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.