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BackgroundInsulin and insulin-like growth factors (IGFs) signal through a highly conserved pathway and control growth and metabolism in both vertebrates and invertebrates. In mammals, insulin-like growth factor binding proteins (IGFBPs) bind IGFs with high affinity and modulate their mitogenic, anti-apoptotic and metabolic actions, but no functional homologs have been identified in invertebrates so far.ResultsHere, we show that the secreted Imaginal morphogenesis protein-Late 2 (Imp-L2) binds Drosophila insulin-like peptide 2 (Dilp2) and inhibits growth non-autonomously. Whereas over-expressing Imp-L2 strongly reduces size, loss of Imp-L2 function results in an increased body size. Imp-L2 is both necessary and sufficient to compensate Dilp2-induced hyperinsulinemia in vivo. Under starvation conditions, Imp-L2 is essential for proper dampening of insulin signaling and larval survival.ConclusionImp-L2, the first functionally characterized insulin-binding protein in invertebrates, serves as a nutritionally controlled suppressor of insulin-mediated growth in Drosophila. Given that Imp-L2 and the human tumor suppressor IGFBP-7 show sequence homology in their carboxy-terminal immunoglobulin-like domains, we suggest that their common precursor was an ancestral insulin-binding protein.

Expansins are extracellular proteins that increase plant cell wall extensibility in vitro and are thought to be involved in cell expansion. We showed in a previous study that administration of an exogenous expansin protein can trigger the initiation of leaflike structures on the shoot apical meristem of tomato. Here, we studied the expression patterns of two tomato expansin genes, LeExp2 and LeExp18. LeExp2 is preferentially expressed in expanding tissues, whereas LeExp18 is expressed preferentially in tissues with meristematic activity. In situ hybridization experiments showed that LeExp18 expression is elevated in a group of cells, called I 1 , which is the site of incipient leaf primordium initiation. Thus, LeExp18 expression is a molecular marker for leaf initiation, predicting the site of primordium formation at a time before histological changes can be detected. We propose a model for the regulation of phyllotaxis that postulates a crucial role for expansin in leaf primordium initiation.

Expansins are extracellular proteins that increase plant cell wall extensibility in vitro and are thought to be involved in cell expansion. We showed in a previous study that administration of an exogenous expansin protein can trigger the initiation of leaflike structures on the shoot apical meristem of tomato. Here, we studied the expression patterns of two tomato expansin genes, LeExp2 and LeExp18. LeExp2 is preferentially expressed in expanding tissues, whereas LeExp18 is expressed preferentially in tissues with meristematic activity. In situ hybridization experiments showed that LeExp18 expression is elevated in a group of cells, called I 1 , which is the site of incipient leaf primordium initiation. Thus, LeExp18 expression is a molecular marker for leaf initiation, predicting the site of primordium formation at a time before histological changes can be detected. We propose a model for the regulation of phyllotaxis that postulates a crucial role for expansin in leaf primordium initiation. INTRODUCTIONIn tomato, leaves are positioned in a spiral phyllotaxis, with a divergence angle of ‫ف‬ 135 Њ . This highly regular pattern originates from the apex of the plant, where leaf primordia are initiated at the peripheral zone of the shoot apical meristem (reviewed in Steeves and Sussex, 1989;Lyndon, 1990;Jean, 1994). At predictable sites on the meristem, groups of cells become engaged in the formation of a bulge that undergoes morphogenesis to become a leaf (Poethig, 1997). All three layers that have been described in the shoot apical meristem, that is, the epidermal (L1), the subepidermal (L2), and the corpus (L3) layers, contribute to the leaf (Szymkowiak and Sussex, 1996).What are the cellular events that induce the local bulging of meristem tissue? Primordium initiation could be driven by a local increase in the rate of cell division or by a reorientation of the plane of cell division, followed by expansion and differentiation processes in the proliferating tissue (Meyerowitz, 1997). Alternatively, bulging could be driven by local expansion of primordium initials, followed by division of the expanded cells.Several lines of evidence suggest that local growth processes in plants can be initiated by cell expansion, with cell division being a secondary event (reviewed in Jacobs, 1997). There is also evidence concerning the importance of cell expansion in the earliest steps of primordium formation. First, when cell division in wheat seedlings was inhibited by ␥ irradiation, primordium initiation continued through local cell expansion (Foard, 1971). Second, transformed tobacco plants with downregulated cell cycle activity had smaller meristems with fewer cells than the controls had, as was expected, but they formed leaves of almost normal shape and size, and at normal rates. In the mature leaves, decreased cell number was compensated by increased cell size (Hemerly et al., 1995). From these experiments, we concluded that regulation of cell expansion rather than cell division determines the rate of primordium initiat...