Symposium on plant development – University of Copenhagen

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Symposium on plant development

Sabine Müller, University of Tübingen, Germany:
Spatial control of cytokinesis in Arabidopsis

The timing and orientation of cell divisions in plants is of utmost importance for cell shape and morphology due to the cell-wall-imposed fixation of plant cells (Müller et al., 2009; Müller, 2012). During cytokinesis, cell plate formation is initiated in the division plane and its expansion progresses laterally towards the parental wall aided by the cytoskeletal phragmoplast.

Intriguingly, the site of cell plate fusion, designated the cortical division zone is already selected at the G2/M transition and delineated at the cell cortex by the preprophase band (PPB). Recently, research in my lab advanced the understanding of this process. Using life cell imaging and fluorescence recovery after photo-bleaching analysis, we revealed the subcellular localization and dynamic behavior of kinesin-12 PHRAGMPOLAST ORIENTING KINESIN (POK) 1 at the cortical division zone throughout mitosis (Lipka et al., 2014).

Simultaneous mutation of the functionally redundant POK1 and POK2 pair lead to oblique cell wall insertions due to the mismatch of PPB and phragmoplast position. cortical division zone markers TANGLED and RanGAP1, which both interact with the carboxy (C)-terminal of POK1 are not retained at the cortical division zone in pok1pok2 mutants (Xu et al., 2008; Lipka et al., 2014) suggesting that POKs act as a molecular framework to preserve the cortical division zone. Despite the recent progress, the molecular pathways that enable maintenance of the cortical division zone and its recognition by the approaching phragmoplast are poorly defined.

To gain further insight, we pursue cell biological characterization of POK dependent pathways using the pok1pok2 double mutant as a genetic tool and dissect POK functions by characterizing POK protein domains. Furthermore, we are using reverse genetics combined with life cell imaging to characterize recently identified POK inter-actors PHGAPs, which are recruited to the cortical division zone. Mutant analysis suggest that they act upstream of POK1 in prophase, but their maintenance is POK-dependent (Stöckle et al., 2016). Our findings suggests that Rop GTPase dependent cell polarity signaling and symmetric division plane establishment share similar signaling pathways.

Fabio Fornara, University of Milano, Italy:
Transcriptional profiling of the shoot apical meristem identifies components of the photoperiodic response network of rice

Flowering of several plant species is induced by exposure to specific photoperiods that promote the expression of florigenic proteins in the leaves and their subsequent translocation to the shoot apex, where they commit the meristem to a reproductive fate. HEADING DATE 3a (Hd3a) and RICE FLOWERING LOCUS T 1 (RFT1) encode components of the rice florigenic signal and move systemically in the phloem to reach the shoot apical meristem (SAM).

Transition to reproductive growth at the apex is often accompanied by stem elongation, to expose flowers above the leaves and facilitate fertilization. However, how growth and inflorescence formation are coupled and how photoperiodic signals coordinate these processes at the apex is still unclear.

We profiled the transcriptome of the SAM using RNA-Seq at different time points during floral commitment, and identified ~500 genes differentially expressed in response to photoperiodic floral induction. Among them, we isolated FLOWERING LOCUS T-LIKE 1 (FT-L1) a homolog of Hd3a and RFT1, whose transcription strongly increases in response to inductive day length. Functional data indicate that FT-L1 encodes a floral promoter induced by Hd3a and RFT1 at the SAM, possibly creating a feed-forward regulatory node among florigens that sustains SAM commitment. 

Conversely, Hd3a and RFT1 are sufficient to repress the expression of PREMATURE INTERNODE ELONGATION 1 (PINE1) at the shoot apex during the transition to flowering, to promote culm elongation. PINE1 encodes a nuclear C2H2 zinc finger transcriptional repressor that controls the mRNA abundance of GA3ox2, encoding a gibberellin (GA) biosynthetic enzyme. These data uncover the existence of a regulatory network coordinating multiple aspects of phase transition at the shoot apex, and indicate that GA-induced growth and activity of florigenic proteins at the shoot apex need to be strictly coupled.