Mechanistic analysis of the hardening process of the thorns on stems of Bougainvillea spectabilis Willdenow

Abstract Background Bougainvillea spectabilis Willdenow is thorny woody vine or shrub. The rigidity of the thorns on the stems should be considered as a horticultural character. In order to find the genes and pathways related to the hardening process of the thorns on the stems of B . spectabilis , the eukaryotic unreferenced transcriptome sequencing analysis is applied to explore the 3 stages of the thorns hardening process. Results This study investigates the transcriptomic changes in B . spectabilis plants during the process of thorns hardening. 3 developmental stages from thorns formation (stage 1) to thorns hardening (stage 2 to stage 3) were examined. Total RNA was extracted from thorns and stems, and transcriptome libraries were constructed and sequenced using unreferenced Illumina sequencing. Gene function annotation was performed using various databases, resulting in 8937 co-annotated genes. The density distribution of Fragments Per Kilobase of transcript per Million mapped reads (FPKM) depicted the overall gene expression patterns. Gene expression correlation analysis confirmed the reliability of the experiment, showing strong similarity among biological replicates. Differential expression analysis revealed that during thorns hardening, 1045 genes significantly up-regulated in thorns and 918 in stems at stage 2 compared to thorns formation (stage 1). At stage 3, as thorns became harder, 98 genes exhibited notable expression increase within thorns, and 46 genes up-regulated in stems, compared to stage 2. These findings highlight stage 2 as the period of highest gene expression activity during the thorns hardening process in B . spectabilis . Phenylpropanoid biosynthesis is a key step in the hardening process of the thorns of B . spectabilis . This transcriptome analysis offers insights into the molecular mechanisms underlying thorns development in this plant species. Conclusion The formation and hardening of thorns on the stem of B . spectabilis is a process in which lignin gradually accumulates in the thorns, and several genes are involved in the process. The phenylalanine ammonia-lyase, trans-cinnamate 4-monooxygenase, reductase4-coumarate-CoA ligase, cinnamoyl-CoA reductase, cinnamyl-alcohol dehydrogenase and peroxidase are the key genes for lignin synthesis and accumulation. The process involves the pathways-phenylpropanoid biosynthesis.

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