Coordinated and High-Level Expression of Biosynthetic Pathway Genes Is Responsible for the Production of a Major Floral Scent Compound Methyl Benzoate in Hedychium coronarium.

Yuechong Yue,Yunyi Yu,Yanping Fan,Rangcai Yu,Feng Chen,Lan Wang,Jieling He,Xinyue Li

doi:10.3389/fpls.2021.650582

Abstract

Methyl benzoate is a constituent of floral scent profile of many flowering plants. However, its biosynthesis, particularly in monocots, is scarcely reported. The monocot Hedychium coronarium is a popular ornamental plant in tropical and subtropical regions partly for its intense and inviting fragrance, which is mainly determined by methyl benzoate and monoterpenes. Interestingly, several related Hedychium species lack floral scent. Here, we studied the molecular mechanism of methyl benzoate biosynthesis in H. coronarium. The emission of methyl benzoate in H. coronarium was found to be flower-specific and developmentally regulated. As such, seven candidate genes associated with methyl benzoate biosynthesis were identified from flower transcriptome of H. coronarium and isolated. Among them, HcBSMT1 and HcBSMT2 were demonstrated to catalyze the methylation of benzoic acid and salicylic acid to form methyl benzoate and methyl salicylate, respectively. Methyl salicylate is a minor constituent of H. coronarium floral scent. Kinetic analysis revealed that HcBSMT2 exhibits a 16.6-fold lower Km value for benzoic acid than HcBSMT1, indicating its dominant role for floral methyl benzoate formation. The seven genes associated with methyl benzoate biosynthesis exhibited flower-specific or flower-preferential expression that was developmentally regulated. The gene expression and correlation analysis suggests that HcCNL and HcBSMT2 play critical roles in the regulation of methyl benzoate biosynthesis. Comparison of emission and gene expression among four Hedychium species suggested that coordinated and high-level expression of biosynthetic pathway genes is responsible for the massive emission of floral methyl benzoate in H. coronarium. Our results provide new insights into the molecular mechanism for methyl benzoate biosynthesis in monocots and identify useful molecular targets for genetic modification of scent-related traits in Hedychium.

Highlights

Floral scent is one of the most important traits for ornamental plants and cut flowers enhancing their esthetic and commercial values (Pichersky and Dudareva, 2007; Tholl and Gershenzon, 2015)
Its emission rates enhanced along with the flower senescence (56–64 h, Figure 2B). These results suggest that the release of floral methyl benzoate in H. coronarium was developmentally regulated
After infiltration of the substrate benzoic acid or salicylic acid into tobacco leaves, the leaves expressing HcBSMT1 or HcBSMT2 synthesized and released methyl benzoate or methyl salicylate, while control leaves harboring EGFP did not (Figure 4). These results demonstrated that HcBSMT1 and HcBSMT2 are benzoic acid/salicylic acid methyltransferases

Summary

Introduction

Floral scent is one of the most important traits for ornamental plants and cut flowers enhancing their esthetic and commercial values (Pichersky and Dudareva, 2007; Tholl and Gershenzon, 2015). Compared to non-scented ones, flowers with pleasant fragrance are more appealing to consumers (Pichersky and Dudareva, 2007; Ben Zvi et al, 2012). Many modern cultivars of ornamental plants, such as rose and chrysanthemum, in flower market lack flower fragrance, which has been attributed to the loss of scent trait in the breeding process or the absence of this trait in their ancestors (Brown, 2002). Potential pollinators can locate and select the host flowers through distinguishing complex floral scent mixtures (Raguso, 2008). Ample evidence supports the idea that benzenoids, such as methyl benzoate and methyl salicylate, serve as important attractants for pollinators, moths, which search and visit flowers at night (Hoballah et al, 2005; Kessler et al, 2013)

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Conclusion