A MOLECULAR MARKER FOR LATERAL ROOT INITIATION - THE RSI-1 GENE OF TOMATO (LYCOPERSICON-ESCULENTUM MILL) IS ACTIVATED IN EARLY LATERAL ROOTPRIMORDIA

The frequency of lateral root initiation in tomato (Lycopersicon escul entum Mill cv. VFN8) seedling roots is increased over eightfold in res ponse to 1.6 mu M alpha-naphthaleneacetic acid (NAA). To identify gene s that are activated during lateral root initiation, a cDNA library wa s made with RNA from roots treated with auxin and differentially scree ned with radioactive probes made from RNA isolated from treated and un treated roots. A cDNA clone, TR132, was identified that hybridized to a transcript that was induced within 4 h of auxin treatment and increa sed tenfold by 72 h. A gene (RSI-1) corresponding to the TR132 cDNA wa s cloned and characterized with regard to its nucleotide sequence, tra nscription start site and chromosomal map position. Approximately 1 kb of the 5' flanking DNA was linked to the beta-glucuronidase (GUS) pro tein coding region and tested for expression in transgenic tomato seed lings. GUS activity was observed in both lateral and adventitious root initials, including very early initials, and persisted until shortly after the lateral emerged from the parent tissue. In roots from seedli ngs with high activity, GUS expression was also observed in the root c ap and vascular tissue. The predicted RSI-1 protein is rich in cystein e, lysine and proline, and includes an N-terminal region with characte ristics of a signal peptide. The putative mature protein exhibits 79% amino acid identity to a protein encoded by a gene (GAST1) that is ind uced by gibberellic acid in tomato shoots.