Modulation of eIF5A1 expression alters xylem abundance in Arabidopsis thaliana

Eukaryotic translation initiation factor 5A (eIF5A) is thoughtto facilitate protein synthesis by participating in the nuclearexport of specific mRNAs. In Arabidopsis, there are three isoformsof eIF5A. One of them, AteIF5A1, has been shown to be expressedin vascular tissue, specifically developing vessel members,using GUS as a reporter. In order to determine whether AteIF5A1plays a role in xylem formation, its full-length cDNA was constitutivelyover-expressed in transgenic Arabidopsis plants. Microscopicanalysis revealed that the cross-sectional area of the xylemin the main inflorescence stems of transgenic plants was 1.9-foldhigher than those of corresponding inflorescence stems of wild-typeplants. In wild-type stems, the primary xylem typically comprisedsix cell layers and was 105 µm thick, but increased to9–11 cell layers, 140–155 µm thick, in transgenicstems. Similarly, the secondary xylem increased from six celllayers, 70 µm thick, in control stems to 9 cell layers,95–105 µm thick, in transgenic stems. Moreover,constitutive down-regulation of AteIF5A1 using antisense technologyresulted in the major suppression of xylem formation comparedwith control plants, and the antisense transgenic plants werealso stunted. These data collectively indicate that eIF5A1 playsa role in xylogenesis. Key words: Arabidopsis thaliana, eukaryotic translation initiation factor 5A, inflorescence stem, xylem Received 5 November 2007; Revised 26 December 2007 Accepted 10 January 2008     

Biomechanics of the columnar cactus Pachycereus pringlei

We report the longitudinal variations in stiffness and bulk density of tissue samples drawn from along the length of two Pachycereus pringlei plants measuring 3.69 and 5.9 m in height to determine how different tissues contribute to the mechanical stability of these massive vertical organs. Each of the two stems was cut into segments of uniform length and subsequently dissected to obtain and mechanically test portions of xylem strands, stem ribs, and a limited number of pith and cortex samples. In each case, morphometric measurements were taken to determine the geometric contribution each tissue likely made to the ability of whole stems to resist bending forces. The stiffness of each xylem strand increased basipetally toward the base of each plant where stiffness sharply decreased, reaching a magnitude comparable to that of strands 1 m beneath the stem apex. The xylem was anisotropic in behavior, i.e., its stiffness measured in the radial and in the tangential directions differed significantly. Despite the abrupt decrease in xylem strand stiffness at the stem base, the contribution made by this tissue to resist bending forces increased exponentially from the tip to the base of each plant due to the accumulation of wood. A basipetal increase in the stiffness of the pith (and, to limited extent, that of the cortex) was also observed. In contrast, the stiffness of stem rib tissues varied little as a function of stem length. These tissues were stiffer than the xylem in the corresponding portions of the stem along the upper two-fifths of the length of either plant. Tissue stiffness and bulk density were not significantly correlated within or across tissue types. However, a weak inverse relationship was observed for these properties in the case of the xylem and stem rib tissues. We present a simple formula that predicts when stem ribs rather than the xylem strands serve as the principal stiffening agents in stems. This formula successfully predicted the observed aspect ratio of the stem ribs (the average quotient of the radial and tangential dimensions of rib transections), and thus provided circumstantial evidence that the ribs are important for mechanical stability for the distal and younger regions of the stems examined.     

Mechanics and Growth Form of the MossHylocomium splendens

The mossHylocomium splendenshas two different growth forms.Sympodial growth occurs where the apical meristem ceases activityannually and growth is continued by a lateral bud. Sympodialplants are vertical and self-supporting. Monopodial growth occurswhen the apex continues growth. Monopodial plants are prostrate.The aims of the study were to examine stem mechanics of thedifferent growth forms and to compare mechanical propertiesalong stems. Stems of annual segments were subjected to threepoint bending tests. In sympodial plants the stiffness of thestem material increased significantly with segment age. Flexuralrigidity increased significantly with age in segments from 1to 4 years old, and then declined. Segment diameter decreasedsignificantly with age in sympodial plants. Monopodial plantsshowed no significant effects of segment age on the diameter,material stiffness or flexural rigidity of stems. Sympodialsegments were significantly wider, stronger, more rigid andcomposed of stiffer material with a higher stress at yield thanmonopodial segments, but did not have a larger proportion ofstrengthening material. Sympodial stems had significantly morecellulose than monopodial stems. The mossHylocomium splendensshowsa range of mechanical adaptations, as a self-supporting or aprostrate plant, which suit it to life in very different environments.Copyright1998 Annals of Botany Company. Hylocomium splendens, growth form, mechanics, cellulose, stiffness, flexural rigidity, bending, monopodial, sympodial, adaptation, anatomy, stems, plant.     

The Mechanical Properties of Xylem Tissue from Tobacco Plants (Nicotiana tabacum'Samsun')

Tobacco plants (Nicotiana tabacum‘Samsun’) havebeen grown with an antisense CAD (cinnamyl alcohol dehydrogenase)gene. This modifies lignin production, resulting in lignin witha greater aldehyde content which is easier to extract chemically.This lignin probably has a reduced crosslink density. The changedproperties of the lignin affect the longitudinal tensile modulusof the xylem tissue (wood), reducing it by one third, from 2.8GPa to 1.9 GPa. Tobacco xylem tissue cell walls are more sensitiveto changes in the properties of the matrix than can be predictedusing current cell wall mechanical models.Copyright 1998 Annalsof Botany Company Tobacco,Nicotiana tabacum, xylem tissue, Young's modulus, matrix polymer connectivity, plant biomechanics.     

Role of Mechanical Loading in Growth of Sunflower (Helianthus annuus) Seedlings

Seedlings of the herbaceous annual Helianthus annuus L. weregrown under three regimens of daily bending. Bending wasadjustedduring ontogeny so that the deflexion of the stem tip dividedby the stem height was constant (elastic similarity). After6weeks, mechanically–treated plants exhibited a significantlygreater ratio of stem diameter to stem height, flexural stiffness,stemgrowth rate, and proportion of collenchymatous tissue inthe stem. Treated plants were also significantly smaller inheight,showed a greater proportion of stem tissue as cortex,and greater initial stem growth rate. No significant differenceswereobserved in the elastic modulus of the tissue composingthe stem, above–ground biomass, and stem diameter. Thedegree ofmechanical loading also had a significant effect onmost parameters. The most highly stressed plants were thicker,shorter, and elastically stiffer with more collenchyma and lesscortex in the stem. Since plants were loaded for only 60 s d^–1it is concludedthat mechanical effects early in life can haveprofound effects on the form and hence ecology of seedlings. Key words: Seedling growth, Helianthus annuus L., stem mechanical properties, thigmomorphogenesis, bending stress     

Effects of Vibration on Mechanical Properties and Biomass Allocation Pattern ofCapsella bursa-pastoris(Cruciferae)

The herbaceous dicot speciesCapsella bursa-pastoris(Cruciferae)was used to determine the influence of chronic mechanical perturbationon the biomass allocation pattern (i.e. dry weight distributionamong roots, stems and reproductive structures) and the mechanicalproperties of roots and stems (i.e. tensile breaking stressand Young's modulus). It was hypothesized that mechanicallystimulated plants would allocate more of their total biomassto root systems and less to shoots compared to control plantsand that the breaking stress (a measure of strength) and Young'smodulus (a measure of material stiffness) would increase forroots and decrease for stems because these responses would adaptivelyreduce the bending moment at the base of shoots and increasethe anchorage strength of root systems. It was also hypothesizedthat mechanical perturbation would maladaptively reduce therelative fitness of individuals by reducing biomass allocationto their reproductive organs and the ability to broadcast seedsby means of elastic stem flexure. These hypotheses were testedby vibrating cultivated plants for 60 s every day during thecourse of growth to maturity and comparing their dry weightdistributions and the mechanical properties of their body parts(measured in tension) to those of undisturbed control plants.Based on a total of 51 experimentally manipulated and 44 controlplants for which mechanical properties were successfully tested,chronic organ flexure resulted in more massive root systemsand less massive vegetative shoots, increased the magnitudesof root breaking stress and Young's modulus and had the reverseeffect on stems, reduced the dry weight of reproductive structuresat maturity, delayed the formation of the first mature flowerand fruit, and accelerated the on-set of plant senescence comparedto control plants. These responses to chronic organ flexureare interpreted to be vegetatively adaptive, since they reducethe probability of stem and root failure as a consequence ofwind-pressure or foraging, and to be reproductively maladaptive,since they reduce reproductive effort and the ability to mechanicallydischarge seeds.Copyright 1998 Annals of Botany Company Adaptation, biomass allocation, biomechanics, elastic properties, roots, stems, thigmomorphogenesis.     

Isolation of 4-coumarate Co-A ligase gene promoter from loblolly pine (Pinus taeda) and characterization of tissue-specific activity in transgenic tobacco

We characterized promoter activity of a phenylpropanoid biosynthetic gene encoding 4-coumarate Co-A ligase (4CL), Pta4Clα, from Pinus taeda. Histochemical- and quantitative assays of GUS expression in the vascular tissue were performed using transgenic tobacco plants expressing promoter-GUS reporters. Deletion analysis of the Pta4Clα promoter showed that the region ?524 to ?252, which has two AC elements, controls the high expression levels in ray-parenchyma cells of older tobacco stems. High activity level of the promoter domain of Pta4CLα was also detected in the xylem cells under bending stress. DNA-protein complexes were detected in the reactions of the Pta4CLα promoter fragments with the nuclear proteins of xylem of P. taeda. The AC elements in the Pta4CLα promoter appeared to have individual roles during xylem development that are activated in a coordinated manner in response to stress in transgenic tobacco.     

Safety and streamlining of woody shoots in wind: an empirical study across 39 species in tropical Australia

? Wind is a key mechanical stress for woody plants, so how do shoot traits affect performance in wind? ? We used a vehicle mounted apparatus to measure drag, streamlining and mechanical safety in 127 vertical lead-shoots, 1.2 m long, across 39 species in tropical Australia. ? Shoot dimensions and stem tissue properties were closely coupled so that shoots with low stem specific gravity or larger projected area had thicker stems. Thicker stems provide larger second moment of area (I), which increased shoot safety and bending stiffness but impeded shoot reconfiguration in strong winds, including frontal area reduction. Nonetheless, increasing I also improved streamlining. Streamlining was unrelated to traits except I. Stem tissue material properties only had small effects. Higher modulus of rupture increased shoot safety and higher Young's modulus impeded shoot reconfiguration. ? We found no conflict between bending stiffness and streamlining for woody shoots. Stiffness might help streamlining by increasing damping and stability, thereby reducing flagging in wind. Tissue-level traits did influence shoot-level mechanical safety and behaviour, but shoot geometry was much more important. Variable shoot and stem traits, which all influenced shoot biomechanics, were integrated in shoots to yield a relatively narrow range of outcomes in wind.     

Thigmomorphogenesis: the Effect of Mechanical Perturbation and Ethrel on Stem Pithiness in Tomato [Lycopersicon esculentum (Mill.) Plants]

The stems of ‘Y’-shaped (double stemmed) tomato(Lycopersicon esculentum Mill.) plants were mechanically perturbed(MP) by stroking for 6 successive days. The treatment reducedelongation of the two stems by 40 per cent. When only one branchof the pair was treated, its length was reduced to the sameextent as the two branches in the previous treatment, whilethe elongation of the untreated branch was increased by 60 percent over that of the control. Withholding irrigation induced stem pithiness due to droughtstress in non-MP-plants. However, in MP-pretreated plants, thenumber of pithy internodes was markedly less and the degreeof severity of the disorder was reduced. Ethrel applicationmimicked the effects of MP on pithiness. In some unknown way,the plants are hardened by MP or Ethrel. Lycopersicon esculentum Mill, tomato, drought stress, thigmomorphogenesis, ethylene, pithiness     

Effect of Disbudding on Root Cytokinin Export and Leaf Senescence in Tomato and Tobacco

Bud removal and decapitation (disbudding) of plants of tomato(Lycopersicon esculentum Miller) and tobacco (Nicotiana tabacumL.) resulted in an increase in both the concentration and thetotal flux of cytokinin in bleeding xylem sap. There was a retardationin the rate of chlorophyll loss from leaves of disbudded plantsof both species. Ultrastructural examination of the chloroplastsof disbudded tomato plants revealed a maintenance of chloroplastintegrity, compared with chloroplasts from control plants. Itis suggested that the delaying of leaf senescence observed indisbudded plants is due to an increased availability of cytokininto these leaves.     

The Effect of Mechanically Induced Stress on the Growth of Cauliflower, Lettuce and Celery Seedlings

Brushing cauliflower, lettuce and celery seedlings with paperfor 1.5 min each day for 11–13 d, 10–12 d or 21–28d, respectively resulted in smaller, more compact, plants thanthe unbrushed controls. In all three species shoot fresh anddry weights and leaf area were reduced following brushing. Incauliflower and celery the largest growth reduction was in petiolelength. In lettuce, which has no discernible petioles, the reductionin leaf length caused by brushing was proportionally greaterthan the reduction in leaf width. Brushing reduced hypocotyllength in cauliflowers and to a lesser extent in lettuce. Petioleand hypocotyl thickness was reduced in cauliflower, whereashypocotyl thickness was increased in lettuce following brushing.Brushing increased leaf thickness in cauliflower, celery andto a lesser extent in lettuce and increased the percentage drymatter content of lettuce shoots. The weight of chlorophyllper fresh weight of leaf tissue increased following brushingin celery and lettuce and declined in cauliflowers. Root length and the number of branches per root system werereduced in all three species following brushing. Root dry weightwas reduced and the root:shoot dry weight ratio was increasedin lettuce, reduced in celery and unaffected in cauliflowers. There were different patterns of response to brushing, the reductionin leaf weight being greatest in the youngest leaf of cauliflowerand least in the youngest leaf of lettuce and celery. Growthresponses to brushing were seen several days after brushinghad ceased, noticeably in leaves which were barely visible atthe time of brushing. It is suggested that growth retardation of cauliflowers, lettuceand celery, induced by mechanical stress such as brushing mayprove valuable as a means of ‘conditioning’ theseedlings to withstand both the physical and physiological stresseswhich occur at and during transplanting. Brassica oleracea, cauliflower, Lactuca sativa L., lettuce, Apium graveolens L., celery, mechanical stress, shoot growth, root growth, chlorophyll     

Why vines have narrow stems: Histological trends in Bauhinia (Fabaceae)

Summary Xylem (wood) tissue in plants functions both for mechanical support and water transport. Since vines are mechanical parasites, they allocate less biomass for xylem tissue than do free-standing trees or shrubs. With-in the genus Bauhinia, stems of vine species were found to have not only less xylem per distal leaf area, but also less phloem and cortical tissue when compared to tree and shrub species. The phloem and cortical reductions are interpreted as an indirect effect of the developmental/geometric constraints imposed by the evolution of a reduced mechanical system. Apparently vines overcame these constraints with the evolution of wider vessels and wider sieve tubes and with many types of variant (anomalous) secondary growth. The long and wide vessels of vines, which compensate hydraulically for the reduced xylem areas, may help limit the distribution of vine species in nature.     

Photosynthetic, hydraulic and biomechanical responses of Juglans californica shoots to wildfire

Leaf gas exchange and stem xylem hydraulic and mechanical properties were studied for unburned adults and resprouting burned Juglans californica (southern California black walnut) trees 1 year after a fire to explore possible trade-offs between mechanical and hydraulic properties of plants. The CO₂ uptake rates and stomatal conductance were 2–3 times greater for resprouting trees than for unburned adults. Both predawn and midday water potentials were more negative for unburned adult trees, indicating that the stems were experiencing greater water stress than the stems of resprouting trees. In addition, the xylem specific conductivity was similar in the two growth forms, even though the stems of resprouting trees were less vulnerable to water-stress-induced embolism than similar diameter, but older, stems of adult trees. The reduced vulnerability may have been due to less cavitation fatigue in stems of resprouts. The modulus of elasticity, modulus of rupture and xylem density were all greater for resprouts, indicating that resprouts have greater mechanical strength than do adult trees. The data suggest that there is no trade-off between stem mechanical strength and shoot hydraulic and photosynthetic efficiency in resprouts, which may have implications for the success of this species in the fire-prone plant communities of southern California.     

Anatomical adaptations of Astragalus gombiformis Pomel. under drought stress

The present study was designed to study the effect of drought on root, stem and leaf anatomy of Astragalus gombiformis Pomel. Several root, stem and leaf anatomical parameters (cross section diameter, cortex, root cortical cells, pith, leaf lamina and mesophyll thickness) were reduced under moderate to severe water deficit (20–30 days of withheld irrigation). The stele/cross section root ratio increased under moderate water deficit. The root’s and stems vascular systems showed reduced xylem vessel diameter and increased wall thickness under water deficit. In addition, the root xylem vessel density was increased in these drought conditions while it was unchanged in the stems. The stomata density was increased under prolonged drought conditions whereas the stomata size was untouched. The leaf vascular system showed reduced xylem and phloem tissue thickness in the main vein under moderate to severe water deficit. However, in the lamina the vascular tissue and the distance between vascular bundle were unaffected. Our findings suggest a complex network of anatomical adaptations such as a reduced vessel size with increased wall thickness, lesser cortical and mesophyll parenchyma formation and increased stomata density. These proprieties are required for the maintenance of water potential and energy storage under water stress which can improve the resistance of A. gombiformis to survive in arid areas.     

Thigmomorphogenesis: On the mechanical properties of mechanically perturbed bean plants

The mechanical properties of control and mechanically perturbed (MP) bean stems ( Phaseolus vulgaris L., ev. Cherokee wax) were compared. The rubbed plants were greatly hardened against mechanical rupture by previous MP. This hardening was due to a dramatic increase in the flexibility of the stems, but not in their stiffness. The MP-plants were able lo bend more than 90° without breaking, whereas the control plants broke after just slight bending. A comparison with other work reveals that different species utilize different tactics for achieving similar resistance to rupture due to mechanical stress.     

Thigmomorphogenesis: Changes in Cell Division and Elongation in the Internodes of Mechanically-perturbed or Ethrel-treated Bean Plants

Examination of first internodes of young Phaseolus vulgarisL. plants which have been subjected to mechanical perturbationshows decreased elongation and increased radial growth. Thedecreased elongation can be attributed to both reduced cellelongation of epidermal and cortical cells and a reduced numberof cells in the vascular and pith tissues. The increased radialenlargement is due to increased cortical cell expansion andincreased secondary xylem production resulting from increasedcambial activity. All of these responses are observable withina few hours of a single mechanical perturbation. Treatment ofplants with ethrel mimics all of these effects of mechanicalperturbation. Phaseolus vulgarisL, Kidney bean, thigmomorphogenesis, mechanical perturbation, ethrel, (2-chloroethyl phosphonic acid), cell division, internode elongation     

How to become a tree without wood – biomechanical analysis of the stem of Carica papaya L

Carica papaya L. does not contain wood, according to the botanical definition of wood as lignified secondary xylem. Despite its parenchymatous secondary xylem, these plants are able to grow up to 10‐m high. This is surprising, as wooden structural elements are the ubiquitous strategy for supporting height growth in plants. Proposed possible alternative principles to explain the compensation for lack of wood in C. papaya are turgor pressure of the parenchyma, lignified phloem fibres in the bark, or a combination of the two. Interestingly, lignified tissue comprises only 5–8% of the entire stem mass. Furthermore, the phloem fibres do not form a compact tube enclosing the xylem, but instead form a mesh tubular structure. To investigate the mechanism of papaya's unusually high mechanical strength, a set of mechanical measurements were undertaken on whole stems and tissue sections of secondary phloem and xylem. The structural Young's modulus of mature stems reached 2.5 GPa. Since this is low compared to woody plants, the flexural rigidity of papaya stem construction may mainly be based on a higher second moment of inertia. Additionally, stem turgor pressure was determined indirectly by immersing specimens in sucrose solutions of different osmolalities, followed by mechanical tests; turgor pressure was between 0.82 and 1.25 MPa, indicating that turgor is essential for flexural rigidity of the entire stem.     

Xylem Hydraulic Characteristics, Water Relations and Wood Anatomy of the Resurrection PlantMyrothamnus flabellifoliusWelw.

Myrothamnus flabellifoliusWelw. is a desiccation-tolerant (‘resurrection’)plant with a woody stem. Xylem vessels are narrow (14 µmmean diameter) and perforation plates are reticulate. This leadsto specific and leaf specific hydraulic conductivities thatare amongst the lowest recorded for angiosperms (k_s0.87 kg m^-1MPa^-1s^-1;k_l3.28x10^-5kg m^-1MPa^-1s^-1, stem diameter 3 mm). Hydraulic conductivitiesdecrease with increasing pressure gradient. Transpiration ratesin well watered plants were moderate to low, generating xylemwater potentials of -1 to -2 MPa. Acoustic emissions indicatedextensive cavitation events that were initiated at xylem waterpotentials of -2 to -3 MPa. The desiccation-tolerant natureof the tissue permits this species to survive this interruptionof the water supply. On rewatering the roots pressures thatwere developed were low (2.4 kPa). However capillary forceswere demonstrated to be adequate to account for the refillingof xylem vessels and re-establishment of hydraulic continuityeven when water was under a tension of -8 kPa. During dehydrationand rehydration cycles stems showed considerable shrinking andswelling. Unusual knob-like structures of unknown chemical compositionwere observed on the outer surface of xylem vessels. These maybe related to the ability of the stem to withstand the mechanicalstresses associated with this shrinkage and swelling.Copyright1998 Annals of Botany Company cavitation, desiccation, hydraulic conductivity, refilling, resurrection plant, root pressure, xylem anatomy,Myrothamnus flabellifolius     

Composition of Amino Compounds Transported in Xylem of Casuarina sp.

Seedlings (180-d-old) of Casuarina cunninghamianaM L., C. equisetifoliaMiq. and C. glauca Sieber inoculated with each of two differentsources of Frankia, were analysed for translocated nitrogenouscompounds in xylem sap. Analyses were also made on sap fromnodulated and non-nodulated plants of C. glauca grown with orwithout a range of levels of combined nitrogen. Xylem exudateswere collected from stems, roots, and individual nodules ofnodulated plants and from stems and roots of non-nodulated plants.While the proportional composition of solutes varied, the samerange of amino compounds was found in xylem sap from the threedifferent symbioses. In C. glauca asparagine was the major aminoacid in the root sap followed by proline, while in symbioticC. cunninghamiana arginine accounted for more than 25% of theamino compounds. Citrulline was the major translocated productfound in the stem exudate of symbiotic C. equisetifolia. Increasingconcentrations of ammonium nitrate in the nutrient solutionresulted in increasing levels of free ammonia and glutaminein xylem sap from stems of nodulated and non-nodulated C. glauca,but there was relatively little change in the prominent solutes,e.g. citrulline, proline, and arginine. The composition of nitrogenoussolutes in stem or root exudates of C. glauca was similar tothat of exudate collected from individual nodules and on thisbasis it was not possible to distinguish specific products ofcurrent N₂ fixation in xylem. The main differences in N solutecomposition between the symbioses were apparently due to hostplant effects rather than nodulation or the levels of combinedN. Also, the data indicate that the use of the proportion ofN in sap as citrulline (or indeed any other organic N solute)could not be used as an index of nitrogen fixation.     

Use of the rice sucrose synthase-1 promoter to direct phloem-specific expression of {beta}-glucuronidase and snowdrop lectin genes in transgenic tobacco plants

The promoter region from the rice sucrose synthase-1 gene (RSs1)was fused with coding sequences for ß-glucuronidase(GUS) and snowdrop (Galanthus nivalis) lectin (GNA). Tobaccoplants were transformed with these chimaenc genes in order todetermine the expression pattern directed by the RSs1 promoter.Histochemical and immunochemical assays demonstrated that theexpression of both GUS and GNA was restricted to phloem tissue,and was not observed in any other tissues. This phloem-specificexpression pattern was consistent in stem, leaf and root, andin different transgenic plants. Chimaeric genes of RSs 1-GUSand RSs1 GNA were stably inherited in T1 plants. In addition,GNA was detected by immunological assay in the honeydew producedby peach potato aphids (Myzus persicae) feeding on RSs1-GNAtransgenic tobacco plants. This provided direct evidence thatGNA was not only expressed in the phloem tissue, but was alsopresent in the phloem sap of transgenic tobacco plants. TheRSs1 promoter can thus be used to direct expression of an insecticidalprotein, such as GNA, in transgenic plants to control phloemsap-feeding insect pests. Key words: Rice sucrose synthase-1 promoter, phloemspecific, transgenic plants, ß-glucuronidase, Galanthus nivalis agglutinin, gene expression