INTRODUCTION
Plant lateral root (LR) development is regulated by both intrinsic and environmental factors. In model plant Arabidopsis , the LR develops along the primary root axis, this process begins when a patch of xylem pole-pericycle (XPP) cells are specified as LR founder cells (LRFCs), and activated to undergo several anticlinal and periclinal divisions to form a dome-shaped LRP, which finally grows outward and emerges from the primary root (De Rybel et al., 2010; Ive De Smet et al., 2008; Dubrovsky et al., 2008; Fernandez et al., 2015; Malamy & Benfey, 1997; J. E. Vermeer et al., 2014). Phytohormone auxin plays a critical role in regulating the whole LR development stages (De Rybel et al., 2010; I. De Smet et al., 2007; Du & Scheres, 2018; Dubrovsky et al., 2008; Lee, Cho, & Kim, 2015). Recent studies usingpDR5:Luciferase reporter line proved that a periodic gene oscillation triggers the repetitive prebranch site formation, and the whole region of rhythmically pulsed expression ofDR5:Luciferase is designated as oscillation zone (OZ) (Moreno-Risueno et al., 2010; Van Norman, Xuan, Beeckman, & Benfey, 2013). The oscillatory gene expression process is also described as the root clock (W. Xuan, De Gernier, & Beeckman, 2020), which can be characterized by amplitude and frequency of DR5:Luciferase expression in OZ. Researches further indicated that lateral root cap (LRC)-derived auxin is critical in the output of the root clock through modulating the oscillation amplitude to determine whether a prebranch site is created or not (Moller, Xuan, & Beeckman, 2017; W. Xuan et al., 2015; W. Xuan et al., 2016).
Environmental factors can affect different LR development stages (Motte, Vanneste, & Beeckman, 2019). Light, heavy metal cadmium, and water deficit has been recently reported to affect LR formation through modulating DR5:Luciferase expression in OZ and subsequent prebranch site formation (Kircher & Schopfer, 2018; Orman-Ligeza et al., 2018; Xie et al., 2019). Beneficial rhizosphere microorganisms can dramatically affect root development by facilitating root cell division and differentiation (Lopez-Bucio et al., 2007; Ortiz-Castro, Martinez-Trujillo, & Lopez-Bucio, 2008; Ortiz-Castro, Valencia-Cantero, & Lopez-Bucio, 2008; Patten & Glick, 2002; Zamioudis, Mastranesti, Dhonukshe, Blilou, & Pieterse, 2013; Zou, Li, & Yu, 2010; Zuniga et al., 2013). Pathogenic bacterium can also strongly induces LR formation (Kong et al., 2020). Moreover, a single bacterial genus (Variovorax ) can manipulate plant hormone levels via metabolic signal interference to maintain root growth in a complex microbiome (Finkel et al., 2020). Nonetheless, how those biotic factors regulate LR formation in spatiotemporal-molding machinery and how they interacted with the intrinsic mechanisms are still not clear.
Bacillus spp., an extracellular plant growth-promoting rhizobacteria (ePGPRs) species, could promote growth of several plant species (Gray & Smith, 2005; Martinez-Viveros, Jorquera, Crowley, Gajardo, & Mora, 2010). VCs produced by some Bacillus spp. affect plant growth and development (Meldau et al., 2013; Perez-Flores et al., 2017; Ryu et al., 2003). In this study, we explored the effects of VCs produced by seven different Bacillus spp. strains on plant root development and chose B. amyloliquefaciens SQR9 as the representative to investigate further the role of its VCs in regulating LR development by employing molecular genetics and pharmacological approaches. Our study provides a tentative model for understanding the PGPR manipulating mechanisms on LR formation regulation.