Genetic buffering of cyclic AMP in Arabidopsis thaliana compromises the plant immune response triggered by an avirulent strain of Pseudomonas syringae pv. tomato.

Cyclic AMP plays important roles in different physiological processes, including plant defence responses. However, as little information is known on plant enzymes responsible for cAMP production/degradation, studies of cAMP functions have relied, to date, on non-specific pharmacological approaches. We therefore developed a more reliable approach, producing transgenic Arabidopsis thaliana lines overexpressing the 'cAMP-sponge' (cAS), a genetic tool that specifically buffers cAMP levels. In response to an avirulent strain of Pseudomonas syringae pv. tomato (PstAvrB), cAS plants showed a higher bacterial growth and a reduced hypersensitive cell death in comparison with wild-type (WT) plants. The low cAMP availability after pathogen infection delayed cytosolic calcium elevation, as well as hydrogen peroxide increase and induction of redox systems. The proteomic analysis, performed 24 h post-infection, indicated that a core of 49 proteins was modulated in both genotypes, while 16 and 42 proteins were uniquely modulated in WT and cAS lines, respectively. The involvement of these proteins in the impairment of defence response in cAS plants is discussed in this paper. Moreover, in silico analysis revealed that the promoter regions of the genes coding for proteins uniquely accumulating in WT plants shared the CGCG motif, a target of the calcium-calmodulin-binding transcription factor AtSR1 (Arabidopsis thaliana signal responsive1). Therefore, following pathogen perception, the low free cAMP content, altering timing and levels of defence signals, and likely acting in part through the mis-regulation of AtSR1 activity, affected the speed and strength of the immune response.

Bacterial selection for biological control of plant disease: criterion determination and validation

This study aimed to evaluate the biocontrol potential of bacteria isolated from different plant species and soils. The production of compounds related to phytopathogen biocontrol and/or promotion of plant growth in bacterial isolates was evaluated by measuring the production of antimicrobial compounds (ammonia and antibiosis) and hydrolytic enzymes (amylases, lipases, proteases, and chitinases) and phosphate solubilization. Of the 1219 bacterial isolates, 92% produced one or more of the eight compounds evaluated, but only 1% of the isolates produced all the compounds. Proteolytic activity was most frequently observed among the bacterial isolates. Among the compounds which often determine the success of biocontrol, 43% produced compounds which inhibit mycelial growth of Monilinia fructicola, but only 11% hydrolyzed chitin. Bacteria from different plant species (rhizosphere or phylloplane) exhibited differences in the ability to produce the compounds evaluated. Most bacterial isolates with biocontrol potential were isolated from rhizospheric soil. The most efficient bacteria (producing at least five compounds related to phytopathogen biocontrol and/or plant growth), 86 in total, were evaluated for their biocontrol potential by observing their ability to kill juvenile Mesocriconema xenoplax. Thus, we clearly observed that bacteria that produced more compounds related to phytopathogen biocontrol and/or plant growth had a higher efficacy for nematode biocontrol, which validated the selection strategy used.(AU)

AFLP analysis of Xanthomonas axonopodis and X. arboricola strains used in xanthan production studies reveal high levels of polymorphism

Amplified fragment length polymorphism (AFLP) was used to analyze the genetic diversity of 14 strains of Xanthomonas arboricola pv. pruni and seven strains of X. axonopodis pv. phaseoli, which are used in xanthan production studies. Relationships identified by the AFLP profiles were assessed for xanthan production capacity, geographical location and host plant. Strains were isolated from 10 different geographic regions in South and Southeast States in Brazil. Data were analyzed for genetic similarity using the Dice coefficient and subjected to UPGMA cluster analysis. A total of 128 AFLP fragments were generated from four primer combinations: EcoRI+C/MseI+0, EcoRI+A/MseI+0, EcoRI+G/MseI+T and EcoRI+G/MseI+A. Of these, 96.1% were polymorphic. X. axonopodis pv. phaseoli (S D = 0.27) was shown to be more polymorphic than X. arboricola pv. pruni (S D = 0.58). All 14 pathovar pruni strains were included in a single main group (S D = 0.58), while the pathovar phaseoli strains were divided into three separate groups, with one group containing five strains (S D = 0.38) and two isolated groups (S D = 0.31 and 0.27) composed of only one strain each. Species were distinguished by three and eight specific AFLP markers present in the pathovar phaseoli and the pathovar pruni, respectively. For the unique strain without xanthan production capacity (X. axonopodis pv. phaseoli str. 48), nine specific AFLP bands were found. There was no evidence that geographic area or host plant influenced genetic heterogeneity. Correlations between AFLP patterns and xanthan production capacity were found in some strains, but were not consistent enough to establish a relationship.