Inactivation of Arabidopsis SIP1 leads to reduced levels of sugars and drought tolerance

Sugars play many roles in land plants including signaling, transport, and environmental adaptation. Plants use carbohydrate based strategies to adapt to water or temperature stress. The SIP1 gene encodes a protein of unknown function that has 81 percnt; similarity to a putative Cucumis raffinose synthase (EC 2.4.1.82). Arabidopsis plants homozygous for an insertion in the SIP1 coding region have no SIP1 mRNA and reduced levels of sucrose, mannitol, and verbascose. These sip1 mutant plants are resistant to levels of raffinose, a precursor of verbascose, that are normally toxic to wild type plants. These results suggest that the SIP1 gene product is responsible for verbascose synthesis rather than raffinose synthase. sip1 plants also have an increased tolerance to drought, indicating that a single loss of function mutation that leads to changes in relative carbohydrate levels can influence dependence on water for survival.