One Two

Research Group Schaaf

Projects

I. Molecular plant nutrition: metal toxicity and metal homeostasis

II. Functions of inositol pyrophosphates

III. Sec14-type lipid binding proteins, organization and morphogenesis of membranes, and phosphoinositide signaling



I. Molecular plant nutrition: metal toxicity and metal homeostasis 

Our lab has a major interest in molecular plant nutrition with a focus on aluminum tolerance/toxicity and iron homeostasis. Aluminum is toxic to plants, animals and microbes and the increased concentration of soluble Al3+ in acid soils represents one of the biggest limitations to crop production worldwide. In contrast, iron is an essential nutrient for all living organisms and iron deficiency is considered to be the most common human nutritional disorder worldwide (https://www.who.int/topics/anaemia/en/). Similar to aluminum, iron solubility strongly depends on soil pH. In consequence, plants often suffer iron deficiency-induced chlorosis when grown in alkaline/calcareous soils. This is especially the case for dicots and non-graminaceous monocots, because they lack the Fe(III)-phytosiderophore uptake system of grasses (and hence have to reduce Fe(III) to Fe(II) prior to uptake – a process that is strongly pH dependent). Synthetic iron chelators applied by farmers to increase iron solubility in calcareous soils, pose a threat to the environment as these chemicals are persistent, mobilize heavy metals and result in ground water pollution.
 

To increase iron availability for human nutrition, breeding and biotech strategies have been initiated by diverse research institutes and companies to reduce plant endogenous phytic acid (InsP6). Because of its stability and its potent ability to chelate mineral cations, phytic acid is a major cause of iron and zinc deficiency whenever grains and legumes are predominant staple foods. Phytic acid is also a major causative agent for environmental pollution due to the eutrophication of aquatic ecosystems with phosphorous and iron caused by run-off waste from non-ruminant livestock (Brinch-Pedersen et al., 2002), because, as humans, non-ruminant animals lack gastric phytase activity and therefore cannot absorb InsP6 and InsP6-chelated metals. Unfortunately, systemic reduction of InsP6 causes negative effects on seed performance and plant immunity as shown by different laboratories. Our work in Arabidopsis suggests that reduced resistance to plant pests such as nectrophic fungi and insect herbivores in low phytic acid plants might not be a direct consequence of reduced InsP6 but rather be caused by a reduction of inositol pyrophosphates such as InsP7 and InsP8 (Laha et al., 2015).
 

We are currently working on 4 independent research projects with the hope to provide tools to allow farmers to produce crops at high yield and with high nutritional value but with reduced input of synthetic metal chelators, insecticides and fungicides, i.e. at reduced environmental cost.
 

i) Role of SEC14-type lipid binding proteins in aluminum tolerance (funded by an Emmy Noether grant)
ii) Strategies to increase Fe-efficiency in crops: Reconstitution of Fe-chelate uptake systems in model dicots
iii) Low phytic acid plants: Trade-offs between iron/zinc efficiency and plant immunity
iv) Epigenetic control of Fe-efficiency
 

Model organisms used for these studies are Arabidopsis, tomato, barley and maize. For more information or if you are interested to work on one of these projects, please contact the lab.

 


 II. Functions of inositol pyrophosphates

Inositol phosphates with diphospho bonds such as InsP7 and InsP8, also referred to as inositol pyrophosphates, are important signaling molecules that are generated in amoebae, yeast and mammalian cells from phytic acid (InsP6) and lower inositol polyphosphates by two distinct enzyme classes, IP6K/Kcs1 enzymes and PPIP5K/Vip1 enzymes.

We recently showed that InsP7 and InsP8 can be readily detected in Arabidopsis extracts and that PPIP5K/VIP1-type enzymes are widespread in plants including diverse taxa such as green algae, mosses lycopods, and monocot and eudicot angiosperms. Recent work in our lab shows that Arabidopsis Vip1 homologs, VIH1 and VIH2, are functional PPIP5K enzymes.

 


 
Simplified scheme of inositol pyrophosphate synthesis in mammals, yeast and plants.


We furthermore found that VIH2 is responsible for bulk InsP8 production in vegetative tissues and plays an important role in jasmonate perception and plant defenses against herbivorous insects and necrotrophic fungi. Our data suggests that coincidence detection of jasmonate and InsP8 by the ASK1-COI1-JAZ jasmonate receptor complex results in recruitment, ubiquitylation and proteasomal degradation of transcriptional JAZ repressors to induce jasmonate-regulated defenses (Laha et al., Plant Cell, 2015).
 

Role of VIH2 and InsP8 in jasmonate perception (modified from Laha et al., Plant Cell, 2015)


We are interested in understanding the precise role of VIH2 and inositol pyrophosphates in jasmonate perception and how they might be involved in hormonal crosstalk. We are hopeful that inositol pyrophosphates can be employed to modulate immunity in crops and are also interested in a potential role of these molecules in nutrient sensing (something that has been observed in non-plant model organisms).

  


III. Sec14-type lipid binding proteins, organization and morphogenesis of membranes, and phosphoinositide signaling

Phosphoinositides – phosphorylated species of phosphatidylinositol (PtdIns) - are important signaling molecules in eukaryotic cells. They regulate the activity of actin binding and membrane (e.g. channel) proteins, and control transcription and mRNA export as well as the transient recruitment of regulatory proteins to distinct membrane sites. Thereby phosphoinositides help to determine membrane identity. Their biosynthesis is regulated by PtdIns kinases and phosphoinositide phosphatases. We and others have found that PtdIns kinases are surprisingly inefficient enzymes on liposomal substrates, raising the question of how phosphoinositide biosynthesis is regulated in living cells.

High resolution crystallography and structure-based functional studies suggest that Sec14, the major yeast phosphatidylinositol (PtdIns)/phosphatidylcholine (PtdCho) transfer protein (PITP), renders PtdIns vulnerable to PtdIns 4-OH kinase attack during a heterotypic lipid exchange reaction with PtdCho (Schaaf et al., Mol Cell, 2008).

In vitro lipid transfer:


In vivo
stimulation of PtdIns 4-OH kinase activity:

In vitro and in vivo activities of yeast Sec14 (compiled from Schaaf et al., Mol Cell, 2008; Schaaf et al., Mol Biol Cell, 2011; Winklbauer et al., Commun Integr Biol, 2011)
 

We are mainly interested in plant Sec14 homologs (more than 30 homologs are encoded by the Arabidopsis thaliana genome). In a project funded by SFB 1101 we investigate how the multidomain Sec14 protein AtSFH1 controls polarized vesicle transport, tip-directed Ca2+ influx and root hair formation in Arabidopsis. Root hairs represent an important soil/plant interface that controls water and nutrient uptake and interactions with microorganisms. We are particularly interested in the C-terminal nodulin domain of this protein, which shares homology with the late nodulin Nlj16 in Lotus japonicus. We were recently able to show that this domain displays high binding specificity for the phosphoinositide PtdIns(4,5)P2 and is essential to cluster PtdIns(4,5)P2 in privileged signaling foci at the plasma membrane of root hairs.

Patterning of phosphoinositide signaling in root hairs by Sec14-nodulin AtSFH1 (modified from Ghosh, de Campos et al., Mol Biol Cell, 2015).
 

Our data suggest that both promotion of lipid synthesis and lipid organization is a major function of Sec14-nodulins and we propose a role of these proteins in priming membranes for diverse signaling functions. Recent isolation of EMS-induced atsfh1-1 suppressor mutants will hopefully provide important insights into how lipid homeostasis controls intracellular vesicle trafficking and homeostasis of the cytoplasmic second messenger Ca2+. A long term goal of this project is to obtain a more comprehensive understanding of lateral/polar organization of lipids within biological membranes and also to obtain tools to modulate root hair development and optimize nutrient uptake and water efficiency in crop plants.

  


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