| Summary: | Flower colour and colour patterns are crucial traits for ornamental species; thus, a comprehensive understanding of their genetic basis is extremely significant for plant breeders. The tulip tree (<i>Liriodendron tulipifera</i> Linn.) is well known for its flowers, odd leave shape and tree form. However, the genetic basis of its colour inheritance remains unknown. In this study, a putative plastid terminal oxidase gene <i>(LtuPTOX)</i> was identified from <i>L. tulipifera</i> based on multiple databases of differentially expressed genes at various developmental stages. Then, the full-length cDNA of <i>LtuPTOX</i> was derived from tepals and leaves using RACE (rapid amplification of cDNA ends) approaches. Furthermore, gene structure and phylogenetic analyses of PTOX as well as AOXs (alternative oxidases), another highly similar homologue in the AOX family, were used to distinguish between the two subfamilies of genes. In addition, transient transformation and qPCR methods were used to determine the subcellular localization and tissue expression pattern of the <i>LtuPTOX</i> gene. Moreover, the expression of <i>LtuPTOX</i> as well as pigment contents was investigated to illustrate the function of this gene during the formation of orange bands on petals. The results showed that the <i>LtuPTOX</i> gene encodes a 358-aa protein that contains a complete AOX domain (PF01786). Accordingly, the <i>Liriodendron</i> <i>PTOX</i> and <i>AOX</i> genes were identified as only paralogs since they were rather similar in sequence. LtuPTOX showed chloroplast localization and was expressed in coloured organs such as petals and leaves. Additionally, an increasing pattern of <i>LtuPTOX</i> transcripts leads to carotenoid accumulation on the orange-band during flower bud development. Taken together, our results suggest that <i>LtuPTOX</i> is involved in petal carotenoid metabolism and orange band formation in <i>L. tulipifera</i>. The identification of this potentially involved gene will lay a foundation for further uncovering the genetic basis of flower colour in <i>L. tulipifera</i>.
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