Cadmium leads to stimulated expression of the lipid transfer protein genes in barley: implications for the involvement of lipid transfer proteins in wax assembly

In order to investigate the nature of genes expressed in leaf epidermal cells of higher plants, we have identified the nucleotide sequence of a cDNA designated Itp 7a2b encoding a novel nonspecific lipid transfer protein of barley (Hordeum vulgare L. cv. Gerbel). The cDNA of 755 basepairs contains an open reading frame of 366 nucleotides coding for a 12.3-kDa polypeptide. The first 29 amino acids constitute the putative signal peptide, characteristic for targeting to the secretory pathway. Analysis of mRNA levels by Northern blotting indicated that ltp 7a2b is preferentially expressed in the leaf epidermis. Levels of mRNA decreased during ageing of leaf tissue. Expression of itp 7a2b was stimulated by a factor of 2-3 when the seedlings were grown in the presence of cadmium (10-1600 mu M). Concomitantly, the primary leaves of Cd-exposed seedlings contained elevated levels of abscisic acid and a thicker wax layer of the cuticle. At 100 mu M Cd in the hydroponic medium, the wax cover was increased by 50%. The increase in abscisic acid content, ltp 7a2b mRNA and wax coverage was either not seen, or seen much less, in Ni-and Zn-stressed seedlings. The data add circumstantial evidence to the recently proposed hypothesis that nonspecific lipid transfer proteins function in transfer of cutin and/or walt monomers from the site of synthesis in the cell to the cuticle.