The leguminous tree genus Inga is thought to be critical for providing N to certified organic coffee shrubs that commonly grow beneath its shade in the state of Chiapas, Mexico. Organic certification prohibits use of synthetic N fertilizers, necessitating understanding of the Inga-rhizobia symbiosis. The objectives of this study were to: (1) establish a collection of isolates from Inga sp. nodules found on organic coffee farms in Chiapas, Mexico; (2) use phenotypic and genotypic approaches to characterize these isolates, and (3) examine cross-inoculation patterns of strains isolated from Inga sp. with other Inga species and N2-fixing trees. Eighty-three strains were isolated from root nodules of Inga oerstediana, Inga pavoniana and an unknown species of Inga. Aside from isolates of I. pavoniana, Inga isolates demonstrate characteristics consistent with Bradyrhizobium. Dendrograms generated using phylogenetic and phenotypic traits showed strains to exhibit significant diversity, even among strains originating from the same farm. Strains isolated from I. pavoniana were 95% similar, and displayed strikingly different characteristics from strains isolated from other species. Cross-inoculation experiments used three Inga strains (cowpea miscellany CB756, and isolates from I. oerstediana and I. sp.) as inoculants with I. edulis, I. fuellii, Acacia mangium, Acacia koa, Vigna unguculata and Gliricidia sepium. All three strains formed nodules containing leghaemoglobin with V. unguculata, while isolates from I. oerstediana, Inga sp. and cowpea miscellany formed nodules with I. feulli and I. edulis, but not with A. mangium and G. sepium. A. koa formed nodules containing leghaemoglobin when inoculated by cowpea miscellany strain CB756 and the strain from Inga sp., but nodules formed with the strain isolated from I. oerstediana did not contain leghaemoglobin. One and two-way ANOVA results showed that no mean biomass of inoculated seedlings was significantly greater than the -N control for each of A. koa, G. sepium, A. mangium (p < 0.001), supporting our hypothesis that inoculation with non-Inga strains would have no effect on biomass production. We suggest that great spatial variation exists among Inga isolates from organic coffee farms in the Chiapas highland region, both between and within farms, and that more research is needed to identify reasons for the genetic distribution and function of Inga symbionts.
Bibliographical noteFunding Information:
This project was made possible by the generous support of the Fulbright US–Mexico Commission for Educational and Cultural Exchange; the Graduate School of the University of Minnesota through the Carolyn M. Crosby, Alexander and Lydia Anderson, and Andrews Hunt endowed fellowships; and the University of Minnesota department of Agronomy and Plant Genetics. Thank you to Ryan Vial for his help with laboratory experiments at the University of Minnesota. El Colegio de la Frontera Sur (ECOSUR) in San Cristobal de Las Casas, Chiapas, provided us with essential institutional support in México, especially David Alvarez-Solis, Luis García-Barrios, Miguel Angel Lopez-Anaya, Manuel de Jesus Gutierrez-Gomez, y Lourdes Herrera. Unión Majomut, notably Walter Anzueto-Anzueto and Victor Perez-Grovas-Garza, arranged invaluable communication with the study communities. Most of all, we offer our greatest sincere gratitude to the producers of Unión Majomut for allowing us to sample from their coffee farms. Without their valuable participation this project could not have been carried out.
Copyright 2008 Elsevier B.V., All rights reserved.
- Root nodule bacteria