Variability in bacterial endophytes in leaves of transgenic Bt and non-Bt cotton crop varieties and their role in suppression of bacterial leaf blight pathogen X. a. pv. malvacearum and the incited disease reaction

Microbial epiphytes and endophytes are an integral part of the plant system and are known to play various roles in crop growth and crop health management. The transgenic crop plays an important role in crop pest management, however, environmentalists and ecologists have reservations about the cultivation of these crops. Whether the transgenic Bt cotton and non-Bt cotton vary in their microbial habitat ecology is not yet studied.

In the present investigations, the leaf endophytic bacteria were detected in the leaves of both transgenic Bt and non-Bt cotton hybrids. However, there were differences in the endophytic bacterial types and their population densities i.e. bacterial colony-forming units (cfu) in the leaves of Bt and non-Bt cotton varieties. At least ten different leaf endophytic bacteria were detected from two cotton varieties i.e. RCH-2 and Bunny (of transgenic Bt and non-Bt versions). A maximum of four types of leaf endophytic bacteria was present in RCH-2 Bt cotton leaves. The population density of leaf endophytic bacteria ranged from 50 cfu/leaf to 5 x 10³ cfu/leaf and varied with individual leaf endophyte and cotton variety.

These bacterial leaf endophytes were observed to inhibit or suppress the growth of bacterial leaf blight pathogen Xanthomonas axonopodis pv. malvacearum (Xam) under in-vitro test. Leaf endophyte no.7 was more effective followed by leaf endophyte no.4 in suppressing the Xam population and population of other endophytes in the interaction studies.

Interaction of leaf endophytes and Xam in cotton leaves suggested that endophytes of transgenic Bunny-Bt were effective on Xam in transgenic Bunny–Bt hybrid only and so these changed the induction of susceptible water-soaking disease reaction into hypersensitive browning resistance reaction (HR). However, these endophytes of Bt-cotton were not effective in the non-Bt version in changing the susceptible reaction of Xam into an HR reaction. This indicated that the endophytes of the respective Bt and non-Bt crops were able to change the susceptible reaction of Xam into a hypersensitive one in their respective host, indicating that the use of leaf endophytes can be effective in their own habitat crop as a biocontrol agent against Xam. The specificity of leaf endophytes has to be considered in biological disease management programs.