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PubMed2004

Plant body weight-induced secondary growth in Arabidopsis and its transcription phenotype revealed by whole-transcriptome profiling.

Wood is an important raw material and environmentally cost-effective renewable source of energy. However, the molecular biology of wood formation (i.e. secondary growth) is surprisingly understudied. A novel experimental system was employed to study the molecular regulation of secondary xylem formation in Arabidopsis. First, we demonstrate that the weight carried by the stem is a primary signal for the induction of cambium differentiation and the plant hormone, auxin, is a downstream carrier of the signal for this process. We used Arabidopsis whole-transcriptome (23 K) GeneChip analysis to examine gene expression profile changes in the inflorescent stems treated for wood formation by cultural manipulation or artificial weight application. Many of the genes up-regulated in wood-forming stems had auxin responsive cis-acting elements in their promoter region, indicating auxin-mediated regulation of secondary growth. We identified 700 genes that were differentially expressed during the transition from primary growth to secondary growth. More than 40% of the genes that were up-regulated (>5x) were associated with signal transduction and transcriptional regulation. Biological significance of these regulatory genes is discussed in light of the induction and development of secondary xylem.

Arabidopsis↗
PubMed2004

FRD3 controls iron localization in Arabidopsis.

The frd3 mutant of Arabidopsis exhibits constitutive expression of its iron uptake responses and is chlorotic. These phenotypes are consistent with defects either in iron deficiency signaling or in iron translocation and localization. Here we present several experiments demonstrating that a functional FRD3 gene is necessary for correct iron localization in both the root and shoot of Arabidopsis plants. Reciprocal grafting experiments with frd3 and wild-type Arabidopsis plants reveal that the phenotype of a grafted plant is determined by the genotype of the root, not by the genotype of the shoot. This indicates that FRD3 function is root-specific and points to a role for FRD3 in delivering iron to the shoot in a usable form. When grown under certain conditions, frd3 mutant plants overaccumulate iron in their shoot tissues. However, we demonstrate by direct measurement of iron levels in shoot protoplasts that intracellular iron levels in frd3 are only about one-half the levels in wild type. Histochemical staining for iron reveals that frd3 mutants accumulate high levels of ferric iron in their root vascular cylinder, the same tissues in which the FRD3 gene is expressed. Taken together, these results clearly indicate a role for FRD3 in iron localization in Arabidopsis. Specifically, FRD3 is likely to function in root xylem loading of an iron chelator or other factor necessary for efficient iron uptake out of the xylem or apoplastic space and into leaf cells.

Arabidopsis↗
PubMed2005

Control of sodium transport in durum wheat.

In many species, salt sensitivity is associated with the accumulation of sodium (Na(+)) in photosynthetic tissues. Na(+) uptake to leaves involves a series of transport steps and so far very few candidate genes have been implicated in the control of these processes. In this study, Na(+) transport was compared in two varieties of durum wheat (Triticum turgidum) L. subsp. durum known to differ in salt tolerance and Na(+) accumulation; the relatively salt tolerant landrace line 149 and the salt sensitive cultivar Tamaroi. Genetic studies indicated that these genotypes differed at two major loci controlling leaf blade Na(+) accumulation (R. Munns, G.J. Rebetzke, S. Husain, R.A. James, R.A. Hare [2003] Aust J Agric Res 54: 627-635). The physiological traits determined by these genetic differences were investigated using measurements of unidirectional (22)Na(+) transport and net Na(+) accumulation. The major differences in Na(+) transport between the genotypes were (1) the rate of transfer from the root to the shoot (xylem loading), which was much lower in the salt tolerant genotype, and (2) the capacity of the leaf sheath to extract and sequester Na(+) as it entered the leaf. The genotypes did not differ significantly in unidirectional root uptake of Na(+) and there was no evidence for recirculation of Na(+) from shoots to roots. It is likely that xylem loading and leaf sheath sequestration are separate genetic traits that interact to control leaf blade Na(+).

Biological Transport, Active↗
PubMed1994

Purification and Characterization of Cinnamyl Alcohol Dehydrogenase Isoforms from the Periderm of Eucalyptus gunnii Hook.

Cinnamyl alcohol dehydrogenase (CAD, EC 1.1.1.195) isoforms were purified from the periderm (containing both suberized and lignified cell layers) of Eucalyptus gunnii Hook stems. Two isoforms (CAD 1P and CAD 2P) were initially characterized, and the major form, CAD 2P, was resolved into three further isoforms by ion-exchange chromatography. Crude extracts contained two aliphatic alcohol dehydrogenases (ADH) and one aromatic ADH, which was later resolved into two further isoforms. Aliphatic ADHs did not use hydroxycinnamyl alcohols as substrates, whereas both aromatic ADH isoforms used coniferyl and sinapyl alcohol as substrates but with a much lower specific activity when compared with benzyl alcohol. The minor form, CAD 1P, was a monomer with a molecular weight of 34,000 that did not co-elute with either aromatic or aliphatic ADH activity. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and western blot analysis demonstrated that this protein was very similar to another CAD isoform purified from Eucalyptus xylem tissue. CAD 2P had a native molecular weight of approximately 84,000 and was a dimer consisting of two heterogenous subunits (with molecular weights of 42,000 and 44,000). These subunits were differentially combined to give the heterodimer and two homodimers. SDS-PAGE, western blots, and nondenaturing PAGE indicated that the CAD 2P heterodimer was very similar to the main CAD isoform previously purified in our laboratory from differentiating xylem tissue of E. gunnii (D. Goffner, I. Joffroy, J. Grima-Pettenati, C. Halpin, M.E. Knight, W. Schuch, A.M. Boudet [1992] Planta 188: 48-53). Kinetic data indicated that the different CAD 2P isoforms may be implicated in the preferential production of different monolignols used in the synthesis of lignin and/or suberin.

Journal Article↗
PubMed2006

Embolism formation during freezing in the wood of Picea abies.

Freeze-thaw events can cause embolism in plant xylem. According to classical theory, gas bubbles are formed during freezing and expand during thawing. Conifers have proved to be very resistant to freeze-thaw induced embolism, because bubbles in tracheids are small and redissolve during thawing. In contrast, increasing embolism rates upon consecutive freeze-thaw events were observed that cannot be explained by the classical mechanism. In this study, embolism formation during freeze-thaw events was analyzed via ultrasonic and Cryo-scanning electron microscope techniques. Twigs of Picea abies L. Karst. were subjected to up to 120 freeze-thaw cycles during which ultrasonic acoustic emissions, xylem temperature, and diameter variations were registered. In addition, the extent and cross-sectional pattern of embolism were analyzed with staining experiments and Cryo-scanning electron microscope observations. Embolism increased with the number of freeze-thaw events in twigs previously dehydrated to a water potential of -2.8 MPa. In these twigs, acoustic emissions were registered, while saturated twigs showed low, and totally dehydrated twigs showed no, acoustic activity. Acoustic emissions were detected only during the freezing process. This means that embolism was formed during freezing, which is in contradiction to the classical theory of freeze-thaw induced embolism. The clustered pattern of embolized tracheids in cross sections indicates that air spread from a dysfunctional tracheid to adjacent functional ones. We hypothesize that the low water potential of the growing ice front led to a decrease of the potential in nearby tracheids. This may result in freezing-induced air seeding.

Cryoelectron Microscopy↗
PubMed1996

Cell-Wall Polysaccharides of Developing Flax Plants.

Flax (Linum usitatissimum L.) fibers originate from procambial cells of the protophloem and develop in cortical bundles that encircle the vascular cylinder. We determined the polysaccharide composition of the cell walls from various organs of the developing flax plant, from fiber-rich strips peeled from the stem, and from the xylem. Ammonium oxalate-soluble polysaccharides from all tissues contained 5-linked arabinans with low degrees of branching, rhamnogalacturonans, and polygalacturonic acid. The fiber-rich peels contained, in addition, substantial amounts of a buffer-soluble, 4-linked galactan branched at the 0-2 and 0-3 positions with nonreducing terminal-galactosyl units. The cross-linking glycans from all tissues were (fucogalacto)xyloglucan, typical of type-I cell walls, xylans containing (1->)-[beta]-D-xylosyl units branched exclusively at the xylosyl O-2 with t-(4-O-methyl)-glucosyluronic acid units, and (galacto)glucomannans. Tissues containing predominantly primary cell wall contained a larger proportion of xyloglucan. The xylem cells were composed of about 60% 4-xylans, 32% cellulose, and small amounts of pectin and the other cross-linking polysaccharides. The noncellulosic polysaccharides of flax exhibit an uncommonly low degree of branching compared to similar polysaccharides from other flowering plants. Although the relative abundance of the various noncellulosic polysaccharides varies widely among the different cell types, the linkage structure and degree of branching of several of the noncellulosic polysaccharides are invariant.

Journal Article↗
PubMed1997

The Propagation of Slow Wave Potentials in Pea Epicotyls.

Slow wave potentials are considered to be electric long-distance signals specific for plants, although there are conflicting ideas about a chemical, electrical, or hydraulic mode of propagation. These ideas were tested by comparing the propagation of hydraulic and electric signals in epicotyls of pea (Pisum sativum L). A hydraulic signal in the form of a defined step increase in xylem pressure (Px) was applied to the root of intact seedlings and propagated nearly instantly through the epicotyl axis while its amplitude decreased with distance from the pressure chamber. This decremental propagation was caused by a leaky xylem and created an axial Px gradient in the epicotyl. Simultaneously along the epicotyl surface, depolarizations appeared with lag times that increased acropetally with distance from the pressure chamber from 5 s to 3 min. When measured at a constant distance, the lag times increased as the size of the applied pressure steps decreased. We conclude that the Px gradient in the epicotyl caused local depolarizations with acropetally increasing lag times, which have the appearance of an electric signal propagating with a rate of 20 to 30 mm min-1. This static description of the slow wave potentials challenges its traditional classification as a propagating electric signal.

Journal Article↗
PubMed1997

Control of Leaf Expansion Rate of Droughted Maize Plants under Fluctuating Evaporative Demand (A Superposition of Hydraulic and Chemical Messages?).

We have analyzed the possibility that chemical signaling does not entirely account for the effect of water deficit on the maize (Zea mays L.) leaf elongation rate (LER) under high evaporative demand. We followed time courses of LER (0.2-h interval) and spatial distribution of elongation rate in leaves of either water-deficient or abscisic acid (ABA)-fed plants subjected to varying transpiration rates in the field, in the greenhouse, and in the growth chamber. At low transpiration rates the effect of the soil water status on LER was related to the concentration of ABA in the xylem sap and could be mimicked by feeding artificial ABA. Transpiring plants experienced a further reduction in LER, directly linked to the transpiration rate or leaf water status. Leaf zones located at more than 20 mm from the ligule stopped expanding during the day and renewed expansion during the night. Neither ABA concentration in the xylem sap, which did not appreciably vary during the day, nor ABA flux into shoots could account for the effect of evaporative demand. In particular, maximum LER was observed simultaneously with a minimum ABA flux in the droughted plants, but with a maximum ABA flux in ABA-fed plants. All data were interpreted as the superposition of two additive effects: the first involved ABA signaling and was observed during the night and in ABA-fed plants, and the second involved the transpiration rate and was observed even in well-watered plants. We suggest that a hydraulic signal is the most likely candidate for this second effect.

Journal Article↗
PubMed1997

Specific localization of the respiratory alternative oxidase in meristematic and xylematic tissues from developing soybean roots and hypocotyls.

We used tissue printing and specific immunostaining to examine the localization of the alternative oxidase (AOX) protein in correlation with measurements of AOX capacity. Selected root and hypocotyl regions were analyzed during the first 14 d of growth. It is shown that AOX protein is localized in the apical meristem and in developing xylem. The temporal pattern of expression is coincident with the evolution of AOX capacity. Data suggest that AOX expression is linked to xylem differentiation. Since heat is a major product of the alternative pathway, we speculate that thermogenesis is implicated in morphogenesis.

Gene Expression Regulation, Developmental↗
PubMed1998

Indole-3-acetic acid controls cambial growth in scots pine by positional signaling

The vascular cambium produces secondary xylem and phloem in plants and is responsible for wood formation in forest trees. In this study we used a microscale mass-spectrometry technique coupled with cryosectioning to visualize the radial concentration gradient of endogenous indole-3-acetic acid (IAA) across the cambial meristem and the differentiating derivatives in Scots pine (Pinus sylvestris L.) trees that had different rates of cambial growth. This approach allowed us to investigate the relationship between growth rate and the concentration of endogenous IAA in the dividing cells. We also tested the hypothesis that IAA is a positional signal in xylem development (C. Uggla, T. Moritz, G. Sandberg, B. Sundberg [1996] Proc Natl Acad Sci USA 93: 9282-9286). This idea postulates that the width of the radial concentration gradient of IAA regulates the radial number of dividing cells in the cambial meristem, which is an important component for determining cambial growth rate. The relationship between IAA concentration in the dividing cells and growth rate was poor, although the highest IAA concentration was observed in the fastest-growing cambia. The radial width of the IAA concentration gradient showed a strong correlation with cambial growth rate. The results indicate that IAA gives positional information in plants.

Journal Article↗
PubMed1999

Mercuric Chloride Effects on Root Water Transport in Aspen Seedlings.

HgCl(2) (0.1 mM) reduced pressure-induced water flux and root hydraulic conductivity in the roots of 1-year-old aspen (Populus tremuloides Michx.) seedlings by about 50%. The inhibition was reversed with 50 mM mercaptoethanol. Mercurial treatment reduced the activation energy of water transport in the roots from 10.82 +/- 0.700 kcal mol(-1) to 6.67 +/- 0.193 kcal mol(-1) when measured over the 4 degrees C to 25 degrees C temperature range. An increase in rhodamine B concentration in the xylem sap of mercury-treated roots suggested a decrease in the symplastic transport of water. However, the apoplastic pathway in both control and mercury-treated roots constituted only a small fraction of the total root water transport. Electrical conductivity and osmotic potentials of the expressed xylem sap suggested that 0.1 mM HgCl(2) and temperature changes over the 4 degrees C to 25 degrees C range did not induce cell membrane leakage. The 0.1 mM HgCl(2) solution applied as a root drench severely reduced stomatal conductance in intact plants, and this reduction was partly reversed by 50 mM mercaptoethanol. In excised shoots, 0.1 mM HgCl(2) did not affect stomatal conductance, suggesting that the signal that triggered stomatal closure originated in the roots. We suggest that mercury-sensitive processes in aspen roots play a significant role in regulating plant water balance by their effects on root hydraulic conductivity.

Journal Article↗
PubMed2000

Ectopic deposition of lignin in the pith of stems of two Arabidopsis mutants.

The biosynthesis of lignin in vascular plants is regulated both developmentally and environmentally. In the inflorescence stems of Arabidopsis, lignin is mainly deposited in the walls of xylem cells and interfascicular fiber cells during normal plant growth and development. The mechanisms controlling the spatial deposition of lignin remain unknown. By screening ethyl methanesulfonate-mutagenized populations of Arabidopsis, we have isolated two allelic elp1 (ectopic deposition of lignin in pith) mutants with altered lignin deposition patterns. In elp1 stems, lignin was ectopically deposited in the walls of pith parenchyma cells in addition to its normal deposition in the walls of xylem and fiber cells. Lignin appeared to be deposited in patches of parenchyma cells in the pith of both young and mature elp1 stems. The ectopic deposition of lignin in the pith of elp1 stems was accompanied by an increase in the activities of enzymes in the lignin biosynthetic pathway and with the ectopic expression of caffeoyl coenzyme A O-methyltransferase in pith cells. These results indicate that the ELP1 locus is involved in the repression of the lignin biosynthetic pathway in the pith. Isolation of the elp1 mutants provides a novel means with which to study the molecular mechanisms underlying the spatial control of lignification.

Arabidopsis↗
PubMed2000

Cloning, developmental, and tissue-specific expression of sucrose:sucrose 1-fructosyl transferase from Taraxacum officinale. Fructan localization in roots.

Sucrose:sucrose 1-fructosyl transferase (1-SST) is the key enzyme initiating fructan synthesis in Asteraceae. Using reverse transcriptase-PCR, we isolated the cDNA for 1-SST from Taraxacum officinale. The cDNA-derived amino acid sequence showed very high homology to other Asteracean 1-SSTs (Cichorium intybus 86%, Cynara scolymus 82%, Helianthus tuberosus 80%), but homology to 1-SST from Allium cepa (46%) and Aspergillus foetidus (18%) was much lower. Fructan concentrations, 1-SST activities, 1-SST protein, and mRNA concentrations were compared in different organs during vegetative and generative development of T. officinale plants. Expression of 1-SST was abundant in young roots but very low in leaves. 1-SST was also expressed at the flowering stages in roots, stalks, and receptacles. A good correlation was found between northern and western blots showing transcriptional regulation of 1-SST. At the pre-flowering stage, 1-SST mRNA concentrations and 1-SST activities were higher in the root phloem than in the xylem, resulting in the higher fructan concentrations in the phloem. Fructan localization studies indicated that fructan is preferentially stored in phloem parenchyma cells in the vicinity of the secondary sieve tube elements. However, inulin-like crystals occasionally appeared in xylem vessels.

Amino Acid Sequence↗
PubMed2000

The forms and sources of cytokinins in developing white lupine seeds and fruits.

A comprehensive range of cytokinins (CK) was identified and quantified by gas chromatography-mass spectrometry in tissues of and in xylem and phloem serving developing white lupine (Lupinus albus) fruits. Analyses were initiated at anthesis and included stages of podset, embryogenesis, and seed filling up to physiological maturation 77 d post anthesis (DPA). In the first 10 DPA, fertilized ovaries destined to set pods accumulated CK. The proportion of cis-CK:trans-CK isomers was initially 10:1 but declined to less than 1:1. In ovaries destined to abort, the ratio of cis-isomers to trans-isomers remained high. During early podset, accumulation of CK (30-40 pmol ovary(-1)) was accounted for by xylem and phloem translocation, both containing more than 90% cis-isomers. During embryogenesis and early seed filling (40-46 DPA), translocation accounted for 1% to 14% of the increases of CK in endosperm (20 nmol fruit(-1)) and seed coat (15 nmol fruit(-1)), indicating synthesis in situ. High CK concentrations in seeds (0.6 micromol g(-1) fresh weight) were transient, declining rapidly to less than 1% of maximum levels by physiological maturity. These data pose new questions about the localization and timing of CK synthesis, the significance of translocation, and the role(s) of CK forms in reproductive development.

Cytokinins↗
PubMed1966

Studies of Frost Hardiness in Woody Plants. II. Effect of Temperature on Hardening.

The effect of temperature on hardening was studied at temperatures ranging from 0 degrees to -20 degrees using twigs of willow and poplar. In October and in late April when the twigs are not very frost hardy, hardening at 0 degrees produced a considerable increase in their frost hardiness, although the effectiveness of hardening at 0 degrees decreased with a decrease in the environmental temperature. In twigs which could withstand continuous freezing without injury, hardening at -3 degrees to -5 degrees was most effective in increasing the frost hardiness of the twigs. Below -20 degrees , only negligible increase was observed either in frost hardiness or sugar content.The rate of starch to sugar conversion differed remarkably in different twig tissues. The starch in xylem was more slowly converted to sugar than that in the cortex. The optimum temperature for converting starch into sugar during frost hardening was also found to be -3 degrees to -5 degrees . In addition, the greater the effectiveness of the hardening treatment, the greater the rate of conversion from starch to sugar. The frost hardiness of a twig is closely related to the sugar content of the twig, especially in the xylem.

Journal Article↗
PubMed1967

Translocation of Indole-3-acetic Acid-1'-C and Tryptophan-1-C in Seedlings of Phaseolus coccineus L. and Zea mays L.

Indole-3-acetic acid-1'-(14)C (IAA-(14)C) and tryptophan-1-(14)C injected in small amounts into cotyledons of Phaseolus coccineus L. seedlings were found to be translocated acropetally into the epicotyls and young shoots. Similarly IAA-(14)C was translocated acropetally into coleoptiles of Zea mays following injection into the endosperms. Labeled metabolites of the injected compounds were also extractable from shoot tissue. However, evidence that IAA-(14)C itself was translocated acropetally was obtained by collection in agar blocks applied to cut surfaces of coleoptiles of injected seedlings. The acropetal translocation in Phaseolus was shown not to occur in the transpiration stream but in living tissue. Cotyledons of Phaseolus coccineus and Phaseolus vulgaris contain extensive vascular tissue.Tryptophan-(14)C was not actively translocated through excised segments of Phaseolus coccineus epicotyl and Zea mays coleoptile when supplied from donor agar blocks in concentrations as high as 100 mum. The small amount of tryptophan-(14)C which did reach receiver blocks when high concentrations were used may be accounted for by passive diffusion through the fluid-filled xylem vessels. Translocation of a non-toxic dye, Light Green SF Yellowish, through xylem vessels was found to occur when supplied from donor blocks placed acropetally or basipetally. Metabolism of the supplied tryptophan-(14)C by the tissue segments was shown to occur during the 3 to 6 hour translocation experiments. IAA-(14)C was transported in a strictly basipetal manner in both tissues. Only 1 labeled compound with an R(F) value of IAA was found in receiver blocks. Composition of a simple green safelight suitable for work in plant physiology is described.

Journal Article↗
PubMed1967

Radial salt transport in corn roots.

Primary roots of solution-grown, 5-day-old or 6-day-old seedlings of corn (Zea mays L.) 10 to 14 cm in length were used to study radial salt transport. Measurements were made of the volume of root pressure exudation, salt concentration of the exudate, and rate of salt movement into the xylem exudate. The (32)P uptake, O(2) consumption, and dehydrogenase activity of the root cortex and stele also were studied.These roots produced copious root pressure exudate containing 4 to 10 times the concentration of (32)P in the external solution. Freshly separated stele from 5-day-old roots accumulated (32)P as rapidly as the cortex from which it was separated and the stele of intact roots also accumulated (32)P. Separated stele has a higher oxygen uptake than cortex. It also shows strong dehydrogenase activity with the tetrazolium test. The high oxygen consumption, (32)P uptake and strong dehydrogenase activity indicate that the cells of the stele probably play a direct role in salt transport.These data raise doubts concerning theories of radial salt transport into the xylem based on the assumption that the stele is unable to accumulate salt vigorously.

Journal Article↗
PubMed1968

Seasonal Course of Photosynthesis, Respiration, and Distribution of C in Young Pinus resinosa Trees as Related to Wood Formation.

Rates of net photosynthesis and dark respiration, and distribution of (14)C were determined for new (current season's) and old (previous season's) needles at 10 times during the seasonal development of young Pinus resinosa Ait. trees. The seasonal changes in these factors associated with the development of the new shoot were related to known seasonal patterns of wood formation.Net photosynthesis per gram of needle dry weight (photosynthetic efficiency) was maximum in the old needles at the time of first new needle elongation; at the same time translocation of (14)C from old to new needles was greatest. Photosynthetic efficiency of new needles was maximum at the end of the period of rapid new needle elongation, when the new needles also began exporting much greater quantities of (14)C to other plant parts. In particular, the amount translocated from the new needles to the stem was greatly increased. At this time thick-walled xylem cells were first observed in the stem.These results, together with those of previous studies, indicate that the production of thick-walled xylem tracheids normally associated with latewood is physiologically correlated with maturation of the current season's needles. Because there is a lesser demand for photosynthate in the new shoot and a high rate of photosynthesis in the whole plant at the time of new needle maturity, a sharply increased amount of photosynthate becomes available for wall synthesis by cambial derivatives.

Journal Article↗