<i>Arabidopsis</i> bHLH60 can regulate leaf development by interacting with and counteracting the AS1-AS2 complex to affect <i>BP1</i> expression

During leaf morphogenesis, various factors interplay to mediate abaxial-adaxial and proximal-distal polarity, along with other factors contributing to organ boundary and leaf expansion. Although significant progress has been made in understanding the genetics of leaf development, there are still gaps in our understanding of leaf morphogenesis. Here, we show that the bHLH60 transcription factor can affect leaf development. Overexpression of bHLH60 leads to pleiotropic phenotypes with increased leaf serration and reduced fertility. A RNA-seq analysis showed that the BREVIPEDICELLUS1 (BP1) gene implicated in leaf development was upregulated as a result of bHLH60 overexpression. Further analysis revealed that bHLH60 directly bound to the BP1 promoter to activate its transcription. Subsequently, bHLH60 interacted with the repressor complex AS1-AS2 and JLO to relieve the inhibition of AS1-AS2 on BP1, thereby indirectly activating the expression of BP1. Genetic analysis indicated that the BP1 mutation partially suppressed the phenotype resulting from bHLH60 overexpression, thereby concluding that the pleiotropic phenotypes of bHLH60-overexpressing plants were partly dependent on BP1. We propose that bHLH60 competes with the AS1-AS2-JLO complex to regulate BP1 expression to modulate leaf development.