Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
18th International Symposium on Iron Nutrition and Interaction in Plants Madrid – May 30-June 3 2016 ROLE OF IRON IN ROOT PHOSPHATE SENSING Jens Müller, Marcus Heisters, Theresa Toev, Ranju Chutia, Jörg Ziegler, and Steffen Abel* Leibniz Institute of Plant Biochemistry, Department of Molecular Signal Processing, Halle (Saale), Germany *corresponding author email address Plant root development is highly responsive to numerous edaphic cues. The mineral nutrient inorganic phosphate (Pi) and its derivative P-anhydrides constitute major nodes in bioenergetics and metabolism. Pi deprivation directly impacts plant performance; however, limited Pi bioavailability is pervasive in soils and caused by complex chemistries involving iron (Fe) and other metals, particularly with increasing soil depth. To cope with Pi shortage, plants activate a set of coordinated biochemical and developmental responses that enhance Pi recycling and Pi acquisition by reprogramming metabolism and redesigning root system architecture. Pi limitation stimulates formation of a shallow root system by attenuating primary root extension and promoting lateral root development (also known as topsoil foraging). Such changes in root development are informed by local Pi supply, and previous studies suggested that Pi sensing at the root apex is modified by external Fe availability (see Abel 2011). To elucidate root Pi sensing, we characterized two functionally interacting genes in Arabidopsis thaliana, PDR2 (PHOSPHATE DEFICIENCY RESPONSE2) and LPR1 (LOW PHOSPHATE ROOT1) as well as their loss-offunction alleles that cause a hypersensitive (pdr2) and insensitive (lpr1) primary root growth response to low Pi, respectively. Using the two epistatic mutants and the wild-type as a reference, we conducted a series of experiments to compare in roots their Pi-responsive cell biology (Ticconi et al., 2009; Müller et al. 2015), their exo-metabolome (Ziegler et al. 2016) as well as their transcriptome and proteome (Müller et al. 2016), which all support a critical role of external Fe for mediating the local response of root meristems to Pi availability. PDR2 codes for the single P5-type ATPase of unknown transport specificity (AtP5A), which is targeted to the ER and likely controls LPR1 biogenesis. LPR1 encodes a cell wall-localized multicopper oxidase, which we demonstrated to express ferroxidase activity. We present a first structure-function analysis of a plant ferroxidase (LPR1). The expression domains of LPR1 and PDR2 overlap in the root meristem; however, LPR1 expression determines the sites of apoplastic Fe, ROS and callose accumulation, which interfere with cell-tocell communication in the root stem cell niche and promote accelerated cell differentiation followed by root growth inhibition when Pi is limiting. As expected, a genetic screen for pdr2 suppressors identified LPR1, but also a transporter of organic acids, which are known to function as Fe chelators. We developed a hydroponic cultivation system to analyze and to compare by non-targeted metabolite profiling semi-polar compounds in root exudates of Pi-replete and Pi-starved wild-type, pdr2 and lpr1 seedlings. Our data uncovered a role for exuded coumarins as potential Fe chelators and for accelerated lignification during the local response to Pi limitation. Similarly, genome-wide transcriptome and proteome analysis of the three genotypes in response to Pi deficiency revealed overrepresentation of genes and proteins that are involved in the regulation of Fe homeostasis, ROS formation and cell wall remodeling. Additional experiments support the hypothesis that IRT1-independent Fe uptake by root tips and apoplastic Fe redistribution, but not intracellular Fe uptake and Fe storage, control Pi-dependent cell wall modification and root growth modulation. Since Pi is the predominant nutrient controlling primary root length, our complementary studies (cell biology, metabolite and comparative gene expression profiling) provide a framework of how the relative abundance of Pi and Fe in soil shape root system architecture. Keywords: Phosphate-iron interactions, ferroxidase activity, meristem maintenance, root development REFERENCES: Abel (2011) 14:303-309; Ticconi et al. (2009) PNAS 106:14174-14179; Müller et al. (2015) Dev Cell 33:216-230; Ziegler et al. (2016) 67:1421-1432; Müller et al. (2016) BMC Plant Biol (in press). ACKNOWLEDGEMENT: Research at the Leibniz Institute of Plant Biochemistry is supported by institutional core funding provided by the state of Saxony-Anhalt and the federal Republic of Germany. 1