Pathogenesis-related protein-la (PR-la) is a protein of unknown function that is strongly induced during the onset of systemic acquired resistance (SAR) in tobacco. The expression of PR-la is under complex regulation that is controlled at least partially by the rate of transcription. In this study, we demonstrated that 661 bp of 5’flanking DNA was sufficient to impart tobacco mosaic virus and salicylic acid inducibility to a reporter gene. The PR-la promoter did not respond significantly to treatments with either auxin or cytokinin. Experiments with the protein synthesis inhibitor cycloheximide indicated that protein synthesis is required for salicylate-dependent mRNA accumulation. At flowering, the PR-la gene was expressed primarily in the mesophyll and epidermal tissues of the leaf blade and the sepals of the flower. Several artifacts, most importantly ectopic expression in pollen, were associated with the use of the P-glucuronidase reporter gene.
Elevated levels of salicylic acid (SA) are required for the induction of systemic acquired resistance (SAR) in plants. Recently, a salicylic acid‐binding protein (SABP) isolated from tobacco was shown to have catalase activity. Based on this finding elevated levels of hydrogen peroxide (H 2 O 2 ) were postulated to act as a second messenger of SA in the SAR signal transduction pathway. A series of experiments have been carried out to clarify the role of H 2 O 2 in SAR‐signaling. No increase of H 2 O 2 was found during the onset of SAR. Induction of the SAR gene, PR‐1, by H 2 O 2 and H 2 O 2 ‐inducing chemicals is strongly suppressed in transgenic tobacco plants that express the bacterial salicylate hydroxylase gene, indicating that H 2 O 2 induction of SAR genes is dependent on SA accumulation. Following treatment of plants with increasing concentrations of H 2 O 2 , a dose‐dependent accumulation of total SA species was found, suggesting that H 2 O 2 may induce PR‐1 gene expression through SA accumulation. While the results do not support a role for H 2 O 2 in SAR signaling, it is suggested that SA inhibition of catalase activity may be important in tissues undergoing a hypersensitive response.
The NIM1 (for noninducible immunity) gene product is involved in the signal transduction cascade leading to both systemic acquired resistance (SAR) and gene-for-gene disease resistance in Arabidopsis. We have isolated and characterized five new alleles of nim1 that show a range of phenotypes from weakly impaired in chemically induced pathogenesis-related protein-1 gene expression and fungal resistance to very strongly blocked. We have isolated the NIM1 gene by using a map-based cloning procedure. Interestingly, the NIM1 protein shows sequence homology to the mammalian signal transduction factor I kappa B subclass alpha. NF-kappa B/I kappa B signaling pathways are implicated in disease resistance responses in a range of organisms from Drosophila to mammals, suggesting that the SAR signaling pathway in plants is representative of an ancient and ubiquitous defense mechanism in higher organisms.
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