Monitoring a beneficial bacterium (Bacillus amyloliquefaciens) in the rhizosphere with arugula herbivory

The plant growth promoting rhizobacterium (PGPR) Bacillus amyloliquefaciens (strain GB03) augments Arabidopsis glucosinolate defenses targeting the generalist herbivore Spodoptera exigua. With the goal to transfer such technology to an agricultural crop, bacterial-defense induction was monitored in the closely related species, Eruca sativa (arugula). Plant growth promotion and leaf protection against the herbivore specialist Plutella xylostella (diamondback-moth) was induced with GB03-seed inoculation. PGPR-induced herbivore protection correlated with greater gene induction encoding for glucosinolate biosynthesis. To monitor rhizosphere-specific GB03 proliferation, PGPR-specific primers were designed for qPCR gene amplification. Rhizosphere colonization was detected with GB03 inoculation for 28 days, while rhizosphere GB03 colonization diminished to below detection level after 21 days for insect-damaged plants. By selectively monitoring GB03 from a complex mix of soil microbes with seed inoculation, rhizosphere PGPR proliferation and foliar glucosinolate transcription for plant protection against the diamondback-moth specialist can be monitored in vivo under authentic field conditions.