Arabidopsis cell walls contain large amounts of pectins and hemicelluloses, which are predominantly synthesized via the common precursor UDP-glucuronic acid. The major enzyme for the formation of this nucleotide-sugar is UDP-glucose dehydrogenase, catalysing the irreversible oxidation of UDP-glucose into UDP-glucuronic acid. Four functional gene family members and one pseudogene are present in the Arabidopsis genome, and they show distinct tissue-specific expression patterns during plant development. The analyses of reporter gene lines indicate gene expression of UDP-glucose dehydrogenases in growing tissues. The biochemical characterization of the different isoforms shows equal affinities for the cofactor NAD(+) ( approximately 40 microM) but variable affinities for the substrate UDP-glucose (120-335 microM) and different catalytic constants, suggesting a regulatory role for the different isoforms in carbon partitioning between cell wall formation and sucrose synthesis as the second major UDP-glucose-consuming pathway. UDP-glucose dehydrogenase is feedback inhibited by UDP-xylose. The relatively (compared with a soybean UDP-glucose dehydrogenase) low affinity of the enzymes for the substrate UDP-glucose is paralleled by the weak inhibition of the enzymes by UDP-xylose. The four Arabidopsis UDP-glucose dehydrogenase isoforms oxidize only UDP-glucose as a substrate. Nucleotide-sugars, which are converted by similar enzymes in bacteria, are not accepted as substrates for the Arabidopsis enzymes.

This paper reports on the identification and characterization of a new ATP-binding cassette (ABC) transporter which was identified as a salicylic acid-induced gene from soybean (Glycine max cv. Williams 82) in a subtractive suppression hybridization approach. A fragment of an ABC-transporter gene was used to isolate a full-length cDNA clone for this gene with a length of 4750 bp. The encoded protein has a length of 1447 amino acids and is composed of two similar repeat units typical of full-size ABC transporters. The sequence displays a close relationship to plant pleiotropic drug resistance (PDR)-type transporters and, on a homology basis, clusters together with the Arabidopsis thaliana PDR12 gene, suggesting GmPDR12 as a name for the gene isolated from soybean. GmPDR12 is rapidly responsive to salicylic acid and methyl jasmonate. The mRNA starts to accumulate 30 min after the addition of the signalling compounds. Salicylic acid is required for the execution of the hypersensitive reaction in soybean cell suspension cultures inoculated with Pseudomonas syringae pv. glycinea. It has been demonstrated previously that salicylic acid can be substituted by a variety of functional analogues of salicylic acid. All of these compounds lead to a strong and rapid transcriptional activation of GmPDR12, suggesting a shared signalling pathway.