Nectary starch degradation affects nectar chemical composition, but not nectar sugars, in Arabidopsis thaliana

Attracting and rewarding pollinators are important for the reproduction of many flowering plants, and floral nectar plays a central role in plant-pollinator relationships as the primary reward. Nectar production by floral organs called nectaries coincides with a buildup and degradation of nectary starch in many species. While this temporal connection might suggest that nectar sugars are produced from nectary starch, direct evidence to support this idea is lacking. Here, we performed genetic manipulations to test how nectary starch contributes to nectar production in Arabidopsis (Arabidopsis thaliana). Additionally, we conducted semi-targeted metabolomics experiments to identify which nectar compounds (NCs) depend on nectary starch for their production and secretion. While nectar sugar levels were not consistently lower in plants defective in nectary starch metabolism, mutants producing more nectary starch consistently produced less nectar sugar. We also detected a number of differentially accumulated NCs connected to biotic stress in starch-related mutants, including salicylic acid. Our results suggest that, in Arabidopsis, nectary starch is not required as a carbohydrate source to produce nectar sugars per se, but nectary starch metabolism is important for the production and secretion of specialized NCs, which may help nectaries respond to stress. NCs identified from our metabolomics experiment provide the foundation for further investigations into the functional and physiological importance of nectary starch in nectar and floral biology.