By: Helen Beilinson
Juliet famously declared her love for Romeo arguing that for her, no meaning lies behind his Montague name, and that she loves him regardless because “a rose by any other name would smell just as sweet”. Although the backstory of this phrase is quite romantic, the science behind it somewhat dims the romance. In fact, roses with different (scientific) names don’t all smell just as sweet. Of note, you might have noticed that most roses you can buy have essentially no scent. This phenomenon has been mostly attributed to the fact that rose cultivars have been selected for their color and longevity once cut.
Color, longevity, and scent are all traits that are controlled by genes. Such genes can either act independently (such that their inheritance is unconnected to the inheritance of other such traits) or can be linked (such that particular traits are inherited together). It’s still unknown why the three aforementioned genes have been selected for in such a pattern, but authors of a recent Science report have made strides in understanding the gene that controls the scent trait in the hopes that it can be genetically returned to those under-scented cultivars that are sold in florists’ shops.
Just as humans sometimes use flowers to attract their mates, flowers use scents to attract pollinators, which are necessary intermediates for plant sexual reproduction (for obvious mobility reasons). There is an entire field devoted to studying flower (and other) scents, called aroma chemistry. Aroma chemistry is incredibly complex. Two important take away points from the field, however, are that first, most floral compounds are aromatic, meaning they contain planar circular components and are volatile (meaning that it easily evaporates) because they need to get from the plant to whomever is smelling them. Second, every floral scent is unique to its particular flower species and is made up of multiple such aromatic compounds. Most rose scents are made of different mixtures of two kinds of aromatic compounds: monoterpene alcohols and 2-phenylethanol. The biochemical pathway of 2-phenylehtanol synthesis is known, but that of monoterpene synthesis in plants was not known. Reinstituting scents in roses that have lost them requires knowing what genes need to be replaced. That is why this group’s goal was to identify what this enzyme (or enzymes) is in roses.
To find the enzyme, the authors compared the genes that are expressed in two different species of roses: the Papa Meilland (PM) cultivar, which produces a heavy rose scent and thus the most amount of aromatic compounds, and the Rouge Meillant (RM) cultivar, which produces minimal scent. They wanted to identify a candidate gene that was highly expressed in the heavy scented rose (PM) and minimally expressed in the non-scented rose. They identified a candidate gene they named RhNUDX1. RhNUDX1 is a Nudix hydrolase, a family of proteins that use water molecules to break their substrate (the molecule for which they are specific) into two. They found that RhNUDX1 is expressed exclusively in petals of the PM cultivar, which is where the aromatic compounds are expected to be expressed, and is most highly expressed during their third stage of growth, a period when maximum scent production occurs. In the RM cultivar, RhNUDX1 expression is minimal in all parts of the plant.
The authors then wanted to verify that this gene’s expression correlated with scent production in various other roses to make sure this wasn’t specific for the two initially studied roses. To do this, they did a survey of 10 different cultivars with different scent potencies. They found that scent intensity directly correlated with RhNUDX1 expression, providing more evidence that this gene was the gene candidate they were indeed looking for.
To directly test whether RhNUDX1 levels directly influence scent and monoterpene production, the authors manipulated the level of RhNUDX1 expression in another heavily scented cultivar, the Old Blush (OB). They found that monoterpene production was impaired following when RhNUDX1 expression was reduced, whereas the level of other aromatic compounds was unaffected. Unfortunately, the authors did not transfer the RhNUDX1 gene into a rose cultivar that had a low level of scent to confirm that the presence of the gene was sufficient to synthesize the monoterpenes. However, the coorelative studies, in addition to showing that RhNUDX1 levels in the OB cultivars is linked to monoterpene levels, show fairly conclusively that this gene is heavily involved in scent production in roses, specifically in monoterpene synthesis.
Although monoterpenes are common aromatic compounds amongst plants, the pathway involving RhNUDX1 in roses is novel. This discovery adds to an increasing line of evidence that shows that although many plants produce similar, or even exactly the same, scent compounds, they independently evolved the proteins needed to do this, pointing at the great importance of having a scent in plant life. This discovery may also mean that readily available roses will soon smell much sweeter, and that we could potentially manipulate how our roses smell.