Rhizobium etli maize populations and their competitiveness for root colonization

Area of Science:

• Microbiology
• Plant-microbe interactions
• Agronomy

Background:

• Rhizobium etli typically nodulates Phaseolus vulgaris (common bean) for nitrogen fixation.
• Maize (Zea mays) is often intercropped with common beans in traditional agricultural systems.
• The endophytic nature of R. etli in maize is not well understood.

Purpose of the Study:

• To analyze the genetic diversity of Rhizobium etli strains in maize and common bean agroecosystems.
• To identify R. etli strains that are adapted to colonize maize as endophytes.
• To investigate the competitive abilities and tolerance of maize-associated R. etli strains.

Main Methods:

• Plasmid profiling and alloenzyme analysis were used to assess genetic diversity.
• Rhizobium etli strains were isolated from soil, bean nodules, maize rhizosphere, roots, and stems.
• Competitive root colonization assays and tolerance tests to 6-methoxy-2-benzoxazolinone (MBOA) were performed.

Main Results:

• Several R. etli types were preferentially found as maize endophytes.
• Some maize-derived R. etli strains showed superior competitive colonization of maize roots compared to rhizosphere or bean nodule strains.
• The dominant strain Ch24-10 exhibited high tolerance to MBOA, a maize antimicrobial compound.
• Rhizobium tropici CIAT899 also proved to be a competitive maize endophyte.

Conclusions:

• Rhizobium etli possesses genetic diversity that allows for specialization as a maize endophyte.
• Specific R. etli strains are adapted for enhanced maize root colonization and survival in the maize rhizosphere.
• MBOA tolerance is a key trait for competitive endophytic colonization of maize by R. etli.