Agrobacterium VirD2 protein interacts with plant host cyclophilins

Agrobacterium tumefaciens induces crown gall tumors on plants by transferring a nucleoprotein complex, the T-complex, from the bacterium to the plant cell. The T-complex consists of T-DNA, a single-stranded DNA segment of the tumor-inducing plasmid, VirD2, an endonuclease covalently bound to the 5′ end of the T-DNA, and perhaps VirE2, a single-stranded DNA binding protein. The yeast two-hybrid system was used to screen for proteins interacting with VirD2 and VirE2 to identify components in Arabidopsis thaliana that interact with the T-complex. Three VirD2- and two VirE2-interacting proteins were identified. Here we characterize the interactions of VirD2 with two isoforms of Arabidopsis cyclophilins identified by using this analysis. The VirD2 domain interacting with the cyclophilins is distinct from the endonuclease, omega, and the nuclear localization signal domains. The VirD2–cyclophilin interaction is disrupted in vitro by cyclosporin A, which also inhibits Agrobacterium-mediated transformation of Arabidopsis and tobacco. These data strongly suggest that host cyclophilins play a role in T-DNA transfer.

AvrPto-dependent Pto-interacting proteins and AvrPto-interacting proteins in tomato

The plant-intracellular interaction of the avirulence protein AvrPto of Pseudomonas syringae pathovar tomato, the agent of bacterial speck disease, and the corresponding tomato resistance protein Pto triggers responses leading to disease resistance. Pto, a serine/threonine protein kinase, also interacts with a putative downstream kinase, Pto-interactor 1, as well as with members of a family of transcription factors Pto-interactors 4, 5, and 6. These proteins are likely involved, respectively, in a phosphorylation cascade resulting in hypersensitive cell death, and in defense gene activation. The mechanism by which the interaction of AvrPto and Pto initiates defense response signaling is not known. To pursue the hypothesis that tertiary interactions are involved, we modified the yeast two-hybrid protein interaction trap and conducted a search for tomato proteins that interact with Pto only in the presence of AvrPto. Five classes of AvrPto-dependent Pto interactors were isolated, and their interaction specificity confirmed. Also, to shed light on a recently demonstrated virulence activity of AvrPto, we conducted a standard two-hybrid screen for tomato proteins in addition to Pto that interact with AvrPto: i.e., potential virulence targets or modifiers of AvrPto. By constructing an N-terminal rather than a C-terminal fusion of AvrPto to the LexA DNA binding domain, we were able to overcome autoactivation by AvrPto and identify four classes of specific AvrPto-interacting proteins.

From seed germination to flowering, light controls plant development via the pigment phytochrome.

Plant growth and development are regulated by interactions between the environment and endogenous developmental programs. Of the various environmental factors controlling plant development, light plays an especially important role, in photosynthesis, in seasonal and diurnal time sensing, and as a cue for altering developmental pattern. Recently, several laboratories have devised a variety of genetic screens using Arabidopsis thaliana to dissect the signal transduction pathways of the various photoreceptor systems. Genetic analysis demonstrates that light responses are not simply endpoints of linear signal transduction pathways but are the result of the integration of information from a variety of photoreceptors through a complex network of interacting signaling components. These signaling components include the red/far-red light receptors, phytochromes, at least one blue light receptor, and negative regulatory genes (DET, COP, and FUS) that act downstream from the photoreceptors in the nucleus. In addition, a steroid hormone, brassinolide, also plays a role in light-regulated development and gene expression in Arabidopsis. These molecular and genetic data are allowing us to construct models of the mechanisms by which light controls development and gene expression in Arabidopsis. In the future, this knowledge can be used as a framework for understanding how all land plants respond to changes in their environment.