Sugars not only serve as energy and cellular carbon skeleton but also function as signaling molecules regulating growth and development in plants. Understanding the molecular mechanisms in sugar signaling pathways will provide more information for improving plant growth and development. Here we describe a sugar hypersensitive recessive mutant tang1. Light-grown tang1 mutants have short roots, increased starch and anthocyanin content as well as reduced chlorophyll content when grown on high sugar concentration media. Dark-grown tang1 plants exhibit sugar hypersensitive hypocotyl elongation and enhanced dark development. The tang1 mutants also show an enhanced response to ABA but reduced response to ethylene. Thus, tang1 displays a range of alterations in sugar signaling related responses. The TANG1 gene was isolated by a map-based cloning approach, and encodes a previously uncharacterized novel protein with a predicted Symplekin tight junction protein C-terminal (Symplekin_C). Expression analysis indicates that TANG1 is ubiquitously expressed at moderate levels in different organs and throughout the Arabidopsis life cycle, however, its expression is not affected by high sugar treatment. Genetic analysis shows that PRL1 and TANG1 have additive effects on sugar related responses. Furthermore, the mutation of TANG1 does not affect the expression of genes involved in known sugar signaling pathways. Taken together, these results suggest that TANG1, a novel gene, plays an important role in sugar responses in Arabidopsis.