Impaired growth in transgenic plants over-expressing an expansin isoform.

Expansins are cell wall proteins characterised by their ability to stimulate wall loosening during cell expansion. The expression of some expansin isoforms is clearly correlated with growth and the external application of expansins can stimulate cell expansion in vivo in several systems. We report here the expression of a heterologous expansin coding sequence in transgenic tomato plants (Lycopersicon esculentum Mill.) under the control of a constitutive promoter. In some transgenic lines with high levels of expansin activity extractable from cell walls, we observed alterations of growth: mature plants were stunted, with shorter leaves and internodes, and dark-grown seedlings had shorter and wider hypocotyls than their wild-type counterparts. Examination of hypocotyl sections revealed similar differences at the cellular 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 reduced in the transgenics. As this observation can seem paradoxical given the known effect of expansins on isolated cell walls, we examined the mechanical behaviour of transgenic tissue. We measured a decrease in hypocotyl elongation 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 reduced susceptibility of transgenic cell walls to expansin action.

Expression of a heterologous expansin in transgenic tomato plants.

Expansins are cell wall proteins thought to play an important role in growth and other events involving cell wall modifications. Whereas the expression patterns of many isoforms have been characterised, the nature of their activity is still poorly understood. Large amounts of active expansins are necessary to undertake biochemical studies and identify their substrates. We report here the successful expression of a recombinant expansin [CsExp1, isolated from cucumber (Cucumis sativus L.) hypocotyls] in transgenic tomato (Lycopersicon esculentum Mill.) plants, under the control of a constitutive promoter. In some transformants, CsExp1 transcript and protein accumulated to high levels, and expansin activity extractable from the cell walls was increased up to about 20-fold the activity measured in wild-type plants. These results confirm the identity of the CsExp1 coding sequence, and will enable large quantities of active expansin to be obtained for further studies.