Characterization of Glycosylphosphatidylinositol-anchored Lipid Transfer Protein 2 (LTPG2) and Overlapping Function Between LTPG/LTPG1 and LTPG2 in Cuticular Wax Export or Accumulation in Arabidopsis Thaliana

Cuticular waxes are synthesized by the extensive export of intracellular lipids from epidermal cells. However, it is still not known how hydrophobic cuticular lipids are exported to the plant surface through the hydrophilic cell wall. The LTPG2 gene was isolated based on Arabidopsis microarray analysis; this gene is predominantly expressed in stem epidermal peels as compared with in stems. The expression of LTPG2 transcripts was observed in various organs, including stem epidermis and silique walls. The composition of the cuticular wax was significantly altered in the stems and siliques of the ltpg2 mutant and ltpg1 ltpg2 double mutant. In particular, the reduced level of the C29 alkane, which is the major component of cuticular waxes in ltpg1 ltpg2 stems and siliques, was similar to the sum of reduced values of either parent. The total cuticular wax load was reduced by approximately 13% and 20% in both ltpg2 and ltpg1 ltpg2 siliques, respectively, and by approximately 14% in ltpg1 ltpg2 stems when compared with the wild-type. Similarly, severe alterations in the cuticular layer structure of epidermal cells of ltpg2 and ltpg1 ltpg2 stems and silique walls were observed. In tobacco epidermal cells, intracellular trafficking of the fluorescent LTPG/LTPG1 and LTPG2 to the plasma membrane was prevented by a dominant-negative mutant form of ADP-ribosylation factor 1, ARF1(T31N). Taken together, these results indicate that LTPG2 is functionally overlapped with LTPG/LTPG1 during cuticular wax export or accumulation and LTPG/LTPG1 and LTPG2 are targeted to the plasma membrane via the vesicular trafficking system.