21st Oct 2016

This weeks Roundup demonstrates a wide breadth of research topics that use Arabidopsis at the model organism. Firstly Matthew Terry and colleagues investigate the factors that control reterograde siganling between chloroplast and nucleus. Secondly researchers from the University of Sheffield demonstrate that a component of the cell wall is involved in stomatal opening. Thirdly Malcolm Bennett and Ranjan Swarup from CPIB are co-authors on research that touches on a familiar topic, the regulation of the AUX1 protein in Arabidopsis roots. Fourthly Alastair Rutherford is a collaborator in a US-led study that investigates the proton motion force across thylakoid membranes. Finally are two studies wherein UK academics, namely David Salt and Malcolm Hawkesford, are co-authors on German-led studies that investigate a plants response to different minerals.

Page MT, McCormac AC, Smith AG, Terry MJ (2016) Singlet oxygen initiates a plastid signal controlling photosynthetic gene expression. New Phytol.

Open Access

Matthew Terry (Southampton) is the lead author on this study that investigates the factors regulating retrograde signaling between chloroplast and nucleus. One model for the control of this process is based around a signal, which is abolished by the herbicide norflurazon, which is related to levels of haem in chloroplasts. In addition this reterograde signalling can be damaged by the transfer of seedlings from far-red to white light. This study investigates whether these external factors influence the same endogenous signal transduction pathway and show, perhaps surprisingly, that different sets of genes are mostly affected in each case. FR pretreatment results in the damaging production of singlet oxygen molecules, which is inhibited by a mutant that lacks a correct signaling pathway that responds to oxygen. The authors suggest that this control process, wherein the production of oxygen in the chloroplast is a signal for reduced photosynthetic gene expression, acts to fine tune this system in response to differing light conditions.

Amsbury S, Hunt L, Elhaddad N, Baillie A, Lundgren M, Verhertbruggen Y, Scheller HV, Knox JP, Fleming AJ, Gray JE (2016) Stomatal Function Requires Pectin De-methyl-esterification of the Guard Cell Wall. Current Biology

Open Access

This manuscript is led by Julie Gray and Andrew Fleming at the University of Sheffield and looks at how the composition of the plant cell wall can affect stomatal opening. They show that guard cell walls are rich in un-esterified pectins and that a pectin methylesterase gene, PME6, is required for stomatal function and appropriately is highly expressed in this tissue. Mutant pme6 plants have an altered cell wall composition alongside a reduction in the dynamics of stomatal opening, indicating that there is a mechanical constraint on the ability of their guard cell walls to move in an appropriate manner. Overall, pme6 plants show decreased growth, due to increased CO2 loss, a phenotype that can be rescued by growth in elevated CO2. Although it is well known that multiple signals converge to control stomatal movement, this manuscript introduces a downstream component to this pathway that links in the physical movement of the guard cell walls.

Street IH, Mathews DE, Yamburkenko MV, Sorooshzadeh A, John RT, Swarup R, Bennett MJ, Kieber JJ, Schaller GE (2016) Cytokinin acts through the auxin influx carrier AUX1 to regulate cell elongation in the root. Development

This paper features Malcolm Bennett and Ranjan Swarup from CPIB as co-authors on a paper led by US colleagues. The requirement for hormone crosstalk has long been known to be critical for the precise control of root development. This study shows the role of cytokinin in modulation of auxin activity in the lateral root cap is dependent on the auxin influx carrier AUX1. This is true of ethylene dependent and independent modes of cytokinin action. Furthermore an autoregulatory signalling loop is discovered that integrates the effects of cytokinin and auxin and features AUX1 as well as the auxin responsive transcription factor ARR10. This study adds further detail to our knowledge of the mechanisms by which cytokinin controls root growth via auxin transport.

Davis GA,, Kanazawa A,, Schöttler MA, Kohzuma K, Froehlich JE, Rutherford AW, Satoh-Cruz M, Minhas D, Tietz S, Dhingra A, Kramer DM (2016) Limitations to photosynthesis by proton motive force-induced photosystem II photodamage. Elife

Open Access

Alastair Rutherford (Imperial College) is a co-author on this study that is led by researchers at Michigan State. They use a range of Arabidopsis mutants that show altered thylakoid lumen proton efflux to investigate their effect on the thylakoid proton motive force (pmf). PMF is required for the production of ATP and concomitantly could be considered essential for life on Earth. These mutants show alterations in photosynthetic regulation as well as on levels of photosystem II photodamage. More detailed measurements show that these phenotypes are dependent on an elevated electric field across the thylakoid, which alters the activity of PSII and its subsequent photodamage through production of destructive oxygen species. This alteration in the electric field across the thylakoid is important in wildtype plants and the authors suggest that the photodamage that naturally occurs in fluctuating light conditions could represent a limiting factor for plant productivity.

Kühnlenz T, Hofmann C, Uraguchi S, Schmidt H, Schempp S, Weber M, Lahner B, Salt DE, Clemens S (2016) Phytochelatin Synthesis Promotes Leaf Zn Accumulation of Arabidopsis thaliana Plants Grown in Soil with Adequate Zn Supply and is Essential for Survival on Zn-contaminated Soil. Plant Cell Physiol.

Open Access

Current GARNet Chairman and a recent arrival at CPIB in Nottingham, David Salt is a co-author on this German-led paper that assesses the role of Phytochelatin (PC) on plant growth in zinc contaiminated soils. PC is known to be essential for metal detoxification yet its role in zinc tolerance is not fully appreciated. Therefore wildtype and PC mutant (atpcs1) Arabidopsis were grown in soil with varying zinc concentrations. High concentrations of zinc cause PC upregulation, a response that is significantly compromised in atpcs1 mutant plants. Interestingly mutant plants show differing zinc levels when compared to wildtype in either normal or zinc-limited conditions. Finally the authors dissect the regulatory elements within AtPCS1 protein, showing that there are distinct domains that are responsible for activation by zinc or cadmium. This is indicative of a level of differential activity and is suggestive of the presence of multiple signaling pathways to deal with metal accumulation.

Forieri I, Sticht C, Reichelt M, Gretz N, Hawkesford MJ, Malagoli M, Wirtz M, Hell R (2016) Systems analysis of metabolism and the transcriptome in Arabidopsis thaliana roots reveals differential co-regulation upon iron, sulfur and potassium deficiency. Plant Cell Environ.

Open Access

This German-led study includes Malcolm Hawkesford (Rothamstead Research) and investigates shoot growth under iron, potassium or sulfur deficiency. When these minerals were adjusted to cause equivalent alterations in shoot growth they showed specific morphological alterations, changes in root metabolic profiles and in transcriptional responses. Iron deficiency causes the strongest gene expression changes, altering up to 18% of the transcriptome. A surprisingly small number of genes (180) were co-regulated by all of the nutrients. Iron deficiency alters a different set of genes involved in sulfur regulation compared to those observed in conditions of sulfur deficiency itself, indicative of mineral-specific cross-regulation of response pathways. Overall these experiments enable the dissection of general stress responses from those induced by specific deficiencies at the metabolic and transcriptomic scales.