Soils of the tropics

The conditions for soil development in the lowland tropics are described, in particular the soil-forming factors, climate, parent material, topography and organisms, and their interactions through time. Of particular importance is the climate of the lowland tropics, which has a major influence on the nature of soil development because of high temperatures and the duration of the periods when the soil is moist. The nature of the parent material is also a major determinant of the nature of the soil. Because soil development has taken place in much of the tropics over long time periods, the soils have distinctive characteristics. Soil-forming processes are described briefly, in particular the nature of the weathering of the inorganic fraction of the soil, and the removal of soluble materials in leaching and the translocation of materials in suspension. Typical soils developed in the humid and seasonally moist tropics are briefly described, and comparisons made between the two most widely used international soil classifications, Soil Taxonomy and World Reference Base for Soil Resources. Some of the other soils found within the tropics are briefly described.

The use of bioluminescence for monitoring in planta growth dynamics of a Pseudomonas syringae plant pathogen

The use of bioluminescence was evaluated as a tool to study Pseudomonas syringae population dynamics in susceptible and resistant plant environments. Plasmid pGLITE, containing the luxCDABE genes from Photorhabdus luminescens, was introduced into Pseudomonas syringae pv. phaseolicola race 7 strain 1449B, a Gram-negative pathogen of bean (Phaseolus vulgaris). Bacteria recovered from plant tissue over a five-day period were enumerated by counting numbers of colony forming units and by measurement of bioluminescence. Direct measurement of bioluminescence from leaf disc homogenates consistently reflected bacterial growth as determined by viable counting, but also detected subtle effects of the plant resistance response on bacterial viability. This bioluminescence procedure enables real time measurement of bacterial metabolism and population dynamics in planta, obviates the need to carry out labour intensive and time consuming traditional enumeration techniques and provides a sensitive assay for studying plant effects on bacterial cells.

In planta conditions induce genomic changes in Pseudomonas syringae pv. phaseolicola

The co-evolution of bacterial plant pathogens and their hosts is a complex and dynamic process. Plant resistance can impose stress on invading pathogens that can lead to, and select for, beneficial changes in the bacterial genome. The Pseudomonas syringae pv. phaseolicola (Pph) genomic island PPHGI-1 carries an effector gene, avrPphB (hopAR1), which triggers the hypersensitive reaction in bean plants carrying the R3 resistance gene. Interaction between avrPphB and R3 generates an antimicrobial environment within the plant, resulting in the excision of PPHGI-1 and its loss from the genome. The loss of PPHGI-1 leads to the generation of a Pph strain able to cause disease in the plant. In this study, we observed that lower bacterial densities inoculated into resistant bean (Phaseolus vulgaris) plants resulted in quicker PPHGI-1 loss from the population, and that loss of the island was strongly influenced by the type of plant resistance encountered by the bacteria. In addition, we found that a number of changes occurred in the bacterial genome during growth in the plant, whether or not PPHGI-1 was lost. We also present evidence that the circular PPHGI-1 episome is able to replicate autonomously when excised from the genome. These results shed more light onto the plasticity of the bacterial genome as it is influenced by in planta conditions.