LATERAL BRANCHING OXIDOREDUCTASE, One Novel Target Gene of Squamosa Promoter Binding Protein-like 2, Regulates Tillering in Switchgrass

Strigolactones (SLs) play a critical role in regulating plant tiller number. LATERAL BRANCHING OXIDOREDUCTASE (LBO) encodes an important late-acting enzyme for SL biosynthesis and regulates shoot branching in Arabidopsis. However, little is known about the function of LBO in monocots including switchgrass (Panicum virgatum L.), a dual-purpose fodder and biofuel crop. We studied the function of PvLBO via the genetic manipulation of its expression levels in both the wild-type and miR156 overexpressing (miR156 ) switchgrass. Co-expression analysis, quantitative real-time polymerase chain reaction (qRT-PCR), transient dual luciferase assay, and chromatin immunoprecipitation-qPCR were all used to determine the activation of PvLBO by miR156-targeted Squamosa Promoter Binding Protein-like 2 (PvSPL2) in regulating tillering of switchgrass. PvLBOtranscripts dramatically declined in miR156 transgenic switchgrass, and the overexpression of PvLBO in the miR156 transgenic line produce fewer tillers than the control. Furthermore, we found that PvSPL2 can directly bind to the promoter of PvLBO and activate its transcription, suggesting that PvLBO is a novel downstream gene of PvSPL2. We propose that PvLBO functions as an SL biosynthetic gene to mediate tillering and acts as an important downstream factor in the crosstalk between the SL biosynthetic pathway and the miR156-SPL module in switchgrass.