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Summary: 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 (miR156OE) 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 miR156OE transgenic switchgrass, and the overexpression of PvLBO in the miR156OE 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. [ABSTRACT FROM AUTHOR]

Grasses possess basal and aerial axillary buds. Previous studies have largely focused on basal bud (tiller) formation but scarcely touched on aerial buds, which may lead to aerial branch development., Genotypes with and without aerial buds were identified in switchgrass ( Panicum virgatum), a dedicated bioenergy crop. Bud development was characterized using scanning electron microscopy. Microarray, RNA-seq and quantitative reverse transcription polymerase chain reaction ( RT-qPCR) were used to identify regulators of bud formation. Gene function was characterized by down-regulation and overexpression., Overexpression of miR156 induced aerial bud formation in switchgrass. Various analyses revealed that SQUAMOSA PROMOTER BINDING PROTEIN LIKE4 ( SPL4), one of the miR156 targets, directly regulated aerial axillary bud initiation. Down-regulation of SPL4 promoted aerial bud formation and increased basal buds, while overexpression of SPL4 seriously suppressed bud formation and tillering. RNA-seq and RT- qPCR identified potential downstream genes of SPL4., Unlike all previously reported genes acting as activators of basal bud initiation, SPL4 acts as a suppressor for the formation of both aerial and basal buds. The miR156- SPL4 module predominantly regulates aerial bud initiation and partially controls basal bud formation. Genetic manipulation of SPL4 led to altered plant architecture with increased branching, enhanced regrowth after cutting and improved biomass yield. [ABSTRACT FROM AUTHOR]