Tobacco mosaic virus helicase domain induces necrosis in N gene-carrying tobacco in the absence of virus replication

Tobacco mosaic virus (TMV) elicits a hypersensitive response (HR) in tobacco plants that carry the N gene. To identify the elicitor of this HR, Agrobacteriumn tumefaciens was used as a vector for the transient expression of TMV replicase proteins, movement protein, and coat protein in NN and nn tobacco. Transient expression of the 126K protein and fragments thereof containing the helicase motifs induced necrosis and systemic expression of the pathogenesis-related PR-1a gene in NN plants but not in nn plants. The results confirm previous evidence that the TMV helicase sequence is the elicitor of the HR (H. S. Padgett, Y. Watanabe, and R. N. Beachy, Mol. Plant-Microbe Interact. 10:709-715, 1997) and demonstrate that this helicase sequence acts as an elicitor of HR in the absence of other viral proteins or RNA replication

A Novel WRKY Transcription Factor Is Required for Induction of PR-1a Gene Expression by Salicylic Acid and Bacterial Elicitors[C][W]

PR-1a is a salicylic acid-inducible defense gene of tobacco (Nicotiana tabacum). One-hybrid screens identified a novel tobacco WRKY transcription factor (NtWRKY12) with specific binding sites in the PR-1a promoter at positions −564 (box WK1) and −859 (box WK2). NtWRKY12 belongs to the class of transcription factors in which the WRKY sequence is followed by a GKK rather than a GQK sequence. The binding sequence of NtWRKY12 (WK box TTTTCCAC) deviated significantly from the consensus sequence (W box TTGAC[C/T]) shown to be recognized by WRKY factors with the GQK sequence. Mutation of the GKK sequence in NtWRKY12 into GQK or GEK abolished binding to the WK box. The WK1 box is in close proximity to binding sites in the PR-1a promoter for transcription factors TGA1a (as-1 box) and Myb1 (MBSII box). Expression studies with PR-1a promoter∷β-glucuronidase (GUS) genes in stably and transiently transformed tobacco indicated that NtWRKY12 and TGA1a act synergistically in PR-1a expression induced by salicylic acid and bacterial elicitors. Cotransfection of Arabidopsis thaliana protoplasts with 35S∷NtWRKY12 and PR-1a∷GUS promoter fusions showed that overexpression of NtWRKY12 resulted in a strong increase in GUS expression, which required functional WK boxes in the PR-1a promoter

Silencing of a gene encoding a protein component of the oxygen-evolving complex of photosystem ii enhances virus replication in plants

It has been suggested that, in addition to viral proteins, host proteins are involved in RNA virus replication. In this study the RNA helicase domain of the Tobacco mosaic virus (TMV) replicase proteins was used as bait in the yeast two-hybrid system to identify tobacco proteins with a putative role in TMV replication. Two host proteins were characterized. One protein (designated #3) belongs to a protein family of ATPases associated with various activities (AAA), while the second host protein (designated #13) is the 33K subunit of the oxygen-evolving complex of photosystem II. Using Tobacco rattle virus vectors, genes #3 and #13 were silenced in Nicotiana benthamiana, after which the plants were challenged by TMV infection. Silencing of gene #13 resulted in a 10-fold increase of TMV accumulation, whereas silencing of gene #3 caused a twofold reduction of TMV accumulation. Additionally, silencing of genes #3 and #13 decreased and increased, respectively, the accumulation of two other viruses. Similar to silencing of gene #13, inhibition of photosystem II by application of an herbicide increased TMV accumulation several fold. Infection of N. benthamiana with TMV resulted in a decrease of #13 mRNA levels. Silencing of gene #13 may reflect a novel strategy of TMV to suppress basal host defense mechanisms. The two-hybrid screenings did not identify tobacco proteins involved in helicase domain-induced N-mediated resistance

Overproduction of salicylic acid in plants by bacterial transgenes enhances pathogen resistance

After a hypersensitive response to invading pathogens, plants show elevated accumulation of salicylic acid (SA), induced expression of plant defense genes, and systemic acquired resistance (SAR) to further infection by a broad range of pathogens. There is compelling evidence that SA plays a crucial role in triggering SAR. We have transformed tobacco with two bacterial genes coding for enzymes that convert chorismate into SA by a two-step process. When the two enzymes were targeted to the chloroplasts, the transgenic (CSA, constitutive SA biosynthesis) plants showed a 500- to 1,000-fold increased accumulation of SA and SA glucoside compared to control plants. Defense genes, particularly those encoding acidic pathogenesis-related (PR) proteins, were constitutively expressed in CSA plants. This expression did not affect the plant phenotype, but the CSA plants showed a resistance to viral and fungal infection resembling SAR in nontransgenic plants.Ministry of Economic Affairs, the Ministry of Education, Culture and Science, and the Ministry of Agriculture, Nature Management and Fishery in the framework of a research program of the Association of Biotechnology Centres in the Netherlands (ABON)Peer reviewe