A Sucrose Transporter and Proper Hormone Response are Essential for Nectary Function in the Brassicaceae

Abstract

Nectar is a reward presented by flowers to attract pollinators to facilitate fertilization. While much is known about the chemical make-up of nectar, little is known about the mechanisms of production and secretion of this pollinator attractant. SWEET9, a nectary enriched gene, was demonstrated to be vital for nectar production in two Brassicaceae species, Arabidopsis thaliana and Brassica rapa as determined by SWEET9pro::GUS histochemical staining and RT-PCR,. The Arabidopsis mutant atsweet9-3, produced no nectar and three independent mutants in B. rapa, (brsweet9-1, -2, and -3) similarly produced no nectar. All four mutants had normal nectary morphology. Transporter assays of SWEET9 expressed in Xenopus oocytes displayed sucrose uniport activity, suggesting a direct role in sugar export. To determine a potential mechanism for the regulation of SWEET9 expression, the plant hormone jasmonic acid (JA) was investigated because it was previously implicated in nectary function. Indeed, JA synthesis (aos-2 and dad1) and response (myb21-4) mutants displayed an absence of floral nectar, in addition to male-sterility. When treated with exogenous MeJA, aos-2 and dad1 mutants regained their nectar production and fertility, while the myb21-4 transcription factor mutant was insensitive to treatment. Significantly, SWEET9 expression was strongly decreased in the JA response mutant myb21-4, in addition to several other genes known to be important in nectary function. For example, all three JA mutants studied displayed decreased expression of PIN6, a nectary enriched gene required for proper auxin homeostasis in the nectaries of Arabidopsis. Additionally auxin response was lost in the JA synthesis mutant aos-2, suggesting an important hormonal crosstalk between JA and auxin. To further investigate the link between JA and the auxin response in nectaries, mutants with altered endogenous auxin levels were created. Mutants with decreased nectary auxin produced 50% less nectar than wild-type plants and had reduced auxin response. Cumulatively, these results identify SWEET9 as a sucrose transporter required for nectar production and that JA plays a major role in the regulation of nectary-specific genes and other hormonal pathways important for nectar production.