Exclusive rewards in mutualisms: ant proteases and plant protease inhibitors create a lock-key system to protect Acacia food bodies from exploitation.

Myrmecophytic Acacia species produce food bodies (FBs) to nourish ants of the Pseudomyrmex ferrugineus group, with which they live in an obligate mutualism. We investigated how the FBs are protected from exploiting nonmutualists. Two-dimensional gel electrophoresis of the FB proteomes and consecutive protein sequencing indicated the presence of several Kunitz-type protease inhibitors (PIs). PIs extracted from Acacia FBs were biologically active, as they effectively reduced the trypsin-like and elastase-like proteolytic activity in the guts of seed-feeding beetles (Prostephanus truncatus and Zabrotes subfasciatus), which were used as nonadapted herbivores representing potential exploiters. By contrast, the legitimate mutualistic consumers maintained high proteolytic activity dominated by chymotrypsin 1, which was insensitive to the FB PIs. Larvae of an exploiter ant (Pseudomyrmex gracilis) taken from Acacia hosts exhibited lower overall proteolytic activity than the mutualists. The proteases of this exploiter exhibited mainly elastase-like and to a lower degree chymotrypsin 1-like activity. We conclude that the mutualist ants possess specifically those proteases that are least sensitive to the PIs in their specific food source, whereas the congeneric exploiter ant appears partly, but not completely, adapted to consume Acacia FBs. By contrast, any consumption of the FBs by nonadapted exploiters would effectively inhibit their digestive capacities. We suggest that the term 'exclusive rewards' can be used to describe situations similar to the one that has evolved in myrmecophytic Acacia species, which reward mutualists with FBs but safeguard the reward from exploitation by generalists by making the FBs difficult for the nonadapted consumer to use.

Medicago truncatula increases its iron-uptake mechanisms in response to volatile organic compounds produced by Sinorhizobium meliloti.

Medicago truncatula represents a model plant species for understanding legume-bacteria interactions. M. truncatula roots form a specific root-nodule symbiosis with the nitrogen-fixing bacterium Sinorhizobium meliloti. Symbiotic nitrogen fixation generates high iron (Fe) demands for bacterial nitrogenase holoenzyme and plant leghemoglobin proteins. Leguminous plants acquire Fe via "Strategy I," which includes mechanisms such as rhizosphere acidification and enhanced ferric reductase activity. In the present work, we analyzed the effect of S. meliloti volatile organic compounds (VOCs) on the Fe-uptake mechanisms of M. truncatula seedlings under Fe-deficient and Fe-rich conditions. Axenic cultures showed that both plant and bacterium modified VOC synthesis in the presence of the respective symbiotic partner. Importantly, in both Fe-rich and -deficient experiments, bacterial VOCs increased the generation of plant biomass, rhizosphere acidification, ferric reductase activity, and chlorophyll content in plants. On the basis of our results, we propose that M. truncatula perceives its symbiont through VOC emissions, and in response, increases Fe-uptake mechanisms to facilitate symbiosis.

Homologous recombination and dynamics of rhizobial genomes.

The nitrogen-fixing bacteria commonly known as rhizobia are attractive organisms due to their symbiotic association with legume plants. Their genomes contain a large number of redundant genetic elements. These reiterations might participate in homologous recombination events and lead to diverse genomic rearrangements. Here we analyze the role of homologous recombination in the dynamics of these bacterial genomes, as well as its possible biological consequences.

Bacillus megaterium rhizobacteria promote growth and alter root-system architecture through an auxin- and ethylene-independent signaling mechanism in Arabidopsis thaliana.

Soil microorganisms are critical players in plant-soil interactions at the rhizosphere. We have identified a Bacillus megaterium strain that promoted growth and development of bean (Phaseolus vulgaris) and Arabidopsis thaliana plants. We used Arabidopsis thaliana as a model to characterize the effects of inoculation with B. megaterium on plant-growth promotion and postembryonic root development. B. megaterium inoculation caused an inhibition in primary-root growth followed by an increase in lateral-root number, lateral-root growth, and root-hair length. Detailed cellular analyses revealed that primary root-growth inhibition was caused both by a reduction in cell elongation and by reduction of cell proliferation in the root meristem. To study the contribution of auxin and ethylene signaling pathways in the alterations in root-system architecture elicited by B. megaterium, a suite of plant hormone mutants of Arabidopsis, including aux1-7, axr4-1, eir1, etr1, ein2, and rhd6, defective in either auxin or ethylene signaling, were evaluated for their responses to inoculation with this bacteria. When inoculated, all mutant lines tested showed increased biomass production. Moreover, aux1-7 and eir1, which sustain limited root-hair and lateral-root formation when grown in uninoculated medium, were found to increase the number of lateral roots and to develop long root hairs when inoculated with B. megaterium. The ethylene-signaling mutants etr1 and ein2 showed an induction in lateral-root formation and root-hair growth in response to bacterial inoculation. Taken together, our results suggest that plant-growth promotion and root-architectural alterations by B. megaterium may involve auxin- and-ethylene independent mechanisms.