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 epiderma l cells of higher plants, we have identified the nucleotide sequence o f a cDNA designated Itp 7a2b encoding a novel nonspecific lipid transf er 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 putat ive signal peptide, characteristic for targeting to the secretory path way. Analysis of mRNA levels by Northern blotting indicated that ltp 7 a2b is preferentially expressed in the leaf epidermis. Levels of mRNA decreased during ageing of leaf tissue. Expression of itp 7a2b was sti mulated by a factor of 2-3 when the seedlings were grown in the presen ce of cadmium (10-1600 mu M). Concomitantly, the primary leaves of Cd- exposed seedlings contained elevated levels of abscisic acid and a thi cker 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 co ntent, ltp 7a2b mRNA and wax coverage was either not seen, or seen muc h less, in Ni-and Zn-stressed seedlings. The data add circumstantial e vidence to the recently proposed hypothesis that nonspecific lipid tra nsfer proteins function in transfer of cutin and/or walt monomers from the site of synthesis in the cell to the cuticle.