Gibberellins signal nuclear import of PHOR1, a photoperiod-responsive protein with homology to Drosophila armadillo

Expression profiling of the whole Arabidopsis shaggy-like kinase multigene family by real-time reverse transcriptase-polymerase chain reaction

The recent publication of the complete sequence of the Arabidopsis genome allowed us to identify and characterize the last two members of the SHAGGY-like kinase (AtSK) gene family. As a result, the study of the overall spatio-temporal organization of the whole AtSK family in Arabidopsis has become an achievable and necessary aim to understand the role of each SHAGGY-like kinase during plant development. An analysis of the transcript level of the 10 members of the family has been performed using the technique of real-time quantitative reverse transcriptase-polymerase chain reaction. Transcript levels in several organs, under different growth conditions, were analyzed. To calibrate the results obtained, a number of other genes, such as those coding for the two MAP3Kepsilons and the two MAP4Kalphas, as well as the stress response marker RD29A; the small subunit of the Rubisco photosynthetic enzyme Ats1A; the MEDEA chromatin remodeling factor; and the SCARECROW, ASYMMETRIC LEAVES 1, and SUPERMAN transcription factors all involved in key steps of plant development were used. The analysis of our data revealed that eight of the 10 genes of the AtSK family displayed a pseudo-constitutive expression pattern at the organ level. Conversely, AtSK13 responded to osmotic changes and saline treatment, whereas AtSK31 was flower specific and responded to osmotic changes and darkness.

Two immediate-early pathogen-responsive members of the AtCMPG gene family in Arabidopsis thaliana and the W-box-containing elicitor-response element of AtCMPG1

The Arabidopsis thaliana genome contains at least 50 predicted AtCMPG genes. The encoded protein family is defined by a common domain possessing four strictly conserved amino acid residues [Cys, Met, Pro, and Gly (CMPG)] that designate the family. Two members, AtCMPG1 and AtCMPG2, with high sequence similarity to the previously described, immediate-early pathogen-responsive PcCMPG1 gene from Petroselinum crispum were selected for analysis of their expression modes and defense-related promoter elements. Among the most striking similarities with PcCMPG1 were immediate-early transcriptional activation on infection or treatment with a pathogen-derived elicitor and the functional importance of a W-box-containing AtCMPG1 promoter element. Remarkably, this strongly pathogen/elicitor-responsive element, F, did not respond to wounding, in contrast to the AtCMPG1 promoter itself. Comparative analysis, both within the A. thaliana genome and across species, provided further insight into the large structural diversity of W-box-containing elements. Possible roles of AtCMPG proteins in regulatory processes are discussed with reference to a large variety of family members, partly with assigned functions, from plants as well as animals.

The cell biology of phytochrome signalling

Phytochrome signal transduction has in the past often been viewed as being a nonspatially separated linear chain of events. However, through a combination of molecular, genetic and cell biological approaches, it is becoming increasingly evident that phytochrome signalling constitutes a highly ordered multidimensional network of events. The discovery that some phytochromes and signalling intermediates show light-dependent nucleo-cytoplasmic partitioning has not only led to the suggestion that early signalling events take place in the nucleus, but also that subcellular localization patterns most probably represent an important signalling control point. Moreover, detailed characterization of signalling intermediates has demonstrated that various branches of the signalling network are spatially separated and take place in different cellular compartments including the nucleus, cytosol, and chloroplasts. In addition, proteasome-mediated degradation of signalling intermediates most probably act in concert with subcellular partitioning events as an integrated checkpoint. An emerging view from this is that phytochrome signalling is separated into several subcellular organelles and that these are interconnected in order to execute accurate responses to changes in the light environment. By integrating the available data, both at the cellular and subcellular level, we should be able to construct a solid foundation for further dissection of phytochrome signal transduction in plants. Contents Summary 553 I. Introduction 554 II. Nucleus vs cytoplasm 556 III. The nucleus 562 IV. The cytoplasm 571 V. Interactions with other signalling pathways 577 VI. Conclusions and the future 582 Acknowledgements 583 References 583.

Gibberellin regulates Arabidopsis seed germination via RGL2, a GAI/RGA-like gene whose expression is up-regulated following imbibition

The germination of Arabidopsis seeds is promoted by gibberellin (GA). Arabidopsis GAI, and RGA are genes encoding key GA signal-transduction components (GAI and RGA) that mediate GA regulation of stem elongation. The Arabidopsis genome contains two further genes, RGL1 and RGL2, that encode proteins (RGL1 and RGL2) that are closely related to GAI and RGA. Here, we show that RGL2 regulates seed germination in response to GA, and that RGL1, GAI, and RGA do not. In addition, we show that RGL2 transcript levels rise rapidly following seed imbibition, and then decline rapidly as germination proceeds. In situ GUS staining revealed that RGL2 expression in imbibed seeds is restricted to elongating regions of pre-emergent and recently emerged radicles. These observations indicate that RGL2 is a negative regulator of GA responses that acts specifically to control seed germination rather than stem elongation. Furthermore, as RGL2 expression is imbibition inducible, RGL2 may function as an integrator of environmental and endogenous cues to control seed germination.

A link between the light and gibberellin signaling cascades

In a recent issue of Cell, Salomé Prat and colleagues describe the characterization of PHOR1, an armadillo-related protein involved in gibberellin signaling and also responsive to light.