SummaryThe oxidative burst is an early response to pathogen attack leading to the production of reactive oxygen species (ROS) including hydrogen peroxide. Two major mechanisms involving either NADPH oxidases or peroxidases that may exist singly or in combination in different plant species have been proposed for the generation of ROS. We identified an Arabidopsis thaliana azide-sensitive but diphenylene iodoniuminsensitive apoplastic oxidative burst that generates H 2 O 2 in response to a Fusarium oxysporum cell-wall preparation. Transgenic Arabidopsis plants expressing an anti-sense cDNA encoding a type III peroxidase, French bean peroxidase type 1 (FBP1) exhibited an impaired oxidative burst and were more susceptible than wild-type plants to both fungal and bacterial pathogens. Transcriptional profiling and RT-PCR analysis showed that the anti-sense (FBP1) transgenic plants had reduced levels of specific peroxidase-encoding mRNAs, including mRNAs corresponding to Arabidopsis genes At3g49120 (AtPCb) and At3g49110 (AtPCa) that encode two class III peroxidases with a high degree of homology to FBP1. These data indicate that peroxidases play a significant role in generating H 2 O 2 during the Arabidopsis defense response and in conferring resistance to a wide range of pathogens.

The oxidative burst, the generation of reactive oxygen species (ROS) in response to microbial pathogen attack, is a ubiquitous early part of the resistance mechanisms of plant cells. It has also become apparent from the study of a number of plant-pathogen interactions and those modelled by elicitor treatment of cultured cells that there may be more than one mechanism operating. However, one mechanism may be dominant in any given species. NADPH oxidases have been implicated in a number of systems and have been cloned and characterized. However, the enzyme system which is the major source of ROS in French bean (Phaseolus vulgaris) cells treated with a cell wall elicitor from Colletotrichum lindemuthianum, appears to be dependent on an exocellular peroxidase. The second component, the extracellular alkalinization, occurs as a result of the Ca(2+) and proton influxes and the K(+) efflux common to most elicitation systems as one of the earliest responses. The third component, the actual reductant/substrate, has remained elusive. The low molecular weight compound composition of apoplastic fluid was compared before and after elicitation. The substrate only becomes available some min after elicitation and can be extracted, so that by comparing the profiles by LC-MS it has been possible to identify possible substrates. The mechanism has proved to be complex and may involve a number of low molecular weight components. Stimulation of H(2)O(2) production was observed with saturated fatty acids such as palmitate and stearate without concomitant oxylipin production. This biochemical evidence is supported by immunolocalization studies on papillae forming at bacterial infection sites that show the peroxidase isoform present at sites of H(2)O(2) production revealed by cerium chloride staining together with the cross-linked wall proteins and callose and callose synthase. The peroxidase has been cloned and expressed in Pichia pastoris and has been shown to catalyse the oxidation reaction with the same kinetics as the purified enzyme. Furthermore, Arabidopsis plants transformed heterologously using the French bean peroxidase in antisense orientation have proved to be highly susceptible to bacterial and fungal pathogens. Thus it is possible that Arabidopsis is another species with the potential to mount an apoplastic oxidative burst and these transformed plant lines may be useful to identify the peroxidase that is responsible.

SummaryMaize actin-depolymerizing factor, ZmADF, binds both Gand F-actin and enhances in vitro actin dynamics. Evidence from studies on vertebrate ADF/cofilin supports the view that this class of protein responds to intracellular and extracellular signals and causes actin reorganization. As a test to determine whether such signal-responsive pathways existed in plants, this study addressed the ability of maize ADF to be phosphorylated and the likely effects of such phosphorylation on its capacity to modulate actin dynamics. It is shown that maize ADF3 (ZmADF3) can be phosphorylated by a calcium-stimulated protein kinase present in a 40-70% ammonium sulphate fraction of a plant cell extract. Phosphorylation is shown to be on Ser6, which is only one of nine amino acids that are fully conserved among the ADF/cofilin proteins across distantly related species. In addition, an analogue of phosphorylated ZmADF3 created by mutating Ser6 to Asp6 (zmadf3-4) does not bind G-or F-actin and has little effect on the enhancement of actin dynamics. These results are discussed in context of the previously observed actin reorganization in root hair cells.

The site of phosphorylation of phenylalanine ammonia-lyase (PAL) has been identified as a threonine residue. A Ca 2+ -stimulated protein kinase of approximately 55 kDa has been partially purified from elicited cells. The kinase can phosphorylate a synthetic peptide derived from PAL and a recombinant poplar PAL. PAL phosphorylation was associated with a decrease in V max in agreement with the suggestion that protein phosphorylation is involved in marking PAL subunits for turnover. The phosphorylation site in French bean PAL is most likely Thr 545 in the sequence VAKRTLTT (539^546). Conservation of the phosphorylation site in PAL from diverse species suggests that phosphorylation of PAL may be a ubiquitous regulatory mechanism in higher plants.z 1999 Federation of European Biochemical Societies.