LOCALIZED UP-REGULATION OF A NEW EXPANSIN GENE PREDICTS THE SITE OF LEAF FORMATION IN THE TOMATO MERISTEM

Expansins are extracellular proteins that increase plant cell wall ext ensibility in vitro and are thought to be involved in cell expansion. We showed in a previous study that administration of an exogenous expa nsin protein can trigger the initiation of leaflike structures on the shoot apical meristem of tomato. Here, we studied the expression patte rns of two tomato expansin genes, LeExp2 and LeExp18. LeExp2 is prefer entially expressed in expanding tissues, whereas LeExp18 is expressed preferentially in tissues with meristematic activity. In situ hybridiz ation experiments showed that LeExp18 expression is elevated in a grou p of cells, called I,, which is the site of incipient leaf primordium initiation. Thus, LeExp18 expression is a molecular marker for leaf in itiation, predicting the site of primordium formation at a time before histological changes can be detected. We propose a model for the regu lation of phyllotaxis that postulates a crucial role for expansin in l eaf primordium initiation.