Functions of PSK-a and analysis of the PSK-signaling pathway in Arabidopsis thaliana

Phytosulfokin-a (PSK-a) ist ein disulfatiertes Pentapeptid mit der Aminosäuresequenz Tyr(SO3H)-Ile-Tyr(SO3H)-Thr-Gln, welches als autokriner Wachstumsfaktor wirkt. Über physiologische und molekulargenetische Ansätze wurden in Arabidopsis thaliana Funktionen von PSK-a und eine mögliche Komponente des Signalweges identifiziert. PSK-Präproproteine werden in Arabidopsis durch fünf Gene kodiert. Die Perzeption des Peptids erfolgt in Arabidopsis durch zwei Rezeptorkinasen mit Leuzinreichen Wiederholungssequenzen, PSKR1 und PSKR2. Spezifische Expressionen von PSK-Präproproteingenen und PSKR1 wurden in Wurzeln, Hypokotylen und reproduktiven Organen und nach Verwundung nachgewiesen. In diesen Geweben kann PSK-a wahrscheinlich synthetisiert und perzipiert werden. PSK- a förderte konzentrationsabhängig die Streckung der Zellen von Wurzeln und Hypokotylen von Arabidopsis-Keimlingen. Die Regulation des Wurzelwachstums durch PSK-a erfolgte über beide PSK-Rezeptoren, während das Hypokotylwachstum und die Expansion von Protoplasten, die aus dem etiolierten Hypokotyl isoliert wurden, nur über PSKR1 reguliert wurden. Analysen von T-DNA-Insertionslinien für PSKR1 zeigten, dass PSK-a und PSKR1 während der Reproduktion die Pollenschlauchorientierung und das synchrone Wachstum von Staubblättern und Fruchtknoten regulieren. PSK-a ist außerdem in die Antwort auf Verwundung und Befall durch Pathogene involviert. Über bioinformatische Analysen wurden Gene identifiziert, die mit PSKR1 und PSKR2 koexprimiert sind. Koexprimierte Kandidatengene wurden an Hand von Knockout Linien auf eine mögliche Funktion bei der Regulation von Hypokotyl- und Wurzelwachstum durch PSK-a untersucht. PSI1 ist ein Protein mit unbekannter Funktion, das möglicherweise eine Komponente des PSK-Signalweges ist. psi1-1 Keimlinge hatten kürzere Wurzeln und Hypokotyle und pskr1-3/pskr2-1/psi1-1 Dreifachmutanten zeigten einen nicht-additiven Wachstumsphänotyp

Phytosulfokine-α Controls Hypocotyl Length and Cell Expansion in Arabidopsis thaliana through Phytosulfokine Receptor 1

The disulfated peptide growth factor phytosulfokine-α (PSK-α) is perceived by LRR receptor kinases. In this study, a role for PSK signaling through PSK receptor PSKR1 in Arabidopsis thaliana hypocotyl cell elongation is established. Hypocotyls of etiolated pskr1-2 and pskr1-3 seedlings, but not of pskr2-1 seedlings were shorter than wt due to reduced cell elongation. Treatment with PSK-α did not promote hypocotyl growth indicating that PSK levels were saturating. Tyrosylprotein sulfotransferase (TPST) is responsible for sulfation and hence activation of the PSK precursor. The tpst-1 mutant displayed shorter hypocotyls with shorter cells than wt. Treatment of tpst-1 seedlings with PSK-α partially restored elongation growth in a dose-dependent manner. Hypocotyl elongation was significantly enhanced in tpst-1 seedlings at nanomolar PSK-α concentrations. Cell expansion was studied in hypocotyl protoplasts. WT and pskr2-1 protoplasts expanded in the presence of PSK-α in a dose-dependent manner. By contrast, pskr1-2 and pskr1-3 protoplasts were unresponsive to PSK-α. Protoplast swelling in response to PSK-α was unaffected by ortho-vanadate, which inhibits the plasma membrane H+-ATPase. In maize (Zea mays L.), coleoptile protoplast expansion was similarly induced by PSK-α in a dose-dependent manner and was dependent on the presence of K+ in the media. In conclusion, PSK-α signaling of hypocotyl elongation and protoplast expansion occurs through PSKR1 and likely involves K+ uptake, but does not require extracellular acidification by the plasma membrane H+-ATPase

Post-translational maturation of IDA, a peptide signal controlling floral organ abscission in Arabidopsis

The abscission of sepals, petals and stamens in Arabidopsis flowers is controlled by a peptide signal called IDA (Inflorescence Deficient in Abscission). IDA belongs to the large group of small post-translationally modified signaling peptides that are synthesized as larger precursors and require proteolytic processing and specific side chain modifications for signal biogenesis. Using tissue-specific expression of proteinase inhibitors as a novel approach for loss-of-function analysis, we recently identified the peptidases responsible for IDA maturation within the large family of subtilisin-like proteinases (subtilases; SBTs). Further biochemical and physiological assays identified three SBTs (AtSBT5.2, AtSBT4.12, AtSBT4.13) that cleave the IDA precursor to generate the N-terminus of the mature peptide. The C-terminal processing enzyme(s) remain(s) to be identified. While proline hydroxylation was suggested as additional post-translational modification required for IDA maturation, hydroxylated and non-hydroxylated IDA peptides were found to be equally active in bioassays for abscission

PSK-alpha promotes root growth in Arabidopsis

Phytosulfokine-alpha (PSK-alpha) is a disulfated pentapeptide described to act as a growth factor in suspension cells. In this study, the involvement of PSK signaling through the PSK receptor gene AtPSKR1 in Arabidopsis root growth was assessed. Expression studies of PSK precursor genes and of AtPSKR1 were performed in roots with RT-PCR and P:GUS analyses. Root elongation, lateral root formation, cell production and root cell elongation were analyzed in wild-type (wt) and in the receptor knockout mutant Atpskr1-T treated with or without synthetic PSK-alpha. Phytosulfokine and AtPSKR1 genes are differentially expressed in roots. PSK-alpha induced root growth in a dose-dependent manner without affecting lateral root density. Kinematic analysis established that enhancement of root growth by PSK-alpha was mainly caused by an increase in cell size. In Atpskr1-T, the primary roots were shorter as a result of reduced mature cell size and a smaller root apical meristem composed of fewer cells than in wt. The results indicate that PSK-alpha signaling through AtPSKR1 affects root elongation primarily via control of mature cell size. Root organogenesis, on the other hand, is not controlled by PSK-alpha