Impaired growth in transgenic plants over-expressing an expansin isoform

Expansins are cell wall proteins characterised by their ability to stimulat e wall loosening during cell expansion. The expression of some expansin iso forms is clearly correlated with growth and the external application of exp ansins can stimulate cell expansion in vivo in several systems. We report h ere the expression of a heterologous expansin coding sequence in transgenic tomato plants (Lycopersicon esculentum Mill.) under the control of a const itutive promoter. In some transgenic lines with high levels of expansin act ivity extractable from cell walls, we observed alterations of growth: matur e plants were stunted, with shorter leaves and internodes, and dark-grown s eedlings had shorter and wider hypocotyls than their wild-type counterparts . Examination of hypocotyl sections revealed similar differences at the cel lular level: cortical and epidermal cells were shorter and wider than those from wild-type seedlings. The observed stimulation of radial expansion did not compensate for the decreased elongation, and overall growth was reduce d in the transgenics. As this observation can seem paradoxical given the kn own effect of expansins on isolated cell walls, we examined the mechanical behaviour of transgenic tissue. We measured a decrease in hypocotyl elongat ion in response to acidic pH in the transformants. This result may account for the alterations in cell expansion, and could itself be explained by a r educed susceptibility of transgenic cell walls to expansin action.