Analysis of expansin-induced morphogenesis on the apical meristem of tomato

Our previous work has shown that localised activity of the cell-wall-loosen ing protein expansin is sufficient to induce primordia on the apical merist em of tomato, consistent with the hypothesis that tissue expansion plays a key role in leaf initiation. In this paper we describe the earliest morphog enic events visible on the surface of the apical meristem of tomato (Lycope rsicon esculentum Mill.) following treatment with expansin and report on th e spectrum of final structures formed. Our observations are consistent with a proposed primary function of expansin effecting morphogenesis via altere d biophysical stress patterns in the meristem. The primordia induced by exp ansin do not complete the full program of leaf development. We present data indicating that one reason for this might be the inability of exogenous ex pansin to mimic the endogenous pattern of expansin activity in the meristem . These data provide the first detailed analysis at the cellular level of e xpansin action on living tissue, the first description of the spectrum of s tructures induced by expansin on the apical meristem, and give an insight i nto a potentially fundamental mechanism in plant development.