RNAi Suppression of Two AGAMOUS Homologs in Sweetgum (Liquidambar) Impairs Male and Female Reproductive Development Under Field Conditions

Many woody ornamentals are grown as exotics outside of their native range, where they have the potential to become invasive.  As street trees, their fruits often create sanitation problems and their pollen exacerbates allergies.  A reduction in fertility of these ornamental plants would help to mitigate these problems.  We used RNA interference (RNAi) to suppress the expression of a key floral development gene, AGAMOUS (AG) in sweetgum trees.  Sweetgum is a popular ornamental tree in the United States known for its vibrant fall foliage, however, it is also known for producing copious amounts of hard, rough fruits termed gum balls or burr balls.  Shed fruits are long-lasting and are an unwelcome nuisance on streets, roofs, and often clog gutters.  We used Agrobacterium to transform sweetgum variety ‘Worplesdon’ with a double-stranded RNAi-inducing transgene that targets the two distinct AG orthologs in sweetgum.  A total of 33 independent transgenic events, plus 12 non-transgenic controls, were planted in the field in 2006.  The RNAi-AG sweetgum trees maintained normal growth, phenology, and vivid fall coloration during 9 years of study.  We found that 8 events had highly-modified floral morphology, which failed to produce seeds or pollen, and had anthers and carpels that were converted to flat leaf-like structures with no ovules.  The female flowers from these events developed into dry papery fruits lacking seeds.  All of these traits were stable across multiple growing seasons.  RNAi against AG is highly effective at modifying fertility and reproductive development in sweetgum.