How Grapes, Oranges, and Corn Create a Sweet-Smelling Compound

Jenn Hoskins
9th July, 2024

How Grapes, Oranges, and Corn Create a Sweet-Smelling Compound

This diagram illustrates the metabolic pathways of anthranilate, highlighting the direct one-step methylation to produce methyl anthranilate by enzymes from sweet orange (Citrus sinensis), grapes (Vitis spp.), and maize (Zea mays) that this study investigates, in contrast to the previously proposed two-step pathway.

Image adapted from: Fallon et al. / CC BY (Source)

Key Findings

  • The study from Williams College focused on the biosynthesis of methyl anthranilate (MeAA) in grapes
  • Researchers discovered that grapes might use a one-step pathway for MeAA synthesis, contrary to the previously believed two-step process
  • The study identified specific enzymes and mutations that enhance the activity of these enzymes, providing insights into the molecular mechanisms of MeAA production
Plants produce a variety of volatile compounds that play crucial roles in their interactions with the environment, such as attracting pollinators, deterring herbivores, and communicating with other plants. One such compound is methyl anthranilate (MeAA), known for its role in grape aroma and its anti-herbivory properties. This volatile is emitted by several agriculturally significant plants, including citrus, grapes, and maize. Recent research from Williams College has shed light on the biosynthesis pathway of MeAA in grapes, challenging previous assumptions and uncovering new enzymatic mechanisms[1]. Previous studies have provided insights into the volatile compounds of citrus flowers, revealing that different citrus cultivars and blooming stages exhibit unique volatile profiles[2]. These studies highlighted the complexity and variability of plant volatile synthesis and suggested that flower volatiles could serve as markers for genetic relationships among citrus cultivars. Similarly, enzymes involved in secondary metabolism, such as those producing plant volatiles, often exhibit moderate catalytic efficiencies compared to enzymes in central metabolism, influenced by evolutionary selection pressures and physicochemical constraints[3]. Furthermore, research on the SABATH gene family of methyltransferases (MTs) has shown how positive selection can drive functional divergence in enzymes that produce key volatile compounds like methyl salicylate, important for plant defense and pollinator attraction[4]. In this context, the new study from Williams College investigated the biosynthesis of MeAA in grapes, traditionally thought to involve a two-step pathway. By analyzing transcriptomics data, the researchers identified two anthranilate methyltransferases (AAMTs) in Vitis vinifera (wine grape) and one ortholog in "Concord" grape. This discovery suggests that, contrary to previous beliefs, grapes may use a one-step pathway for MeAA synthesis, similar to maize. To further understand the molecular basis of AAMT activity, the researchers examined the Citrus sinensis (sweet orange) SA methyltransferase, known to methylate both anthranilate (AA) and salicylic acid (SA). By introducing specific mutations, they identified several active site residues that enhance activity with AA. Conversely, they introduced mutations in the maize AAMT to impart activity with SA, revealing different active site residues from those in the citrus enzyme. This approach demonstrated the molecular mechanisms underpinning AA activity across various methyltransferases. Phylogenetic analysis indicated that one of the Vitis AAMTs shares an ancestor with jasmonic acid methyltransferases, similar to the AAMT from strawberry (Frageria sp.). This evolutionary insight aligns with previous findings that enzymes involved in secondary metabolism, such as those producing plant volatiles, are subject to different evolutionary pressures compared to enzymes in central metabolism[3]. The identification of one-step enzymes for MeAA synthesis in grapes not only challenges the two-step pathway hypothesis but also highlights the evolutionary adaptability of plant enzymes to optimize specific functions. Collectively, these findings from Williams College advance our understanding of the enzymatic and molecular basis of volatile compound biosynthesis in plants. They reveal the potential for more efficient production of key volatiles like MeAA through evolutionary and biochemical adaptations, offering new avenues for enhancing plant defense and aroma in agricultural practices. By integrating insights from previous studies on plant volatiles[2], enzyme kinetics[3], and methyltransferase evolution[4], this research provides a comprehensive view of the complex interplay between plant biochemistry and evolution in shaping ecological interactions.

FruitsBiochemPlant Science

References

Main Study

1) Molecular basis of one-step methyl anthranilate biosynthesis in grapes, sweet orange, and maize.

Published 8th July, 2024

https://doi.org/10.1111/tpj.16922

Related Studies

2) Comparative analysis of flower volatiles from nine citrus at three blooming stages.

https://doi.org/10.3390/ijms141122346

3) The moderately efficient enzyme: evolutionary and physicochemical trends shaping enzyme parameters.

https://doi.org/10.1021/bi2002289

4) Positive selection for single amino acid change promotes substrate discrimination of a plant volatile-producing enzyme.

Journal: Molecular biology and evolution, Issue: Vol 24, Issue 6, Jun 2007


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