Impairment in a Member of AP2/ERF and F-box Family Protein Enhances Complete Panicle Exsertion in Rice

Complete panicle exsertion (CPE) is an economically important quantitative trait that contributes to grain yield in rice. We deployed an integrated approach for understanding the molecular mechanism of CPE using a stable ethyl methanesulfonate mutant line, CPE-109 of the Samba Mahsuri (SM) variety of rice (Oryza sativa), which exhibits CPE. Two consistent genomic regions were identified for CPE through quantitative trait locus (QTL) mapping [qCPE-4 (28.24-31.22 Mb) and qCPE-12 (2.30-3.18 Mb)] and QTL-sequencing [chr 4 (31.21-33.69 Mb) and chr 12 (0.12-3.15 Mb)]. Two non-synonymous single nucleotide polymorphisms, namely KASP 12-12 (T -> C; chr12:1269983) in Os12g0126300, encoding an AP2/ERF transcription factor, and KASP 12-16 (G -> A; chr12:1515198) in Os12g0131400, encoding an F-box domain-containing protein, explained 81.05% and 59.61% of the phenotypic variance, respectively, and exhibited strong co-segregation with CPE in F2 mapping populations, advanced generation lines, and CPE-exhibiting SM mutants through KASP assays. Down-regulation of these genes in CPE-109 compared with SM (wild type) was observed in transcriptome sequencing of flag leaves, which was validated through qRT-PCR. We propose that the abrogation of Os12g0126300 and Os12g0131400 in CPE-109 combinatorially influences down-regulation of ethylene biosynthetic genes, Os01g0192900 (ACC synthase) and Os05g0497300 (ethylene-responsive factor-2), and up-regulation of a gibberellic acid synthetic gene, Os06g0569900 (ent-kaurene synthase) and the two cytokinin biosynthetic genes Os07g0486700 (cytokinin-O-glucosyltransferase 2) and Os10g0479500 (similar to carboxy-lyase), which results in complete panicle exsertion. Complete panicle exsertion in CPE-109 rice resulted from a missense mutation (T -> C) in the sixth exon of the gene for AP2/ERF and a mutation (G -> A) at the third exon of a gene encoding an F-box family protein.