Natural occurrence of Azospirillum brasilense in petunia with capacity to improve plant growth and flowering.

To evaluate the natural occurrence of the plant growth-promoting bacterium Azospirillum brasilense and petunia plants, local strains were isolated and characterized by biochemical and molecular methods. Three strains were assessed in greenhouse conditions using Petunia × hybrida Ultra™. Treatments: Plants without bacterial inoculation or chemical fertilization; fertilized with NPK and KNO3 ; and independently inoculated with the strains 2A1, 2A2, and 2E1 by submerging their roots in a bacterial suspension (~106 CFU·ml-1 ). Root length, dry weight of roots and shoots, leaf area, leaf greenness, and nutrient content were evaluated. The number of days from transplanting to the opening of the first flower and the number of flowers per plant were also determined. As a result, five isolates were characterized as A. brasilense, showing the capacity to produce indoles and siderophores, to solubilize phosphate, nitrogenase activity, and nifH-PCR amplification. In general, all the parameters of the plant assay were improved in plants inoculated with A. brasilense, with variations among the strains, as well as the onset of flowering and the number of flowers per plant, compared with uninoculated or fertilized plants. This is the first report on the natural occurrence of A. brasilense in petunia with the capacity to improve plant growth and flowering.

Cross-talk between the RcsCDB and RstAB systems to control STM1485 gene expression in Salmonella Typhimurium during acid-resistance response.

Bacterial survive and respond to adverse changes in the environment by regulating gene transcription through two-component regulatory systems. In Salmonella Typhimurium the STM1485 gene expression is induced under low pH (4.5) during replication inside the epithelial host cell, but it is not involved in sensing or resisting to this condition. Since the RcsCDB system is activated under acidic conditions, we investigated whether this system is able to modulate STM1485 expression. We demonstrated that acid-induced activation of the RcsB represses STM1485 transcription by directly binding to the promoter. Under the same condition, the RstA regulator activates the expression of this gene. Physiologically, we observed that RcsB-dependent repression is required for the survival of bacteria when they are exposed to pancreatic fluids. We hypothesized that STM1485 plays an important role in Salmonella adaptation to pH changes, during transition in the gastrointestinal tract. We suggest that bacteria surviving the gastrointestinal environment invade the epithelial cells, where they can remain in vacuoles. In this new environment, acidity and magnesium starvation activate the expression of the RstA regulator in a PhoPQ-dependent manner, which in turn induces STM1485 expression. These levels of STM1485 allow increased bacterial replication within vacuoles to continue the course of infection.

Purification and characterization of AsES protein: a subtilisin secreted by Acremonium strictum is a novel plant defense elicitor.

In this work, the purification and characterization of an extracellular elicitor protein, designated AsES, produced by an avirulent isolate of the strawberry pathogen Acremonium strictum, are reported. The defense eliciting activity present in culture filtrates was recovered and purified by ultrafiltration (cutoff, 30 kDa), anionic exchange (Q-Sepharose, pH 7.5), and hydrophobic interaction (phenyl-Sepharose) chromatographies. Two-dimensional SDS-PAGE of the purified active fraction revealed a single spot of 34 kDa and pI 8.8. HPLC (C2/C18) and MS/MS analysis confirmed purification to homogeneity. Foliar spray with AsES provided a total systemic protection against anthracnose disease in strawberry, accompanied by the expression of defense-related genes (i.e. PR1 and Chi2-1). Accumulation of reactive oxygen species (e.g. H2O2 and O2(˙)) and callose was also observed in Arabidopsis. By using degenerate primers designed from the partial amino acid sequences and rapid amplification reactions of cDNA ends, the complete AsES-coding cDNA of 1167 nucleotides was obtained. The deduced amino acid sequence showed significant identity with fungal serine proteinases of the subtilisin family, indicating that AsES is synthesized as a larger precursor containing a 15-residue secretory signal peptide and a 90-residue peptidase inhibitor I9 domain in addition to the 283-residue mature protein. AsES exhibited proteolytic activity in vitro, and its resistance eliciting activity was eliminated when inhibited with PMSF, suggesting that its proteolytic activity is required to induce the defense response. This is, to our knowledge, the first report of a fungal subtilisin that shows eliciting activity in plants. This finding could contribute to develop disease biocontrol strategies in plants by activating its innate immunity.

Temporal accumulation of salicylic acid activates the defense response against Colletotrichum in strawberry.

Many authors have reported interactions between strawberry cultivars and pathogenic microorganisms, yet little is known about the mechanisms triggered in the plant. In this paper we examine the participation of the salicylic acid (SA) signaling pathway involved in the response of Fragaria x ananassa cv. Pájaro plants to pathogens. Strawberry plants were challenged with the virulent strain M11 of Colletotrichum acutatum, or with the avirulent strain M23 of Colletotrichum fragariae which confers resistance to the former. Our study showed that the isolate M23 induced a temporal SA accumulation that was accompanied with the induction of PR-1 gene expression in strawberry plants. Such events occured after the oxidative burst, evaluated as the accumulation of hydrogen peroxide and superoxide anion, and many hours before the protection could be detected. Similar results were obtained with exogenously applied SA. Results obtained supports the hypothesis that strawberry plants activate a SA mediated defense mechanisms that is effective against a causal agent of anthracnose. In contrast, plants inoculated with M11 did not show oxidative burst, SA accumulation or PR1 gene induction. This is the first report about a defense response signaling pathway studied in strawberry plants.

White-fruited Duchesnea indica (Rosaceae) is impaired in ANS gene expression.

PREMISE OF THE STUDY: Duchesnea indica is a wild strawberry-like species that has red fruits. In a recent survey in the highlands of Tucumán (Argentina), a plant of D. indica with white fruits was discovered. The aim of this study was to investigate whether the white-fruited character was due to a phenotypic or genotypic change. The stability and heritability of the character and the expression of genes involved in anthocyanins synthesis were studied and compared with red-fruited genotypes. This study contributes to understanding the molecular basis of some factors involved in fruit pigmentation, a horticulturally and taxonomically important trait. METHODS: Stability and heritability of the white-fruited character were evaluated in plants obtained by asexual propagation or by sexual crosses between the white- and red-fruited genotypes. Asexual multiplications were carried out by stolon rooting and sexual multiplications by germination of achenes obtained from crosses. The expression level of the genes involved in the synthesis and regulation of the anthocyanins pathway (CHS, F3H, DFR, ANS, and MYB10) were evaluated by RT-PCR using specific primers. KEY RESULTS: Plants with the white-fruited character always yielded white-fruited progeny when propagated asexually, whereas in sexually propagated plants fruit color depended on the mother. Red-fruited mothers yielded red-fruited progeny, and white-fruited mothers yielded fruits ranging from dark pink to white. Molecular analysis suggested that the white-fruited character was due to the low expression of the ANS gene. CONCLUSIONS: Results obtained indicate that the white-fruited character was stable. Mother progenitors exert a strong influence on the expression of the white-fruited character. The white-fruited phenotype is due to the impairment or downregulation of the ANS gene.