Characterization of the Molecular Events Underlying the Establishment of Axillary Meristem Region in Pepper

Abstract

Plant architecture is a major motif of plant diversity, and shoot branching patterns primarily determine the aerial architecture of plants. In this study, we identified an inbred pepper line with fewer lateral branches, 20C1734, which was free of lateral branches at the middle and upper nodes of the main stem with smooth and flat leaf axils. Successive leaf axil sections confirmed that in normal pepper plants, for either node n, Pn (Primordium n) < 1 cm and Pn+1 < 1 cm were the critical periods between the identification of axillary meristems and the establishment of the region, whereas Pn+3 < 1 cm was fully developed and formed a completely new organ. In 20C1734, the normal axillary meristematic tissue region establishment and meristematic cell identity confirmation could not be performed on the axils without axillary buds. Comparative transcriptome analysis revealed that “auxin-activated signaling pathway”, “response to auxin”, “response to abscisic acid”, “auxin biosynthetic process”, and the biosynthesis of the terms/pathways, such as “secondary metabolites”, were differentially enriched in different types of leaf axils at critical periods of axillary meristem development. The accuracy of RNA-seq was verified using RT-PCR for some genes in the pathway. Several differentially expressed genes (DEGs) related to endogenous phytohormones were targeted, including several genes of the PINs family. The endogenous hormone assay showed extremely high levels of IAA and ABA in leaf axils without axillary buds. ABA content in particular was unusually high. At the same time, there is no regular change in IAA level in this type of leaf axils (normal leaf axils will be accompanied by AM formation and IAA content will be low). Based on this, we speculated that the contents of endogenous hormones IAA and ABA in 20C1734 plant increased sharply, which led to the abnormal expression of genes in related pathways, which affected the formation of Ams in leaf axils in the middle and late vegetative growth period, and finally, nodes without axillary buds and side branches appeared.

All-Year High IAA and ABA Contents in Rhizome Buds May Contribute to Natural Four-Season Shooting in Woody Bamboo Cephalostachyum pingbianense

“…Various hormones, such as auxin/IAA, cytokinin (CTK), gibberellin (GA), and abscisic acid (ABA), are involved in the process of lateral bud germination [1,5]. For example, ABA can affect the expression of key transcription factors that maintain bud dormancy [6,7], while IAA [8][9][10], CTK [11,12], and GA [13] cooperatively promote germination and development of lateral buds in multiple plants.…”

“…Both types start shooting soon after the beginning of the rainy season [15,16]. Therefore, empirically, the rhizome buds germination of woody bamboos with a single shooting peak is generally considered to be induced by the regulation of endogenous hormone synthesis and transport following an increase in soil temperature and precipitation in the rainy season [8]. The previous studies indicated the mass fractions of various phytohormones fluctuated regularly during the shooting period, implying that antagonism or coordination of endogenous hormones, such as IAA, GA, and ABA, were key signaling factors for bamboo shoot initiation [17][18][19].…”