No abstract

Reactive oxygen species (ROS) are both signal molecules and direct participants in plant defense against pathogens. Many fungi synthesize mannitol, a potent quencher of ROS, and there is growing evidence that at least some phytopathogenic fungi use mannitol to suppress ROSmediated plant defenses. Here we show induction of mannitol production and secretion in the phytopathogenic fungus Alternaria alternata in the presence of host-plant extracts. Conversely, we show that the catabolic enzyme mannitol dehydrogenase is induced in a non-mannitol-producing plant in response to both fungal infection and specific inducers of plant defense responses. This provides a mechanism whereby the plant can counteract fungal suppression of ROS-mediated defenses by catabolizing mannitol of fungal origin.Compelling evidence has arisen over the last decade demonstrating that reactive oxygen species (ROS) play a central role in pathogen defense in both animals and plants. In animals, ROS production by phagocytic leukocytes (macrophages͞ neutrophils) is a well characterized antimicrobial defense mechanism (1). Plants produce an analogous, localized oxidative burst (2), wherein massive amounts of antimicrobial ROS [e.g., superoxide, ⅐O 2 Ϫ ; and hydrogen peroxide (H 2 O 2 )] are generated by a pathogen-induced NADPH oxidase localized on the plant plasma membrane (3). In addition to its direct antimicrobial activity, H 2 O 2 also triggers the hypersensitive response, in which plant programmed, localized cell death at the site of infection limits pathogen spread (4). H 2 O 2 also plays a central role in signaling a unique phenomenon known as systemic acquired resistance, in which localized infection of a plant confers enhanced systemic resistance to subsequent attack by the same or unrelated pathogens (5, 6). Systemic acquired resistance is correlated with the systemic induction of a large number of defense-related proteins collectively labeled pathogenesis-related (PR) proteins. In addition to H 2 O 2 , the endogenous signal molecule salicylic acid (SA) is implicated in PR protein induction and has been used extensively as an exogenous stimulator of the systemic acquired resistance response (7,8).A successful pathogen must be able to overcome or suppress this complex array of ROS-mediated host defenses. In fact, microbial suppression of ROS-mediated defenses by secretion of ROS-scavenging enzymes such as superoxide dismutase and catalase, which convert ROS into less reactive species, has been extensively documented in both plant and animal pathogens (9-12). Evidence also is emerging that pathogens suppress ROS-mediated defenses by nonenzymatic quenching of ROS. Mannitol has long been recognized as a potent ROS quencher in vitro (13) and has widely been used as a laboratory reagent to scavenge hydroxyl radicals (HO⅐) generated by the phagocyte respiratory burst or by cell-free oxidant systems (14). In vivo, increased mannitol production protects Saccharomyces cerevisiae from oxidative injury (15). Furthermore, it was recently show...

Published studies focused on characterizing the allelopathy-based weed suppression by rye cover crop mulch have provided varying and inconsistent estimates of weed suppression. Studies were initiated to examine several factors that could influence the weed suppressiveness of rye: kill date, cultivar, and soil fertility. Ten cultivars of rye were planted with four rates of nitrogen fertilization, and tissue from each of these treatment combinations was harvested three times during the growing season. Concentrations of a known rye allelochemical DIBOA (2,4-dihydroxy-1,4-(2H)benzoxazine-3-one) were quantified from the harvested rye tissue using high performance liquid chromatography (HPLC). Phytotoxicity observed from aqueous extracts of the harvested rye tissue correlated with the levels of DIBOA recovered in harvested tissue. The amount of DIBOA in rye tissue varied depending on harvest date and rye cultivar, but was generally lower with all cultivars when rye was harvested later in the season. However, the late maturing variety 'Wheeler' retained greater concentrations of DIBOA in comparison to other rye cultivars when harvested later in the season. The decline in DIBOA concentrations as rye matures, and the fact that many rye cultivars mature at different rates may help explain why estimates of weed suppression from allelopathic agents in rye have varied so widely in the literature.

The plant cell wall is a dynamic cellular compartment consisting of a complex matrix of components that can change dramatically in response to environmental stresses. During pathogen attack, for instance, a wide spectrum of proteins that participate in various sequential processes involved in plant defense is secreted into the cell wall. In this study, a mass spectrometry, data-independent acquisition approach known as LC/MS (E) was used to assess temporal changes in the cell wall proteome in response to different levels of an endogenous inducer of plant disease defense responses, salicylic acid (SA). LC/MS (E) was used as a label-free method that enabled simultaneous protein identification and absolute femtomole quantification of each protein secreted into the extracellular matrix. A total of 74 secreted proteins were identified, 63 of which showed increased specific secretion in response to SA. A majority of this induced secretion occurred within 2 h of treatment, indicating that many proteins are involved in the early stages of plant defenses. We also identified a number of apparently nonclassically secreted proteins, suggesting that, as in many nonplant systems, Golgi/ER-independent mechanisms exist for plant protein secretion. These results provide new insight into plant apoplastic defense mechanisms and demonstrate that LC/MS (E) is a powerful tool for obtaining both relative and absolute proteome-scale quantification that can be applied to complex, time- and dose-dependent experimental designs.