Nitric oxide and salicylic acid signaling in plant defense

Salicylic acid (SA) plays a critical signaling role in the activation of plant defense responses after pathogen attack. We have identified several potential components of the SA signaling pathway, including (i) the H2O2-scavenging enzymes catalase and ascorbate peroxidase, (ii) a high affinity SA-binding protein (SABP2), (iii) a SA-inducible protein kinase (SIPK), (iv) NPR1, an ankyrin repeat-containing protein that exhibits limited homology to IκBα and is required for SA signaling, and (v) members of the TGA/OBF family of bZIP transcription factors. These bZIP factors physically interact with NPR1 and bind the SA-responsive element in promoters of several defense genes, such as the pathogenesis-related 1 gene (PR-1). Recent studies have demonstrated that nitric oxide (NO) is another signal that activates defense responses after pathogen attack. NO has been shown to play a critical role in the activation of innate immune and inflammatory responses in animals. Increases in NO synthase (NOS)-like activity occurred in resistant but not susceptible tobacco after infection with tobacco mosaic virus. Here we demonstrate that this increase in activity participates in PR-1 gene induction. Two signaling molecules, cGMP and cyclic ADP ribose (cADPR), which function downstream of NO in animals, also appear to mediate plant defense gene activation (e.g., PR-1). Additionally, NO may activate PR-1 expression via an NO-dependent, cADPR-independent pathway. Several targets of NO in animals, including guanylate cyclase, aconitase, and mitogen-activated protein kinases (e.g., SIPK), are also modulated by NO in plants. Thus, at least portions of NO signaling pathways appear to be shared between plants and animals.

Dual Targeting of Osh1p, a Yeast Homologue of Oxysterol-binding Protein, to both the Golgi and the Nucleus-Vacuole Junction

Oxysterol binding protein (OSBP) is the only protein known to bind specifically to the group of oxysterols with potent effects on cholesterol homeostasis. Although the function of OSBP is currently unknown, an important role is implicated by the existence of multiple homologues in all eukaryotes so far examined. OSBP and a subset of homologues contain pleckstrin homology (PH) domains. Such domains are responsible for the targeting of a wide range of proteins to the plasma membrane. In contrast, OSBP is a peripheral protein of Golgi membranes, and its PH domain targets to the trans-Golgi network of mammalian cells. In this article, we have characterized Osh1p, Osh2p, and Osh3p, the three homologues of OSBP in Saccharomyces cerevisiae that contain PH domains. Examination of a green fluorescent protein (GFP) fusion to Osh1p revealed a striking dual localization with the protein present on both the late Golgi, and in the recently described nucleus-vacuole (NV) junction. Deletion mapping revealed that the PH domain of Osh1p specified targeting to the late Golgi, and an ankyrin repeat domain targeting to the NV junction, the first such targeting domain identified for this structure. GFP fusions to Osh2p and Osh3p showed intracellular distributions distinct from that of Osh1p, and their PH domains appear to contribute to their differing localizations.