Gene structure and expression characteristics of the auxin receptor TIR1 ortholog in Momordica charantia and developmental analysis of its promoter in transgenic plants

Abstract

It is known that phytohormone auxin affects floral sexual determination in monoecious Cucurbitaceae such as cucumber and melon. TRANSPORT INHIBITOR RESPONSE 1 (TIR1) was first identified in Arabidopsis thaliana as the auxin receptor but few reports have correlated it with fruit initiation and development. As the first step to understanding the regulatory mechanism of auxin in flower sex expression, fruit set, and fruit development in bitter gourd, a TIR1 ortholog named McTIR1, was isolated and characterized in monoecious bitter gourd (Momordica charantia L.). The translation start codon locates in the second exon of McTIR1. Based on phylogenetic analysis, the similarity of deduced amino acid sequence was closer between the McTIR1 and AtTIR1 than between McTIR1 and AUXIN SIGNALING F-BOX (AFB) family. Northern analysis indicated McTIR1 expressed in the early fruit developmental stages of bitter gourd. The McTIR1 mRNA was most abundant in young leaves and stems and lowest in roots of bitter gourd. Expression of McTIR1 was further characterized using transcriptional fusion construct containing β-glucuronidase reporter gene fused to the gene promoter and 5′-untranslated region of McTIR1. The expression of McTIR1 promoter decreased dramatically during the late stages of pistil development in transgenic tobacco. These results indicate that expression of auxin receptor McTIR1 is associated with the pistil development of unisexual flowers in bitter gourd.