On the cover: The p70 ribosomal S6 kinase (p70s6k) signalling pathway plays a key role in regulating the cell cycle via translational regulation of specific 5'TOP mRNAs. However, the function of this signaling pathway is still poorly understood in plants. In this issue, Tzeng et al. (pp. 1695–1709) describe the functional analysis of a lily (Lilium longiflorum) putative p70s6k gene LS6K1 in Arabidopsis. Ectopic expression of the LS6K1 caused up-regulation of NAP (NAC-LIKE, ACTIVATED BY AP3/PI) and PISTILLATA (PI) expression, and significantly inhibited cell expansion for petals and stamens, resulting in the male sterility phenotype in transgenic Arabidopsis. Sequence analysis revealed that the genes involved in petal and stamen development, such as APETALA3 (AP3), PI and SUPERMAN (SUP), as well as their orthologues in rice probably encode 5'TOP mRNAs. This assumption was supported by the ability of the chimeric green fluorescent protein (GFP) mRNAs to acquire the typical behaviour of the 5'TOP mRNAs in human cells in response to mitogen stimulation and inhibition by the macrolide antibiotic rapamycin when fused with the oligopyrimidine tract sequences of AP3, PI and SUP at their 5' UTR. Furthermore, pAP3::GUS was translationally up-regulated in the presence of the 35S::LS6K1 transgene in transgenic plants when GUS was preceded by the 5' UTR of AP3. These results have identified a mechanism to demonstrate that the p70s6k signalling pathway may translationally regulate genes in controlling cell division and the expansion of petal and stamen among higher plant species.
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