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Many plant species excrete organic acids into the rhizosphere in response to aluminum stress to protect sensitive cells from aluminum rhizotoxicity. When the roots of Eucalyptus camaldulensis, a major source of pulp production, were incubated in aluminum-toxic medium, citrate released into the solution increased as a function of time. Citrate excretion was inducible by aluminum, but not by copper or sodium chloride stresses. This indicated that citrate is the major responsive organic acid released from the roots of this plant species to protect the root tips from aluminum damage. Four genes highly homologs to known citrate-transporting multidrugs and toxic compounds exclusion proteins, named EcMATE1-4, were isolated using polymerase chain reaction-based cloning techniques. Their predicted proteins included 12 membrane spanning domains, a common structural feature of citrate-transporting MATE proteins, and consisted of 502-579 amino acids with >60 % homology to orthologous genes in other plant species. One of the homologs, designated EcMATE1, was expressed in the roots more abundantly than in the shoots and in response to both Al and low pH stresses. Ectopic expression of EcMATE1 and 3 in tobacco hairy roots enhanced Al-responsive citrate excretion. Pharmacological characterization indicated that Al-responsive citrate excretion involved a protein phosphorylation/dephosphorylation process. These results indicate that citrate excretion through citrate-transporting multidrugs and toxic compounds exclusion proteins is one of the important aluminum-tolerance mechanisms in Eucalyptus camaldulensis.

A quantitative approach to characterize lignin-carbohydrate complex (LCC) linkages using a combination of quantitative ¹³C NMR and HSQC 2D NMR techniques has been developed. Crude milled wood lignin (MWLc), LCC extracted from MWLc with acetic acid (LCC-AcOH) and cellulolytic enzyme lignin (CEL) preparations were isolated from loblolly pine (Pinus taeda) and white birch (Betula pendula) woods and characterized using this methodology on a routine 300 MHz NMR spectrometer and on a 950 MHz spectrometer equipped with a cryogenic probe. Structural variations in the pine and birch LCC preparations of different types (MWL, CEL and LCC-AcOH) were elucidated. The use of the high field NMR spectrometer equipped with the cryogenic probe resulted in a remarkable improvement in the resolution of the LCC signals and, therefore, is of primary importance for an accurate quantification of LCC linkages. The preparations investigated showed the presence of different amounts of benzyl ether, γ-ester and phenyl glycoside LCC bonds. Benzyl ester moieties were not detected. Pine LCC-AcOH and birch MWLc preparations were preferable for the analysis of phenyl glycoside and ester LCC linkages in pine and birch, correspondingly, whereas CEL preparations were the best to study benzyl ether LCC structures. The data obtained indicate that pinewood contains higher amounts of benzyl ether LCC linkages, but lower amounts of phenyl glycoside and γ-ester LCC moieties as compared to birch wood.

Radiochemical studies of Populus tremuloides xylem tissue administered L-[1-(3)H]arabinose, D-[1-(3)H]glucose, and D-[6-(3)H]glucose demonstrate that L-[1-(3)H]arabinose is an excellent precursor for pentosan in this tissue. Transverse sections of first-year xylem (from cambial zone to pith) were examined by light and electron microscope autoradiography. Relatively large amounts of labeled pentosan are found in parenchyma cell walls, including the "protective layer" of ray parenchyma. Computeraided analyses of grain distributions in electron micrographs of cell walls of individual fibers localized the labeled wall components after different periods of incubation by comparison to model behavior. These analyses indicate that pentosan is added to the secondary cell wall of developing fibers by an appositional mechanism.

Native-state organization and distribution of cell-wall components in the secondary wall of woody tissue from P. mariana (Black Spruce) have been investigated using polarized Raman microspectroscopy. Evidence for orientation is detected through Raman intensity variations resulting from rotations of the exciting electric vector with respect to cell-wall geometry. Spectral features associated with cellulose and lignin were studied. The changes in cellulose bands indicate that the pyranose rings of the anhydroglucose repeat units are in planes perpendicular to the cross section, while methine C-H bonds are in planes parallel to the cross section. Changes in bands associated with lignin indicate that the aromatic rings of the phenyl-propane units are most often in the plane of the cell-wall surface. However, regions where lignin orientation departs from this pattern also occur. These results represent direct evidence of molecular organization with respect to cellular morphological features in woody tissue, and indicate that cell-wall components are more highly organized than had been recognized. Studies carried out in order to establish the usefulness and sensitivity of the Raman technique to differences of composition within the cell walls provide evidence of variations in the distribution of cellulose and lignin. Such compositional differences were more prominent between the walls of different cells than within a particular cell wall.