An endo-1,4-beta-glucanase expressed at high levels in rapidly expanding tissues

Plant developmental processes involving modifications to cell wall structure, such as cell expansion, organ abscission and fruit ripening, are accompanied by increased enzyme activity and mRNA abundance of endo-1,4-beta-glucanases (EGases). An EGase cDNA clone, Cel4, isolated from tomato (Lycopersicon esculentum) has been shown to be identical to a tomato pistil-predominant EGase cDNA, TPP18. In addition to its previously reported expression during certain stages of early pistil development, Cel4 mRNA was also detected at high levels in the growing zones of etiolated hypocotyls (about 2.5-fold less than in pistils) and in young expanding leaves (about 3.5-fold less than in pistils). The abundance of Cel4 mRNA declined precipitously in older tissues as cells became fully expanded, and was barely detectable in mature vegetative tissues. Cel4 mRNA abundance was also low in abscission zones, and did not increase as abscission progressed. In fruit, Cel4 mRNA was present at low levels during fruit expansion, but was essentially absent during subsequent fruit development and ripening. Treatment of etiolated hypocotyls with ethylene or high concentrations of auxin sufficient to induce rapid lateral cell expansion and hypocotyl swelling also brought about an approximate doubling of Cel4 mRNA abundance, suggesting that Cel4 mRNA accumulation may be promoted directly or indirectly by ethylene. Thus, accumulation of Cel4 mRNA was found to be correlated with rapid cell expansion in pistils, hypocotyls and leaves.