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

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