The expression of an extensin-like protein correlates with cellular tip growth in tomato

Extensins are abundant proteins presumed to determine physical characteristics of the plant cell wall. We have cloned a cDNA encoding LeExt1 from a tomato (Lycopersicon esculentum Mill.) root hair cDNA library. The deduced sequence of the LeExt1 polypeptide defined a novel type of extensin-like proteins in tomato. Patterns of mRNA distribution indicated that expression of the LeExt1 gene was initiated in the root hair differentiation zone of the tomato rhizodermis. Cloning of the corresponding promoter and fusion to the -glucuronidase (GUS) reporter gene allowed detailed examination of LeExt1 expression in transgenic tomato plants. Evidence is presented for a direct correlation between LeExt1 expression and cellular tip growth. LeExt1/GUS expression was detectable in trichoblasts (=root hair-bearing cells), but not in atrichoblasts of the tomato rhizodermis. Both hair formation and LeExt1 expression was inducible by the plant hormone ethylene. Comparative analysis of the LeExt1/GUS expression was preformed in transgenic tomato, potato (Solanum tuberosum), tobacco (Nicotiana tabacum), and Arabidopsis plants. In the apical/basal dimension, GUS staining was absent from the root cap and undifferentiated cells at the root tip in all species investigated. It was induced at the distal end of the differentiation zone and remained high proximally to the root/hypocotyl boundary. In the radial dimension, GUS expression was root hair specific in the solanaceous species. Whereas LeExt1 mRNA was exclusively detectable in the rhizodermis, root hair-specific expression correlated with GUS expression in germinating pollen tubes. This is correlative evidence for the role of LeExt1 in root hair tip growth.