enhanced ADH activity compared with unacclimated root tips, through induction of Adh2. Despite this, enzyme activity was still only about 5% that of acclimated, wild-type root tips and about half that of unacclimated, wild-type root tips. During anoxia, acclimated Adh1- root tips showed a higher rate of anaerobic respiration and ethanol production, greater concentrations of ATP and total adenylates, and a greater adenylate energy charge compared with unacclimated root tips. These results suggest that although enhanced ADH activity may have raised fermentation rates in acclimated Adh1- tissues and thereby contributed to energy metabolism and viability, the high levels of ADH activity inducible in acclimated, wild-type maize root tips appear to be in excess of that required to increase rates of fermentation.     

Improved Cytoplasmic pH Regulation,Increased Lactate Efflux,and Reduced Cytoplasmic Lactate Levels Are Biochemical Traits Expressed in Root Tips of Whole Maize Seedlings Acclimated to a Low-Oxygen Environment

We tested the hypothesis (J.-H. Xia and P.H. Saglio [1992] Plant Physiol 100: 40-46) that the enhanced ability of maize (Zea mays) root tips to survive anoxia, elicited by a 4-h exposure to 3% O2 ("acclimation"), is due to less cytoplasmic acidosis early in anoxia. Cytoplasmic pH and fermentation reactions were monitored in excised and intact (attached) maize root tips by simultaneous in vivo 13C- and 31P-NMR spectroscopy. We demonstrate that both excised and intact acclimated root tips have significantly higher cytoplasmic pH values under anoxia. This reduction in cytoplasmic acidosis is greater in intact root tips. Remarkably, cytoplasmic pH does not change when root tips are transferred from 3% O2 to anoxia. The earlier observation of considerable lactate efflux and lowered intracellular lactate in excised, acclimated root tips (ibid.) was extended to intact seedlings. The predominant fermentation end product retained in the cells of acclimated root tips is alanine. We discuss the relationship between cytoplasmic pH and levels of intracellular lactate and alanine in sugar-replete roots, and the role of cytoplasmic pH in determining survival under anoxia.     

Rescue of shoot apical meristems of chrysanthemum by culturing on root tips

A new method to regenerate plants from leaf primordia-free shoot apical meristem domes (LP-free SAMs) was developed by establishing the meristem dome on the cut surface of root tips. Ten days after culture, the viable rate of LP-free SAMs of chrysanthemum Piato attached to chrysanthemum root tips was >40%. Shoot regeneration was not observed from LP-free SAMs without the root tips. When LP-free SAMs of chrysanthemum were transferred to root tips of either petunia, cabbage, or carnation, the highest shoot regeneration rate was observed with cabbage root tips. Microscopic observation documented that the LP-free SAM temporarily adhered to the cut surface of the root tip of cabbage.Communicated by K.K. Kamo     

Nucleotide Levels Do Not Critically Determine Survival of Maize Root Tips Acclimated to a Low-Oxygen Environment

We tested the hypothesis that ATP levels and energy charge determine the resistance of maize (Zea mays) root tips to anoxia. We focused on root tips of whole maize seedlings that had been acclimated to low O2 by exposure to an atmosphere of 3% (v/v) O2 in N2. Acclimated anoxic root tips characteristically have higher ATP levels and energy charge and survive longer under anoxia than nonacclimated tips. We poisoned intact, acclimated root tips with either fluoride or mannose, causing decreases in ATP and energy charge to values similar to or, in most cases, below those found in nonacclimated anoxic tips. With the exception of the highest fluoride concentration used, the poisoned, acclimated tips remained much more tolerant of anoxia than nonacclimated root tips. We conclude that high ATP and energy charge are not components critical for the survival of acclimated root tips during anoxia. The reduced nucleotide status in poisoned, acclimated root tips had little effect on cytoplasmic pH regulation during anoxia. This result indicates that in anoxic, acclimated root tips either cytoplasmic pH regulation is not dominated by ATP-dependent processes or these processes can continue in vivo largely independently of any changes in ATP levels in the physiological range. The role of glycolytic flux in survival under anoxia is discussed.     

Fertility of Triticum aestivum plants derived from anther culture

The fertility of plants from wheat anther culture was studied. It was found that one half of the 36 plants with diploid root tips didn't set seeds at all, and that 41 of the 42 plants with haploid root tips were completely sterile. It was surprising that sterility was so widely distributed even among the plants with diploid root tips.     

Effect of boron on the expression of aluminium toxicity in Phaseolus vulgaris

The interaction of boron (B) and aluminium (Al) was investigated in 5-day-old seedlings of soybean cv. Maple Arrow. Al treatment inhibited root elongation and callose formation in root tips particularly after 4-h Al treatment. After 10 and 24 h, both parameters indicated increasing recovery from Al stress. B deficiency aggravated Al toxicity compared with B sufficiency. B deficiency did lead to an increase in unmethylated pectin in the first 3 mm of the root tip. This increase in potential binding sites is reflected in generally higher Al contents in root tips of B-deficient plants. A fractionated extraction of Al from the root tips showed that citrate-exchangeable and non-exchangeable Al steeply increased up to 4 h, but then decreased after 10- and 24-h Al treatment faster in B-sufficient than in B-deficient plants. This decrease of Al contents can be explained by an Al-enhanced release of citrate from the root tips after 10-h Al treatment. However, the citrate exudation rate was the same (after 10 h) or even lower (after 24 h) in B-sufficient plants and thus cannot explain the faster decrease in Al contents of the root tips compared with the B-deficient plants. We, therefore, propose that under B deficiency, Al is more strongly bound by the pectic network of the cell wall of the root tips, which delays or prevents the recovery from initial Al stress through exudation of citrate, and thus explains the greater Al sensitivity of B-deficient common bean roots.     

DEVELOPMENT OF THE WOODY PORTION OF THE ROOT SYSTEM OF BETULA PAPYRIFERA

Our ontogenetic analysis of paper birch root systems shows that the fate of a root tip is related to its relative primary xylem diam (PXD). Lateral root tips with an initial PXD less than about 25 % of the parent root PXD are ephemeral. Some tips having a PXD of more than about 25 % of the parent root PXD become the permanent portions of the root system, enlarging over time as they elongate. In seedling root systems, the primary root and first-formed laterals are initially about the same size, and their PXD's all enlarge with increasing distance from the stem as the tips elongate to form the initial horizontal woody framework. Permanent lateral root branches with a large relative PXD develop after root tip injury, when the root tip is forced to grow in a curve, or from other unaccountable causes. Our observations show the importance of using relative diameters when classifying roots and when applying the concept of heterorhizy to paper birch root systems. Evidence points to the existence of some form of apical dominance in roots.     

Distribution of G-actin is related to root hair growth of wheat

BACKGROUND AND AIMS: Actin distribution in root hair tips is a controversial topic. Although the relationship between Ca2+ gradient and actin dynamics in plant tip-growth has been a focus of study, there is still little direct evidence on the exact relationship in root hair tip-growth. METHODS: G-actin was labelled by fluorescein isothiocyanate-DNase I. F-actin was labelled by tetramethylrhodamine isothiocyanate-phalloidin. Actin in root hairs of Triticum aestivum (wheat) was investigated using confocal laser-scanning microscopy. KEY RESULTS: Thick F-actin bundles did not extend into a region of approx. 5-10 microm from the tip of the growing root hairs, although they gave off branches of fine actin filaments in the hair tips. A tip-focused G-actin gradient was shown at the extreme apex of growing root hairs. In full-grown wheat root hairs, the tip-focused G-actin gradient disappeared while the thick F-actin bundles extended into the tips. BAPTA-AM, a Ca2+ disruption agent, also caused the tip-focused G-actin gradient to disappear and the diffuse F-actin bundles to appear in the tips of wheat root hairs. CONCLUSIONS: These results suggest that the tip-focused gradient of intracellular G-actin concentration at the extreme apex may be essential for root hair growth, and that preserving the tip-focused gradient needs a high Ca2+ concentration in the root hair tips.     

Changes in the expression and the enzymic properties of the 20S proteasome in sugar-starved maize roots. evidence for an in vivo oxidation of the proteasome

The 20S proteasome (multicatalytic proteinase) was purified from maize (Zea mays L. cv DEA 1992) roots through a five-step procedure. After biochemical characterization, it was shown to be similar to most eukaryotic proteasomes. We investigated the involvement of the 20S proteasome in the response to carbon starvation in excised maize root tips. Using polyclonal antibodies, we showed that the amount of proteasome increased in 24-h-carbon-starved root tips compared with freshly excised tips, whereas the mRNA levels of alpha 3 and beta 6 subunits of 20S proteasome decreased. Moreover, in carbon-starved tissues, chymotrypsin-like and caseinolytic activities of the 20S proteasome were found to increase, whereas trypsin-like activities decreased. The measurement of specific activities and kinetic parameters of 20S proteasome purified from 24-h-starved root tips suggested that it was subjected to posttranslational modifications. Using dinitrophenylhydrazine, a carbonyl-specific reagent, we observed an increase in carbonyl residues in 20S proteasome purified from starved root tips. This means that 20S proteasome was oxidized during starvation treatment. Moreover, an in vitro mild oxidative treatment of 20S proteasome from non-starved material resulted in the activation of chymotrypsin-like, peptidyl-glutamyl-peptide hydrolase and caseinolytic-specific activities and in the inhibition of trypsin-like specific activities, similar to that observed for proteasome from starved root tips. Our results provide the first evidence, to our knowledge, for an in vivo carbonylation of the 20S proteasome. They suggest that sugar deprivation induces an oxidative stress, and that oxidized 20S proteasome could be associated to the degradation of oxidatively damaged proteins in carbon starvation situations.     

In vitro synthesis of Pisolithus-Eucalyptus ectomycorrhizae: synchronization of lateral tip emergence and ectomycorrhizal development

 A simple and reproducible in vitro system is described for the synthesis of Pisolithus-Eucalyptus grandis ectomycorrhizae. Hyphal discs from actively growing colonies were placed in large petri dishes containing minimum nutrient agar overlaid with cellophane and allowed to grow for 7 days. Seeds were then surface sterilized and placed above the expanding fungal colonies and the plates slanted. Seedlings that germinated and grew in the presence of fungal hyphae had twice as many lateral root tips as seedlings that germinated before they were transferred onto hyphal mats. In addition, the lateral root tips of inoculated seedlings had a faster maturation rate and emerged closer to the primary root apex than non-inoculated seedlings. All lateral tips emerged in contact with fungal hyphae and the differentiation of ectomycorrhizae was followed by examining lateral tips basipetally along a single primary root. Typical ectomycorrhizae had formed on 4-day-old lateral tips, i.e. a mantle, radially elongated epidermal cells and a Hartig net commencing about 0.3 mm behind the lateral root apex. Thereafter, the mantle continued to thicken and the apical meristem diminished. The Hartig net often surrounded the apex of 11- to 12-day-old lateral root tips. This model system will facilitate detailed studies on synchronized ectomycorrhizal development and associated molecular and biochemical changes. Accepted: 12 January 1996     

Physiological aspects of aluminium tolerance associated with the long arm of chromosome 2D of the wheat (Triticum aestivum L.) genome

Aluminum (Al) uptake in roots of wheat nearisogenic lines having differing tolerances to aluminium toxicity was studied using roots and root segments immersed in a nutrient solution at a controlled pH and temperature. At low Al concentrations a mechanism preventing root tips from accumulating too much Al was observed in an Al-tolerant isoline and a BH1146 euploid. This mechanism was more efficient when divalent cations of calcium or magnesium were present in the nutrient medium. Al accumulation steadily increased in root tips of the Al-sensitive wheat isoline during all 24 h of incubation, and the presence of divalent cations in the medium even increased Al concentration in root tissue. However, at higher Al concentrations in the medium the mechanism preventing the root tips of Al-tolerant genotypes from accumulating too much Al was not observed, and in effect Al concentration in root tips of both Al-tolerant and Al-sensitive isolines increased. It is concluded that genetical factors are located on the long arm of chromosome 2D from the BH1146 euploid that control the mechanism preventing root apical meristems from accumulating too much Al at low Al concentrations in the medium. However, there must be other genetical factors also located on this chromosome segment that control Al detoxication in root tips of Al-tolerant lines at higher external Al concentrations.     

Attachment of Agrobacterium tumefaciens B6 and A. radiobacter K84 to Tomato Root Tips

Agrobacterium tumefaciens B6 and the avirulent Agrobacterium radiobacter strain K84 attached to in vitro-cultured tomato root tips, but the binding of strain B6 to root tips was greater than the binding of strain K84. Strain K84 was not able to block the attachment of A. tumefaciens B6 to in vitro-cultured tomato root tips.     

Changes in growth and structure of pea primary roots (Pisum sativum L. cv. Alaska) as a result of sudden flooding

Pea (Pisum sativum L. cv. Alaska) primary roots were exposed to flooding after growth for 4 or 5 d at 25 degrees C under relatively dry conditions. Flooding after 4 d growth reduced, but did not stop, primary root growth, and cavities caused by degradation of central vascular cells were typically found from 10-60 mm from the tips. Flooding after 5 d stopped primary root growth and caused cell death in the tips, and vascular cavities formed that typically were 20-60 mm from the tips of the roots. Degradation of root tip cells in 5-day-roots was very rapid and began in the elongation zone and later in the apical zone. Root tips discolored, narrowed or curled before growth arrest. The mitotic indices of 5-day-root tips were suppressed by the flooding treatment. A few mitotic figures were observed in roots treated with flooding after 4 d growth. Affected cells had condensed nuclei, but cytoplasms appeared to be normal in the early stages of cell degradation. Later these cells became very vacuolated. The relationship of flooding to root growth, vascular cavity formation, and the morphology of pea primary roots is described with regard to the ability to resist flooding stress.     

草甘膦和百草枯对毛桃幼苗根系形态及地上部生长的影响

以砧木毛桃幼苗为研究对象,通过土施草甘膦和百草枯研究2种桃园常用除草剂对毛桃营养生长、根系结构、根尖细胞分裂、叶片光合特性等的影响,为除草剂在桃生产中的安全使用提供科学依据。结果表明: 草甘膦处理显著抑制毛桃地上部和根生长,与对照相比,株高降低31.5%,总根系长度、总根表面积、总根体积和总根尖数分别降低了39.5%、39.5%、49.8%和44.6%,而百草枯处理以上指标与对照差异均不显著;草甘膦和百草枯处理后毛桃根尖细胞有丝分裂指数分别降低38.0%和35.9%,且草甘膦处理分裂中期细胞数占分裂细胞总数的比例显著低于对照和百草枯处理;毛桃根尖细胞对2种除草剂响应迅速,从处理第2天开始根尖细胞电解质渗漏率始终显著高于对照。叶片细胞电解质渗漏率则从处理5 d后开始显著升高,且草甘膦处理出现幼叶基部变黄并向叶尖蔓延,同时部分叶尖逐渐焦枯的现象;2种除草剂处理导致毛桃叶片净光合速率、气孔导度、蒸腾速率有不同程度的降低,其中草甘膦处理下降更明显。综上,使用草甘膦和百草枯均会降低毛桃幼苗根尖细胞分裂指数,提高根尖细胞电解质渗透率,总体降低叶片净光合速率。草甘膦对毛桃营养生长、叶片光合作用影响更大,而且会造成幼叶变黄、叶尖焦枯等现象。     

Aluminum stress and protein synthesis in near isogenic lines of Triticum aestivum differing in aluminum tolerance

Aluminum (Al) stress was examined in three lines of wheat ( Triticum aestivum L.) by measuring root lengths, protein synthesis and protein accumulation in seedling root tips grown in a hydroponic system. An Al-sensitive, recurrent wheat parent (cv. Katepwa) showed very little root growth in low Al concentrations. In contrast, an Al-tolerant near isogenic line (Alikat) and Al-tolerant donor (cv. Maringa) had much greater root growth. Segregation data from an F₂ population (Katepwa × Alikat) showed that one major gene controlled Al tolerance based on root growth ( X ²= 0.651). All three lines showed an approximately 2-fold increase in [³⁵S]-Met incorporation in root tips after 3 days in Al and a comparable increase in root-tip dry weight. Maringa and Alikat root tips showed an increased total protein content while Katepwa root tips showed no increase in total protein content during the Al stress. Based on higher specific activities, insoluble proteins were preferentially translated in all three lines during Al stress. Proteinase activity in Katepwa root tips was 1.7-fold higher during Al stress, with Maringa and Alikat showing no change in proteinase activity. The Al-induced, increased proteinase activity in Katepwa appeared to inhibit soluble protein accumulation.