A NEW CDNA-ENCODING A XYLOGLUCAN ENDO-TRANSGLYCOSYLASE-RELATED POLYPEPTIDE (ATXTR8) PREFERENTIALLY EXPRESSED IN SEEDLING, ROOT AND STEM OF ARABIDOPSIS-THALIANA

A new cDNA clone (AtXTR8) encoding a xyloglucan endo-transglycosylase (XET)-related protein was isolated from an Arabidopsis thaliana germin ating seed cDNA library, using a differential screen. The predicted am ino acid sequence displays only 38 to 42% amino acid identity with oth er members of the Arabidopsis XTR (XET-related) protein family but sho ws 64% identity to a xyloglucan endo-transglycosylase enzyme (TmNXG1) expressed in nasturtium (Tropaeolum majus) germinating seeds. The AtXT R8 transcipts are expressed in germinating seeds as early as 24 h afte r imbibition with highest level of accumulation reached at about 72 h. In adult plants, transcripts are mainly detected in roots and to a lo wer extent in inflorescence stems. During imbibition of the severely g ibberellin-deficient gal-3 mutant seeds, transcripts are detected only if exogenous gibberellins are supplied in the medium, suggesting that gibberellins are directly or indirectly required for AtXTR8 gene expr ession. Unlike other XET genes such as TCH4, EXT and XTR3 which are ra pidly activated by auxins in wild type Arabidopsis, the steady state l evel of AtXTR8 transcripts is not markedly increased by auxin treatmen t of 6-day-old seedlings. Phylogenetic analysis confirms that the XET and XTR protein sequences so far identified in various plant species s plit in three groups and Arabidopsis AtXTR8, which enzymatic activity has not been assessed, belongs to the least related group III. Copyrig ht (C) 1996 Elsevier Science Ireland Ltd