Plant disease resistance to pathogens such as fungi, bacteria, and viruses often depends on whether the plant is able to recognize the pathogen early in the infection process. The recognition event leads to a rapid tissue necrosis at the site of infection, which is called the HR. The HR deprives the pathogen of nutrients and/or releases toxic molecules, thereby confining pathogen growth to a small region of the plant. This response provides resistance to the great majority of potential pathogens (nonhost or species resistance). For a given plant species, a much more limited number of true pathogens exhibit the ability to evade the host recognition system and grow extensively within the plant without evoking host necrosis at a11 or only after considerable delay. In this case, the plant exhibits susceptibility and the extensive growth of the successful pathogen can cause varying degrees of damage. However, certain races within pathogenic bacteria1 or funga1 species are recognized by certain cultivars or genotypes of the host plant species and the HR is triggered. These observations indicate that there is an ongqing evolution of the host plant's ability to recognize pathogen races that were previously unrecognized while the pathogen evolves to avoid recognition by a previously resistant host.Recognition of pathogens triggers a large range of inducible defense mechanisms that are believed to contribute to overall resistance in the plant. The mechanisms induced at the site of infection and associated with the HR include synthesis of antimicrobial compounds called phytoalexins, synthesis of hydrolytic enzymes that attack fungi and bacteria, and alterations in the synthesis of cell-wall structural proteins (for review, see Lamb et al., 1989). Many of these responses are due to transcriptional activation of specific genes that are collectively known as plant defense or defense-related genes. Defense gene regulation has been extensively studied both in intact plant-pathogen interactions and in model systems in which plant cell suspensions are treated with pathogenderived molecules termed elicitors.Severa1 rapid processes characteristic of the HR appear to involve primarily activation of preexisting components rather than changes in gene expression. One of these rapid processes is the striking release of AOS, which is termed the oxidative burst. This response to pathogens or elicitors has been observed in diverse monocotyledonous and dicotyledonous species including rice, tobacco, soybean, and spruce. The AOS are toxic intermediates that result from successive oneelectron steps in the reduction of molecular 02. The predominant species detected in plant-pathogen interactions are superoxide anion (O2-), hydrogen peroxide (H202), and hydroxyl radical (OH). The oxidative burst is correlated with the HR in a number of plant-pathogen interactions and therefore may be an important element contributing to disease resistance.This review first describes the occurrence of the oxidative burst in severa1 plant-pathogen interactions. S...

No abstract

We are studying cell differentiation in Dictyostelium discoideum by examining the regulation of genes that are preferentially expressed in different cell types. A system has been established in which prestalk-and prespore-cell-specific genes are expressed in single cells in response to culture conditions. We confirm our previous results showing that cyclic AMP induces prestalk genes and now show that it is also required for prespore gene induction. The expression of both classes of genes is additionally dependent on the presence of a factor(s) secreted by developing cells which we call conditioned medium factor(s). An assay for conditioned medium factor(s) shows that it is detectable within 2.5 h after the onset of development. Conditioned medium factor(s) also promotes the expression of genes induced early in development, but has no detectable effect on the expression of actin genes and a gene expressed maximally in vegetative cells. In the presence of conditioned medium factor(s), exogenous cyclic AMP at the onset of starvation fails to induce the prespore and prestalk genes. The addition of cyclic AMP between 2 and 12 h of starvation results in rapid prestalk gene expression, whereas prespore genes are induced at an invarient time (-18 h after the onset of starvation). These data suggest that cyclic AMP and conditioned medium factor(s) are sufficient for prestalk gene induction, whereas an additional parameter(s) is involved in the control of prespore gene induction. In contrast to several previous studies, we show that multicellularity is not essential for the expression of either prespore or prestalk genes. These data indicate that prespore and prestalk genes have cell-type-specific as well as shared regulatory factors.The structures and expression of numerous developmentally regulated genes have been analyzed in a variety of organisms. However, the biochemical factors that induce cell-type-specific gene expression and concomitant cell differentiation are not well understood in most systems. Several factors affecting cell differentiation have been recently elucidated in the simple eucaryote, Dictyostelium discoideum. Upon starvation, amoebae aggregate via cyclic AMP (cAMP)-mediated chemotaxis into multicellular aggregates.

No abstract

The structure and expression of a cDNA clone (PvPRP1) isolated from a cDNA library prepared from bean (Phaseolus vulgaris) cells treated with fungal elicitor have been characterized. Sequence analysis of the 1.1 kb insert revealed a complete open reading frame which encodes a 32 kDa protein. The protein resembles other proline-rich proteins in plants but possesses several unique features: (i) the N-terminal half of the protein is proline rich and contains three identical repeats of Pro-Val-His-Pro-Pro-Val-Lys-Pro-Pro-Val and six related repeats; (ii) the proline-rich region contains two tracts of six histidine residues; and (iii) the C-terminal half is low in proline and lacks repeats. Genomic blotting experiments suggest the presence of a single PvPRP1 gene as well as more distantly related genes within the bean genome. A dramatic decrease in PvPRP1 mRNA levels occurs within 2 h of elicitor treatment of cell cultures. The PvPRP1 mRNA is present at a moderate level in hypocotyls. Upon wounding, the mRNA level initially decreases over 5 h and then accumulates over 25 h to levels which are higher than the basal level in unwounded hypocotyls. Based on the similarity to other proline-rich proteins with repeated motifs, including the presence of a putative signal peptide, it is likely that the PvPRP1 protein is targetted to the cell wall. The expression of the PvPRP1 gene appears to be integrated with the remodeling of the plant cell wall during the defense response.