The Arabidopsis vacuolar sugar transporter SWEET 2 limits carbon sequestration from roots and restricts Pythium infection

Abstract

Summary: Plant roots secrete a significant portion of their assimilated carbon into the rhizosphere. The putative sugar transporter SWEET 2 is highly expressed in Arabidopsis roots. Expression patterns of SWEET 2–β‐glucuronidase fusions confirmed that SWEET 2 accumulates highly in root cells and thus may contribute to sugar secretion, specifically from epidermal cells of the root apex. SWEET 2–green fluorescent protein fusions localized to the tonoplast, which engulfs the major sugar storage compartment. Functional analysis of SWEET 2 activity in yeast showed low uptake activity for the glucose analog 2‐deoxyglucose, consistent with a role in the transport of glucose across the tonoplast. Loss‐of‐function sweet2 mutants showed reduced tolerance to excess glucose, lower glucose accumulation in leaves, and 15–25% higher glucose‐derived carbon efflux from roots, suggesting that SWEET 2 has a role in preventing the loss of sugar from root tissue. SWEET2 root expression was induced more than 10‐fold during Pythium infection. Importantly, sweet2 mutants were more susceptible to the oomycete, showing impaired growth after infection. We propose that root‐expressed vacuolar SWEET 2 modulates sugar secretion, possibly by reducing the availability of glucose sequestered in the vacuole, thereby limiting carbon loss to the rhizosphere. Moreover, the reduced availability of sugar in the rhizosphere due to SWEET 2 activity contributes to resistance to Pythium.