Characterization of Pseudomonas chlororaphis from Theobroma cacao L. rhizosphere with antagonistic activity against Phytophthora palmivora (Butler).

AIM: To isolate and characterize rhizobacteria from Theobroma cacao with antagonistic activity against Phytophthora palmivora, the causal agent of the black pod rot, which is one of the most important diseases of T. cacao. METHODS AND RESULTS: Among 127 rhizobacteria isolated from cacao rhizosphere, three isolates (CP07, CP24 and CP30) identified as Pseudomonas chlororaphis, showed in vitro antagonistic activity against P. palmivora. Direct antagonism tested in cacao detached leaves revealed that the isolated rhizobacteria were able to reduce symptom severity upon infection with P. palmivora Mab1, with Ps. chlororaphis CP07 standing out as a potential biocontrol agent. Besides, reduced symptom severity on leaves was also observed in planta where cacao root system was pretreated with the isolated rhizobacteria followed by leaf infection with P. palmivora Mab1. The production of lytic enzymes, siderophores, biosurfactants and HCN, as well as the detection of genes encoding antibiotics, the formation of biofilm, and bacterial motility were also assessed for all three rhizobacterial strains. By using a mutant impaired in viscosin production, derived from CP07, it was found that this particular biosurfactant turned out to be crucial for both motility and biofilm formation, but not for the in vitro antagonism against Phytophthora, although it may contribute to the bioprotection of T. cacao. CONCLUSIONS: In the rhizosphere of T. cacao, there are rhizobacteria, such as Ps. chlororaphis, able to protect plants against P. palmivora. SIGNIFICANCE AND IMPACT OF THE STUDY: This study provides a theoretical basis for the potential use of Ps. chlororaphis CP07 as a biocontrol agent for the protection of cacao plants from P. palmivora infection.

A role for the miR396/GRF network in specification of organ type during flower development, as supported by ectopic expression of Populus trichocarpa miR396c in transgenic tobacco.

The MIR396 family, composed of ath-miR396a and ath-miR396b in Arabidopsis, is conserved among plant species and is known to target the Growth-Regulating Factor (GRF) gene family. ath-miR396 overexpressors or grf mutants are characterised by small and narrow leaves and show embryogenic defects such as cotyledon fusion. Heterologous expression of ath-miR396a has been reported in tobacco and resulted in reduction of the expression of three NtGRF genes. In this study, the precursor of the Populus trichocarpa ptc-miR396c, with a mature sequence identical to ath-miR396b, was expressed under control of the CaMV35S promoter in tobacco. Typical phenotypes of GRF down-regulation were observed, including cotyledon fusion and lack of shoot apical meristem (SAM). At later stage of growth, transgenic plants had delayed development and altered specification of organ type during flower development. The third and fourth whorls of floral organs were modified into stigmatoid anthers and fasciated carpels, respectively. Several NtGRF genes containing a miR396 binding site were found to be down-regulated, and the cleavage of their corresponding mRNA at the miR396 binding site was confirmed for two of them using RACE-PCR analysis. The data obtained agree with the functional conservation of the miR396 family in plants and suggest a role for the miR396/GRF network in determination of floral organ specification.

Endophytic fungi from leaves of Centella asiatica: occurrence and potential interactions within leaves.

Fungal endophytes were isolated from leaves of Centella asiatica (Apiaceae) collected at Mangoro (middle eastern region of Madagascar, 200 km from Antananarivo). Forty- five different taxa were recovered. The overall foliar colonization rate was 78%. The most common endophytes were the non-sporulating species 1 (isolation frequency IF 19.2%) followed by Colletotrichum sp.1 (IF 13.2%), Guignardia sp. (IF 8.5%), Glomerella sp. (IF 7.7%), an unidentified ascomycete (IF 7.2%), the non-sporulating species 2 (IF 3.7%) and Phialophora sp. (IF 3.5%). Using sequences of the ribosomal DNA internal transcribed spacer (ITS) regions, major endophytes (IF > 7%) were identified as xylariaceous taxa or as Colletotrichum higginsianum, Guignardia mangiferae and Glomerella cingulata. Results from in vitro fungal disk experiments showed a strong inhibitory activity of the xylariaceous non-sporulating species 1 against G. mangiferae and C. higginsianum and of C. higginsianum against G. mangiferae. This can be explained by antagonism between dominant taxa.

Investigation of the mutagenic and antimutagenic effects of Origanum compactum essential oil and some of its constituents.

In the present study, the chemical composition of Origanum compactum essential oil was determined by gas chromatography and mass spectrometry, and its mutagenic and antimutagenic activities were investigated by the somatic mutation and recombination test (SMART) in Drosophila melanogaster. No significant increase in the number of somatic mutations was observed with the essential oil tested using both the standard (ST) and high bio-activation (HB) cross. In order to investigate the antimutagenic effect of the essential oil, we have tested the effect on the indirect-acting mutagen urethane (URE), as well as the direct-acting mutagen methyl methanesulfonate (MMS). O. compactum essential oil showed a strong inhibitory effect against URE-induced mutagenicity, especially with the HB cross. However, only a weak inhibitory effect on the mutagenicity induced by MMS was observed. These results suggest that the detected antimutagenicity could be mediated by an inhibitory effect on metabolic activation. The essential oil was fractionated to identify the components responsible of the suppressing effect detected. Seven fractions were obtained: two of them showed the most potent inhibitory effect against URE-induced mutagenicity and were further fractionated. The sub-fractions obtained from the second chromatographic fractionation were tested for their antimutagenic activity, together with carvacrol and thymol. The highest antimutagenic effect obtained with the sub-fractions was similar to the effect of the crude essential oil, as well as to the effect of carvacrol alone. These results suggest the absence of a synergic antimutagenic effect between the components of O. compactum essential oil and indicate that carvacrol was the most active oil component.