Ascorbic acid and development of xylem and phloem cells in the pine trunk

Changes in the levels of ascorbic acid (AA), its oxidized form, dehydroascorbic acid (DHA), and uronic acids as initial precursors for the AA synthesis were studied as related to the degree of xylem and phloem cell development in the course of early and late wood formation in the trunks of Scots pine (Pinus sylvestris L.). The cells of mature and conducting phloem, cambial zone, differently developed cells in the zones of cell enlargement and maturation were obtained by successive scraping tissue layers from trunk segments of 20–25-year-old trees; tissue identification was checked anatomically and histochemically. The contents of compounds tested were calculated per dry weight and per cell basis. We found great differences in the contents of AA and DHA and also in their ratio in dependence of the wood type developing in the pine trunks during growth period and on the stage of differentiation of xylem and phloem cells. Changes in the AA content during xylem cell differentiation were accompanied by changes in the content of uronic acids. The amounts of AA, DHA, and uronic acids were the highest at the stage of early lignification and reduced with tracheid maturation. The AA to DHA ratio changed differently in the course of early and late xylem lignification. It reduced from the start of lignification to the formation of early mature xylem and, in contrast, increased in mature late wood; this indicates a difference in the level of redox processes in these tissues.     

Drought tolerance of selected Eragrostis species correlates with leaf tensile properties

BACKGROUND AND AIMS: Previous studies on grass leaf tensile properties (behaviour during mechanical stress) have focused on agricultural applications such as resistance to trampling and palatability; no investigations have directly addressed mechanical properties during water stress, and hence these are the subject of this study. METHODS: Critical (lethal) relative water contents were determined for three species of grass in the genus Eragrostis varying in their tolerance to drought. Measurements were taken for leaf tensile strength, elastic modulus, toughness and failure load under different conditions of hydration, and light microscopy and histochemical analyses were undertaken. KEY RESULTS: Leaf tensile strength of fully hydrated leaves for the drought-intolerant E. capensis, the moderately drought-tolerant E. tef and the drought-tolerant E. curvula correlated well with drought tolerance (critical relative water content). Eragrostis curvula had higher tensile strength values than E. tef, which in turn had higher values than E. capensis. Measurements on the drought-tolerant grass E. curvula when fully hydrated and when dried to below its turgor loss point showed that tensile strength, toughness and the elastic modulus all increased under conditions of turgor loss, while the failure load remained unchanged. Additional tests of 100 mm segments along the lamina of E. curvula showed that tensile strength, toughness and the elastic modulus all decreased with distance from the base of the lamina, while again the failure load was unaffected. This decrease in mechanical parameters correlated with a reduction in the size of the vascular bundles and the amount of lignification, as viewed in lamina cross-sections. CONCLUSIONS: The results confirm that leaf mechanical properties are affected by both water status and position along the lamina, and suggest a positive correlation between leaf internal architecture, tensile strength, cell wall chemistry and tolerance to dehydration for grasses.     

Touch- and Methyl Jasmonate-induced Lignification in Tendrils of Bryonia dioica Jacq.

The touch-induced free coiling response of Bryonia dioica tendrils is accompanied by the differentiation of supporting tissue at the ventral side of the organ, becoming the inner (concave) side of the coiled tendril. As part of this process, the Bianconi plate, a continuous sclerenchyma sheath stretching along the ventral face of the five bicollateral vascular bundles, becomes strongly lignified. During this reaction, the extractable activity of phenylalanine ammonia-lyase in the tendrils increases four- to five-fold, while the amount of PAL, as demonstrated by immunoblotting, remains unchanged. This touch-induced process can also be elicited by airborne application of methyl jasmonate. The PAL inhibitor, 2-aminoindan-2-phosphonic acid (AIP) completely inhibits both the touch- and methyl jasmonate-induced deposition of lignin-like material in the Bianconi plate, but has no effect on coiling. From these results, it can be concluded that the cessation of growth at the ventral side of a free-coiling tendril is not due to induced lignification but rather, lignification seems to serve the function of increasing the mechanical strength of the coiled tendril.     

Induction of Isoperoxidases in Resistant and Susceptible Tomato Cultivars by Meloidogyne incognita

Isoperoxidases were detected in resistant Rossol and susceptible Roma VF tomato roots uninfected and infected by Meloidogyne incognita. Syringaldazine, guaiacol, p-phenylenediamine-pyrocatechol (PPD-PC), and indoleacetic acid (IAA) were used as substrates, and the corresponding peroxidative activities were detected either in cytoplasmic or in cell wall fractions, except for IAA oxidase, which was measured in soluble and microsomal fractions. Isoperoxidase activities and cellular locations were induced differently in resistant and susceptible cultivars by nematodes. Nematode infestation markedly enhanced syringaldazine oxidase activity in cell walls of the resistant cultivar. This isoperoxidase is involved in the last step of lignin deposition in plants. Conversely, the susceptible cultivar reacted to M. incognita infection with an increase in cytoplasmic PPD-PC oxidase activity, which presumedly is involved in ethylene production; no changes in cell wall isoperoxidases were observed. IAA oxidase was inhibited in susceptible plants after nematode inoculation, whereas in resistant plants this activity increased in the soluble fraction and decreased in the microsomal fraction.     

Induction of peroxidases in cucumber hypocotyls by wounding and fungal infection

The induction of peroxidases (EC 1.11.1.7) during elicitation of lignification by α-1,4-linked oligogalacturonides in cucumber hypocotyl segments ( Cucumis sativus L. cv, Wisconsin SMR 58) was investigated. The wounding associated with the preparation of hypocotyl segments induced a 19-fold increase in peroxidase activity during the following 72 h. The increase was partially due to an increase in activity of a constitutive peroxidase with a pI of 8.9 and partially due to the expression of new peroxidase isozymes with pIs of 3.8, 5.4, 6.2, 9.1 and 9.4. The oligogalacturonides did not induce any peroxidase activity in addition to the wound-induced activity. These results suggest that either the constitutive peroxidase isozyme (pI 8.9) of intact hypocotyls or some of the wound-induced peroxidases are involved in the oligogalacturonide-induced lignification.
Induction of the peroxidases by wounding was inhibited by cycloheximide. This indicates that they accumulate as a result of de novo protein synthesis. Actinomycin D caused only a modest inhibition of the wound-induction peroxidases, indicating that the process is regulated at the level of translation.
Peroxidase activity increased more rapidly in resistant than in susceptible cucumber hypocotyls after inoculation with the pathogen Cladosporium cucumerinum Ellis & Arthur. The pattern of isozymes which was induced by fungal infection of resistant hypocotyls was similar to the pattern of isozymes induced by wounding. This suggests that similar induction mechanisms may be involved in the two processes.     

Peroxidase involvement in lignification in water-impermeable seed coats of weedy leguminous and malvaceous species

Abstract. The development of water impermeable seed coats of two members each of the leguminoseae family [ Crotalaria spectabilis Roth, Sesbania exaltata (Raf) Cory] and the malvaceae family [Anoda cristata (L.) Schlecht, Abutilon theophrasti Medic.] was investigated. Highest peroxidase (POD) activity of Anoda and Abutilon seed coat extracts was highly correlated with the developmental stages when soluble phenolics were maximally converted into lignin. Although extensive lignification occurred during seed coat development in both legumes, the patterns of POD activity, soluble phenolic levels and time of lignification were different from those of the malvaceous species. POD activity levels in developing coats of the malvaceous seeds increased as phenolics decreased. Both POD activity and phenolic levels decreased during seed coat development of the legumes. POD was immunocytochemically and immunochemically detected in seed coats of all four species; however, results for polyphenol oxidase were negative. The results confirmed POD involvement in lignification of leguminous and malvaecous species and support and extend our earlier view that POD is involved in lignin formation during development of impermeable seed coats.     

Orientation nouvelle du métabolisme des acides hydroxycinnamiques dans les fruits de tomates blessés (Lycopersicon esculentum)

A change in the metabolism of hydroxycinnamic acids in wounded tomato fruits (Lycopersicon esculentum) .
Healing of lesions in tomato fruits (Lycopersicon esculentum Mill. var. cerasiforme ) is partly due to lignification of cells bordering the wounded zones. The pericarp of healthy fruits contains a high level of hydroxycinnamic derivatives but never shows lignification. Thus, the reaction of the fruit to wounding seems to be a change in the metabolism, leading to the formation of monomeric units of which lignins are constituted. Hydroxycinnamate:CoA ligase (EC 6.2.1.12; CL) and O-methyltransferase (EC 2.1.1.6; OMT) appear to play an important role in this change. In wounded fruits CL acts preferentially on p-coumarate and ferulate as compared to caffeate, and OMT is particularly active with 5-OH ferulate as substrate. These changes lead to the formation of p-coumaroyl CoA, feruloyl CoA and sinapate, which are incorporated into lignin. Phenylalanine ammonialyase (EC 4.3.1.5) and glucosyltransferase activities increase greatly after wounding, whereas the activity of hydroxycinnamoyl-CoA:quinate hydroxycinnamoyl transferase decreases. These data complement those previously reported on peroxidases and suggest that, after the increase of enzyme activities, monomeric units are formed and then polymerized by some peroxidases specific for lignification.     

Cell wall-bound malate dehydrogenase from horseradish

Isolated cell walls from horseradish contain NAD-specific malate dehydrogenase which is not released on treatment with 2 M NaCl. This enzyme catalyses a rapid reduction of oxalacetate. Its physiological role, however, is assumed to be the oxidation of malate, thus providing NADH as electron donor in the formation of H₂O₂, by a wall-bound peroxidase. In the presence of malate, NAD and Mn²⁺ ions, cell walls catalyse the synthesis of H₂O₂ which might be utilized in lignin formation. In analogy to the known malate-oxalacetate shuttles, the possibility is discussed that this cell wall-associated malate dehydrogenase is involved in the transport of cytoplasmic reducing equivalents through the plasmalemma into the cell wall.     

Caffeic acid-O-methyltransferase in a suspension of cell aggregates of tobacco

Aggregates of tobacco cells in suspension in 2,4-D (10^?6 M) and kinetin (10^?5 M) cultures were fractionated by size, then their O-methyltransferase (OMT) activities were assayed. Only the kinetin culture showed high OMT activity, which was higher in the larger than the smaller aggregates at all stages of cell growth. The contents of phenolic acids were also greater in the larger cell aggregates in the kinetin culture. However, when the kinetin cultured cells were transferred to a medium containing 10^?6 M of 2,4-D, the relationships between the cell size of the aggregates and OMT, lignin and the phenolic acids disappeared. The importance of kinetin and cell association for OMT and the subsequent lignification of the cells is discussed.