This weeks Arabidopsis Research Roundup includes a study from the University of Warwick investigating the role of the ubiquitin-proteasome system in the defense response. Secondly researchers from Warwick (again), Durham and Rothamsted are involved in a paper that links cell cycle progression to production of endomembranes. Finally are two studies that include researchers from Edinburgh that firstly provide an insight into the non-specific transport of molecules into the phloem and secondly looks at the cellular basis of endosperm breakdown.
The latter study is part of a special issue of the journal Development that focuses on plant science and includes tributes to the late Ian Sussex. This issue contains paper that have been highlighted in the ARR together the summer.
Üstün S, Sheikh A, Gimenez-Ibanez S, Jones AM, Ntoukakis V, Börnke F (2016) The proteasome acts as a hub for plant immunity and is targeted by Pseudomonas type-III effectors Plant Physiology http://dx.doi.org/10.1104/pp.16.00808
Open Access
Alex Jones and Vardis Ntoukakis (University of Warwick) are co-authors on this German-led study that investigates the hypothesis that plant pathogens enhance their virulence by targeting the ubiquitin-proteasome system (UPS). The authors show that the UPS is induced upon basal defence and that proteasomal subunit mutants are more sensitive to growth of Pseudomonas species. This indicates that the UPS is involved in defence priming and also establishment of systemic-acquired resistance (SAR). A screen for bacterial effectors that interact with the proteasome yielded four known candidates. Subsequently they show HopM1 interacts with several E3 ligases and proteasome subunits. Therefore this study further demonstrates that the proteasome is a key component of the defence response and might be a potential target for future generation of pathogen resistant plants.
Craddock CP, Adams N, Kroon JT, Bryant FM, Hussey PJ, Kurup S, Eastmond PJ (2016) Cyclin-dependent kinase activity enhances phosphatidylcholine biosynthesis in Arabidopsis by repressing phosphatidic acid phosphohydrolase activity Plant Journal http://dx.doi.org/10.1111/tpj.13321 Open Access
This collaborative research between the Universities of Warwick and Durham with Rothamsted Research is led by Peter Eastmond. They investigate the coordinated biogenesis of endomembranes throughout the cell cycle, building on previous work that had demonstrated that disruption of the PHOSPHATIDIC ACID PHOSPHOHYDROLASE (PAH) enzyme caused expansion of the ER. In this work they show PAH expression is dependent on phosphorylation by the cell cycle regulatory CYCLIN-DEPENDENT KINASE A;1 (CDKA;1). Use of a CDKA;1 insensitive versions of PAH results in less endomembrane production and a reduced rate of cell division. This demonstrates that growth of the endomembrane is tightly linked to cell division, a control mechanism that ensures the cell is fully prepared with sufficient endomembranes to support the daughter cells that result from division.
Paultre DS, Gustin MP, Molnar A, Oparka KJ (2016) Lost in transit: long-distance trafficking and phloem unloading of protein signals in Arabidopsis homografts Plant Cell http://dx.doi.org/10.1105/tpc.16.00249
Karl Oparka (University of Edinburgh) is the corresponding author of this paper that uses micrografting to assess the long-distance movement of GFP-tagged proteins. They found that it took 10 days for GFP-tagged proteins to move from transgenic scions into non-transgenic roots, targeted to either chloroplasts, peroxisomes, actin, or to the nucleus. However, proteins targeted to endoplasmic reticulum or Golgi did not move through the plant. This demonstrates that there is extensive movement of proteins throughout the plant, thus explaining the many macromolecules that can be found in the phloem. However they also some that the recipient roots demonstrate limited movement outward from the phloem, the selective blockage occurring at the plasmodesmata between stele and cortex.
Fourquin C, Beauzamy L, Chamot S, Creff A, Goodrich J, Boudaoud A, Ingram G (2016) Mechanical stress mediated by both endosperm softening and embryo growth underlies endosperm elimination in Arabidopsis seeds Development. 143(18):3300-5. http://dx.doi.org/10.1242/dev.137224
Justin Goodrich (University of Edinburgh) is a co-author on this study from the lab of Gwyneth Ingram in Lyon that looks in detail at the mechanisms that control the breakdown of the coenocytic endosperm, which is essential for the nourishment of the growing embryo. The programmed cell death of the endosperm is controlled by the basic helix-loop-helix transcription factor ZHOUPI. This study shows that this is not a direct effect but rather occurs via the expression of cell wall-modifying enzymes, which alter the physical properties of the endosperm, enabling its compression as the developing embryo expands.
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