Exploring linear and cyclic (DI)-nucleotides as messengers for regulation of T3SS and biofilm formation in erwinia amylovora

The hypersensitive response and pathogenicity (hrp)-type III secretion system (T3SS) is a key pathogenicity factor in Erwinia amylovora, the causal agent of fire blight disease, which has recently reached Central/East Asia, the native origin of apple germplasm resources. It is well understood that the hrp-T3SS in E. amylovora is transcrip-tionally regulated by the master regulator HrpL, an ECF alternative sigma factor. Recent genetic and biochemical evidence has further demonstrated that transcription of hrpL is positively regulated by alternative sigma factor 54 (RpoN), its modulation protein (YhbH), a σ⁵⁴ -enhancer binding protein (HrpS), and integration host factor (IHF). However, the bacterial/host signals that activate the sigma factor cascade remain elusive. Biofilm formation also plays an important role in E. amylovora virulence and is the main contributing factor to population growth in xylem. Three exopolysaccharides, amylovoran, levan, and cellulose, are required for biofilm formation, and amylovoran and levan also contribute to virulence in other aspects of plant infection. In this short review, we summarized recent findings on the regulation of the T3SS and biofilm formation in E. amylovora by bacterial secondary messengers, including both linear and cyclic nucleotides. As an internal “switch”, the linear nucleotide second messengers (p)ppGpp mediates a stringent response under nutrient stress, which then activates RpoN-dependent regulation of the hrpL gene. Furthermore, cyclic di-GMP has been elucidated as a positive regulator of both amylovoran and cellulose biosynthesis and as a negative regulator of the T3SS. We present models for the T3SS and biofilm regulatory pathways in E. amylovora and discuss potential future research directions.