From coexistence to antagonism: nutrient- and temperature-dependent interactions between Bacillus atrophaeus JunSE1L and Pseudomonas chlororaphis O6 with implications for the wheat rhizosphere.

backgroundInteractions between beneficial plant-associated microbes influence microbial community structure and plant health, yet antagonistic relationships among co-occurring beneficial bacteria remain poorly understood. This study examined interactions between Pseudomonas chlororaphis O6 (PcO6), a wheat root-colonizing epiphyte, and JunSE1L, a Bacillus atrophaeus seed-borne endophyte isolated from winter wheat (Triticum aestivum var. Juniper).

resultsFor wheat grown in a sterile sand matrix, JunSE1L emerged from seeds and colonized roots; however, when PcO6 was introduced at the bottom of the growth boxes, it migrated and became the dominant rhizoplane colonizer. JunSE1L emerged from both damaged roots and shoots, and produced biosurfactants that lowered the surface tension of water to 30 mN/m. On agar plates at 22 °C PcO6 strongly inhibited JunSE1L growth on minimal medium (MM) and to a limited extent on rich LB medium; inhibition was absent on both media at 37 °C. On MM, JunSE1L colonies adopted dendritic, highly spread morphologies, potentially from biosurfactant release that may have also facilitated PcO6 swarming over established JunSE1L colonies. Whole PcO6 supernatant and a < 3 kDa filtrate each suppressed JunSE1L growth in liquid MM at 22 ˚C. The inhibitory compound(s) was resistant to proteinase K digestion and heat deactivation, exhibiting bacteriostatic activity against JunSE1L.

conclusionsPcO6 inhibits JunSE1L in a temperature-, nutrient-, and density-dependent manner: On rich growth media they coexist, while on defined minimal media, PcO6 dominates, suggesting that resource limitation intensifies competitive interactions and favors epiphyte (PcO6) - mediated suppression of endophyte (JunSE1L). While both bacteria comprise the wheat root microbiome, under conditions recapitulating the rhizosphere (defined nutrients, moderate temperature, and PcO6 already present in the soil) PcO6 release of a bacteriostatic agent may repress JunSE1L emergence. To the contrary, JunSE1L release of biosurfactants may facilitate PcO6 migration to the rhizoplane.