The role of Pantoea stewartii subsp. stewartii leucine-responsive regulatory protein (Lrp) during maize xylem growth.

The bacterium Pantoea stewartii subsp. stewartii (Pss) causes Stewart's wilt disease in maize. Pss is introduced into maize via the corn flea beetle vector, Chaetocnema pulicaria, when beetle feces enter wounds created during feeding. The infection begins in the apoplast of the leaf where Pss causes leaf blight. Subsequently, the bacteria move to the xylem and form a biofilm, preventing water transport. This causes wilting and leads to necrosis, consequently affecting both crop yield and survival. A previous Tn-Seq experiment identified genes essential for Pss in planta survival. One essential gene, lrp, encodes the global transcription factor leucine-responsive regulatory protein (Lrp). The Lrp protein family is found across many bacterial and archaeal species where it regulates multiple critical physiological functions. In Pss, Lrp is known to positively control motility and capsule production, which are important for the in planta lifestyle and virulence of Pss. In this study, the genes within the Pss Lrp regulon were defined through bioinformatic analyses of RNA-Seq data that measured differential gene expression between wild-type Pss and a ∆lrp strain grown in planta. Lrp was found to regulate genes involved in capsule biosynthesis and nitrogen-associated assimilation and metabolism. Biolog plates were subsequently used to link the regulatory role of Lrp with regard to Pss metabolism by examining the capacity of Pss to grow using sole carbon or nitrogen sources in vitro. Collectively, this work has provided insights into how Pss recognizes and exploits the maize xylem environment.IMPORTANCEThe bacterium Pantoea stewartii subsp. stewartii (Pss) causes Stewart's wilt disease in maize when it forms a biofilm in the xylem that prevents water flow. Little is known about how Pss is able to colonize and grow within the maize xylem. Previous work identified the Lrp regulatory protein as being important for the survival of the bacterium inside maize. This study determined the genes whose transcription is under Lrp control and predicted the physiological functions associated with those genes to learn more about the bacterial growth inside the plant. The ability to transport and metabolize organic compounds containing nitrogen and the ability to produce capsule were found to be regulated by Lrp. Additional laboratory experiments demonstrated that Lrp also controls the metabolism of certain sole carbon and nitrogen sources. Together, these findings provide new insights into how Lrp enables Pss to respond to nutrient availability in the maize xylem environment.