;Mitogen-activated protein kinase (MAPK) cascades play important roles in signal transduction of extracellular stimuli in eukaryotes. However, stimulatory signals for plant MAPKs have not been well elucidated. Here, a cDNA clone, termed Oryza sativa MAPK4 (OsMAPK4), from rice encoding a protein that showed homology with the eukaryotic MAPKs was isolated. According to the phylogenetic analysis, OsMAPK4 belongs to subgroup IV MAPK in plants. OsMAPK4 transcripts were expressed strongly in mature leaves and weakly in young leaves and panicles. The gene was also differentially expressed in roots at different developmental stages. In addition, the mRNA level of OsMAPK4 was up-regulated under sugar starvation, high salinity and cold treatments. These results suggest that this OsMAPK4 functions not only in developmental programs but also in stress-signaling pathways.

Eukaryotic translation initiation factor 5 A (eIF5A) is the only cellular protein known to contain the unusual amino acid hypusine. However, the precise cellular function of eIF5A is to date unknown. In the present study, we report on the characterization of two cDNA clones encoding eIF5A in rice (Oryza sativa). Sequence analysis revealed that the two cDNAs share 93% amino acid sequence identity. Phylogenetic analysis of the eIF5A genes revealed paraphyly of OseIF5A-1 and OseIF5A-2. Analysis at the mRNA level has shown that OseIF5A-1 and OseIF5A-2 are expressed in rice leaves and panicles and high relative amounts of both genes were detected in old leaves. In addition, both OseIF5A-1 and OseIF5A-2 were spatially regulated during rice leaf development. In suspension-cultured cells, the transcripts of OseIF5A genes were strongly reduced after sugar starvation. Abiotic stresses, salt and heavy metal, induce the accumulation of OseIF5A-1 and OseIF5A-2 mRNAs in rice cells. These results suggested that both OseIF5A genes might be regulated by plant development and environmental stresses.

Mitogen-activated protein kinase (MAPK) pathways are modules involved in the transduction of extracellular signals to intracellular targets in all eukaryotes. In plants, there is evidence for MAPKs playing a role in the signalling of abiotic stresses, pathogens, plant hormones, and cell cycle cues. The large number and divergence of plant MAPKs indicates that this ancient mechanism of signal transduction is extensively used in plants. However, there have been no reports of classical MAPK module in rice. In this report, we have isolated a MAPK from rice (Oryza sativa) termed OsMAPK2. The cloned cDNA is 1457 nucleotides long and the deduced amino acid sequence comprised 369 amino acid residues. Sequence analysis revealed that the predicted amino acid sequence is 72% identical to tobacco wound-induced protein kinase (WIPK). Southern analysis suggested a single OsMAPK2 gene in rice. Analysis at the mRNA level has shown that OsMAPK2 is expressed in all plant organs and high relative amounts of OsMAPK2 were detected in the mature panicles in comparison with in the immature panicles. In suspension-cultured cells, the OsMAPK2 mRNA transcript increased markedly upon temperature downshift from 26 degrees C to 4 degrees C and sucrose starvation. In contrast, the OsMAPK2 mRNA level rapidly declined in rice cell challenged by high temperature. A similarly rapid response of OsMAPK2 was observed in stress-treated seedlings, demonstrating that response of the MAPK pathway occurs also in intact plants. These results suggest that this OsMAPK2 may function in the stress-signalling pathway as well as panicle development in rice.

;Very little is known about the molecular events triggering differentiated cells to re-enter the cell cycle. We have investigated the possible role of tyrosine phosphorylation in this process with hypocotyl explants of Arabidopsis thaliana. Phytohormone-stimulated cell cycle reactivation in hypocotyls was accompanied by tyrosine phosphorylation of several proteins. Such regulation of the tyrosine phosphorylation in these proteins was not observed in a callusformation-deficient mutant, srd2, a result which suggests that the induction of tyrosine phosphorylation occurs as a specific event in callus cell proliferation. The promoter activity of cyclin-dependent kinase, CDKA;1, was also examined in phytohormone-stimulated hypocotyls. This study highlighted that protein tyrosine phosphorylation may play an important regulatory role in phytohormonestimulated cell proliferation.

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