Insights into the hormonal regulation of nectar production and the biochemical characteristics of antimicrobial nectar proteins

Abstract

Nectar is a complex, carbohydrate rich solution that facilitates important plant-biotic relationships. Of the utmost importance is nectars ability to effectively attract and manipulate pollinators to maximize plant reproductive success while deterring unwanted consumers that could be deleterious to a plant’s fecundity. Therefore, the regulation of nectar secretion during times of peak pollinator activity, as well as other essential plant reproductive processes, is essential. Very little is known about the detailed mechanisms of nectar regulation and whether these mechanisms are conserved in the plant kingdom. Through the examination of null alleles of JA biosynthesis and response genes in Arabidopsis, this report finds that the octadecanoid pathway plays an important role in nectar production. The nectar-less JA synthesis mutant aos-2 showed no auxin response in nectaries, but both nectar production and the auxin response were restored upon exogenous JA and auxin treatment. Cumulatively, these observations strongly suggest an indispensable role for an octadecanoic acid- and auxin-dependent, but JA- and COI1-dispensible, pathway in regulating nectar production in Arabidopsis. Another important aspect of nectar biology is the chemical constituents of nectar droplets. While sugars are the predominant solutes of nectar, non-sugar solutes have been shown to provide additional levels of functionality to nectar. Some of these non-sugar solutes include amino acids, lipids, ions, secondary metabolites and proteins. Nectars generally secrete small arrays of proteins. Because nectar is a nutrient dense solution, it must be protected against non-mutualistic consumers such as deleterious communities of microorganisms. Nectar proteins generally exhibit direct and indirect antimicrobial activity and are thought to serve as a defense mechanism for nectar. To this end, I characterized non-specific lipid transfer proteins (nsLTPs) that are secreted into the nectar of Arabidopsis (AZI7), Cucurbita pepo (CpLTP1.1), and Brassica rapa (BrLTP2.1). Broadly, these nectar specific nsLTPs appear to have strong antimicrobial activity, particularly against pathogenic plant fungi, are very heat stable, and have some capacity to bind free fatty acids. Overall, these data improve our understanding of the hormonal regulation of nectar and takes the initial steps to gaining broad insight to the biological function of nectar proteins.