Endoxyloglucan transferase cDNA isolated from pea roots and its fluctuating expression in hydrotropically responding roots

We isolated an endoxyloglucan transferase cDNA (Ps-EXGT1) from the roots of an agravitropic pea mutant, ageotropum. The putative product of the cDNA w as 34.1 kDa and consisted of 293 amino acid residues. The predicted amino a cid sequence was 75.1-88.6% identical to those of EXGT genes in other plant s. The Ps-EXGT1 cDNA was strongly expressed in elongating roots and stems b ut not in either mature stems or young leaves. In roots, the transcription level of Ps-EXGT1 was most abundant in the rapidly growing region. When roo t elongation was inhibited by a water stress, Ps-EXGT1 transcription was re pressed. The roots curved hydrotropically due to differential growth of the cortical cells in the elongation zone when the root cap was exposed to a g radient of water potential; the length of the cells on the side of lower wa ter potential was much longer than those on the side of higher water potent ial. The expression pattern of Ps-EXGT1 in the hydrotropically responding r oots fluctuated between the side of the higher water potential and that of the lower water potential in the elongation zone. In other words, the accum ulation of Ps-EXGT1 mRNA was much greater on the side of lower water potent ial than on that of higher potential just prior to the commencement of posi tive hydrotropism. When the roots started to curve slightly away from the s ide of higher water potential causing a rhythmic oscillatory movement [Taka no et al, (1995) Planta 197: 410], there was more transcription of Ps-EXGT1 on the side of higher water potential. These results suggest that the tran scription of Ps-EXGT1 is involved in cell growth and that this regulation o f transcription plays a role in the differential growth of hydrotropically responding roots.