Boron(B)is an essential micronutrient for vascular plant growth.Both B deficiency and toxicity can impair tissue development in diverse plant species,but little is known about the effect of B on reproduc-tive panicle development and grain yield.In this study,a mutant of Setaria italica exhibiting necrotic pan-icle apices was identified and designated as sibor1.Sequencing revealed a candidate gene,SiBOR1,with a G-to-A alteration at the seventh exon.Knockout transgenic lines generated by clustered regularly inter-spaced short palindromic repeats and their associated protein-9 also had necrotic panicles,verifying the function of SiBOR1.SiBOR1 encoded a membrane-localized B efflux transporter,co-orthologous to the rice BOR1 protein.SiBOR1 was dominantly expressed in panicles and displayed a distinct expression pattern from those of its orthologs in other species.The induced mutation in SiBOR1 caused a reduction in the B content of panicle primary branches,and B deficiency-associated phenotypes such as thicker cell walls and higher cell porosity compared with Yugu 1.Transcriptome analysis indicated that differentially expressed genes involved in cell wall biogenesis,jasmonic acid synthesis,and programmed cell death response pathways were enriched in sibor1.qPCR analysis identified several key genes,including pheny-lalanine ammonia-lyase(SiPAL)and jasmonate-ZIM-domain(SiJAZ)genes,responsive to B-deficient con-ditions.These results indicate that SiBOR1 helps to regulate panicle primary branch development to maintain grain yield in S.italica.Our findings shed light on molecular mechanisms underlying the rela-tionship between B transport and plant development in S.italica.