SummaryAn Arabidopsis thaliana line bearing a transposon insertion in the gene coding for the isozyme form of the plant-specific cap-binding protein, eukaryotic initiation factor (iso) 4E (eIF (iso) 4E), has been isolated. This mutant line completely lacks both eIF(iso)4E mRNA and protein, but was found to have a phenotype and fertility indistinguishable from wild-type plants under standard laboratory conditions. In contrast, the amount of the related eIF4E protein was found to increase in seedling extracts. Furthermore, polysome analysis shows that the mRNA encoding eIF4E was being translated at increased levels. Given the known interaction between cap-binding proteins and potyviral genome-linked proteins (VPg), this plant line was challenged with two potyviruses, Turnip mosaic virus (TuMV) and Lettuce mosaic virus (LMV) and was found resistant to both, but not to the Nepovirus, Tomato black ring virus (TBRV) and the Cucumovirus, Cucumber mosaic virus (CMV). Together with previous data showing that the VPg-eIF4E interaction is necessary for virus infectivity and upregulates genome amplification, this shows that the eIF4E proteins are specifically recruited for the replication cycle of potyviruses.

The molecular mechanism of neural induction is still unknown and the identity of the natural inducer remains elusive. It has been suggested that both the protein kinase C and cAMP signal transduction pathways may be involved in mediating its action. On the other hand, induction of the ectoderm toward the neural pathway can be provoked simply by modifying the extracellular concentration ofdivalent cations (16,17). These observations and the availability of microspectrofluorimetric techniques prompted us to examine the role of Ca2+ in transduction of the neuralizing signal. We triggered in vitro neural induction on excised dorsal ectoderm explants of the urodele amphibian Pleurodeles waltl with the lectin Con A. This lectin is an inducer for both anurans and urodeles, acting at the plasma membrane level. So far, it is the only lectin known to have inducing activity (7,8,18).Here we demonstrate that neural induction triggered by Con A involved the activation of L-type Ca2+ channels. In our system L-type Ca2+ channels are upmodulated by PKC, suggesting that inductive effects ofphorbol esters, previously observed on Xenopus ectoderm (9, 10), may be partly explained by the activation of L-type Ca2+ channels. In addition, an increase in intracellular Ca2+ concentration ([Ca2+],) obtained by direct stimulation ofinternal stores by caffeine or ryanodine triggers neural induction. We conclude that an increase in [Ca2+]i is sufficient to induce the competent ectoderm toward the neural pathway.