RESUMO
The Arabidopsis mutant cad1 (constitutively activated cell death 1) shows a phenotype that mimics hypersensitive response (HR)-like cell death. The CAD1 gene, which encodes a protein containing a domain with significant homology to the MACPF (membrane attach complex and perforin) domain of complement components and perforin, is likely to control plant immunity negatively and has a W-box cis-element in its promoter region. We found that expression of the CAD1 gene and other W-box containing genes, such as NPR1 and PR2, was promoted by salicylic acid (SA) and benzothiadiazole (BTH) as a SA agonist. The CAD1 gene was also stimulated by a purified chitin oligosaccharide elicitor (degree of polymerization = 8). This latter control was not under SA, because CAD1 expression was not suppressed in 35SnahG transgenic plants, which are unable to accumulate SA. These expression profiles were confirmed by promoter analysis using pCAD1::GUS transgenic plants. The CAD1 expression promoted by BTH and the chitin elicitor was not suppressed in the npr1 mutant, which is insensitive to SA signaling. These results indicate that the CAD1 gene is regulated by two distinct pathways involving SA and a chitin elicitor: viz., SA signaling mediated through an NPR1-independent pathway, and chitin elicitor signaling, through an SA-independent pathway. Three CAD1 homologs that have multiple W-box elements in their promoters were also found to be under the control of SA.
Assuntos
Proteínas de Arabidopsis/genética , Arabidopsis/genética , Quitina/farmacologia , Regulação da Expressão Gênica de Plantas/efeitos dos fármacos , Ácido Salicílico/farmacologia , Sequência de Aminoácidos , Arabidopsis/imunologia , Proteínas de Arabidopsis/biossíntese , Proteínas de Arabidopsis/imunologia , Northern Blotting , Southern Blotting , Complexo de Ataque à Membrana do Sistema Complemento/biossíntese , Complexo de Ataque à Membrana do Sistema Complemento/genética , Complexo de Ataque à Membrana do Sistema Complemento/imunologia , Dados de Sequência Molecular , Oligossacarídeos/farmacologia , Regiões Promotoras Genéticas , Reação em Cadeia da Polimerase Via Transcriptase Reversa , Alinhamento de Sequência , Tiadiazóis/farmacologia , Regulação para Cima/efeitos dos fármacosRESUMO
To clarify the processes involved in plant immunity, we have isolated and characterized a single recessive Arabidopsis mutant, cad1 (constitutively activated cell death 1), which shows a phenotype that mimics the lesions seen in the hypersensitive response (HR). This mutant shows spontaneously activated expression of pathogenesis-related (PR) genes, and leading to a 32-fold increase in salicylic acid (SA). Inoculation of cad1 mutant plants with Pseudomonas syringae pv tomato DC3000 shows that the cad1 mutation results in the restriction of bacterial growth. Cloning of CAD1 reveals that this gene encodes a protein containing a domain with significant homology to the MACPF (membrane attack complex and perforin) domain of complement components and perforin proteins that are involved in innate immunity in animals. Furthermore, cell death is suppressed in transgenic cad1 plants expressing nahG, which encodes an SA-degrading enzyme. We therefore conclude that the CAD1 protein negatively controls the SA-mediated pathway of programmed cell death in plant immunity.
Assuntos
Arabidopsis/genética , Genes de Plantas , Sequência de Aminoácidos , Apoptose/genética , Arabidopsis/citologia , Arabidopsis/imunologia , Sequência de Bases , DNA de Plantas/genética , Dados de Sequência Molecular , Mutação , Fenótipo , Filogenia , Doenças das Plantas/genética , Doenças das Plantas/virologia , Proteínas de Plantas/química , Proteínas de Plantas/genética , Proteínas de Plantas/imunologia , Estrutura Terciária de Proteína , Ácido Salicílico/metabolismo , Homologia de Sequência de Aminoácidos , Transdução de SinaisRESUMO
To clarify the mechanism of sugar-response of higher plants, the ghs1 (glucose hypersensitive) mutant of Arabidopsis was isolated and characterized. The ghs1 mutant had an increased sensitivity to glucose, showing a dramatic inhibition of chlorophyll synthesis and developmental arrest of leaves when grown on medium containing more than 5% glucose; the wild type required exposure to 7% glucose to show the same response. The ghs1 mutant is a single recessive loss-of-function mutation caused by a T-DNA insertion in the GHS1 gene (At3g27160), which encodes the plastid 30S ribosomal protein S21. The mutant showed: (1) reduction in the translation product but not the transcript for plastid-encoded rbcL, (2) reduction in photosynthetic activity monitored with pulse-amplitude modulated fluorometry, (3) impaired chloroplast development, as observed by electron microscopy. These results indicate that the deficiency of such chloroplast functions as photosynthetic activity observed in the ghs1 mutant is caused by impaired plastid protein synthesis associated with loss of ribosomal S21 protein. Relationships between the GHS1 gene and sugar-response are discussed.
