These authors contributed equally to this work. SUMMARYRecognition of microbe-associated molecular patterns (MAMPs) initiates pattern-triggered immunity in host plants. Pattern recognition receptors (PRRs) and receptor-like cytoplasmic kinases (RLCKs) are the major components required for sensing and transduction of these molecular patterns. However, the regulation of RLCKs by PRRs and their specificity remain obscure. In this study we show that PBL27, an Arabidopsis ortholog of OsRLCK185, is an immediate downstream component of the chitin receptor CERK1 and contributes to the regulation of chitin-induced immunity in Arabidopsis. Knockout of PBL27 resulted in the suppression of several chitin-induced defense responses, including the activation of MPK3/6 and callose deposition as well as in disease resistance against fungal and bacterial infections. On the other hand, the contribution of PBL27 to flg22 signaling appears to be very limited, suggesting that PBL27 selectively regulates defense signaling downstream of specific PRR complexes. In vitro phosphorylation experiments showed that CERK1 preferentially phosphorylated PBL27 in comparison to BIK1, whereas phosphorylation of PBL27 by BAK1 was very low compared with that of BIK1. Thus, the substrate specificity of the signaling receptor-like kinases, CERK1 and BAK1, may determine the preference of downstream RLCKs.

The mechanism of reverse saturable absorption of meso-substituted tetrabenzoporphyrins was studied by means of picosecond transient spectroscopy. Characteristic 1 (π,π*), 3 (π,π*), and (d,d) transitions have been observed. The kinetics of the excited states vary with metal substitution. The nonlinear transmission of benzoporphyrins was measured, and the excited-state absorption was determined to be the dominant optical limiting mechanism. The results agreed very well with a five-level model.

We examined, by using a specific PGE receptor subtype EP4 agonist and antagonist, the involvement of EP4 receptors in duodenal HCO 3 Ϫ secretion induced by PGE2 and mucosal acidification in rats. Mucosal acidification was achieved by exposing a duodenal loop to 10 mM HCl for 10 min, and various EP agonists were given intravenously 10 min before the acidification. Secretion of HCO 3 Ϫ was dose-dependently stimulated by AE1-329 (EP4 agonist), the maximal response being equivalent to that induced by sulprostone (EP1/EP3 agonist) or PGE2. The stimulatory action of AE1-329 and PGE2 but not sulprostone was attenuated by AE3-208, a specific EP4 antagonist. This antagonist also significantly mitigated the acidinduced HCO 3 Ϫ secretion. Coadministration of sulprostone and AE1-329 caused a greater secretory response than either agent alone. IBMX potentiated the stimulatory action of both sulprostone and AE1-329, whereas verapamil mitigated the effect of sulprostone but not AE1-329. Chemical ablation of capsaicin-sensitive afferent neurons did not affect the response to any of the EP agonists used. We conclude that EP4 receptors are involved in the duodenal HCO 3 Ϫ response induced by PGE2 or acidification in addition to EP3 receptors. The process by which HCO 3 Ϫ is secreted through these receptors differs regarding secondmessenger coupling. Stimulation through EP4 receptors is mediated by cAMP, whereas that through EP3 receptors is regulated by both cAMP and Ca 2ϩ ; yet there is cooperation between the actions mediated by these two receptors. The neuronal reflex pathway is not involved in stimulatory actions of these prostanoids. prostaglandin E2; EP receptor subtype; EP4 receptor SECRETION OF HCO 3 Ϫ FROM surface epithelial cells is one of the mucosal defensive mechanisms and plays an important role in protecting the duodenal mucosa against acid injury (6,4,18). Although the physiological regulation of this secretion involves several factors such as PGs, peptides, and neuronal factors (4,10,19,20), endogenous PGs are particularly important in the local control of the process. We (23) previously investigated the relationship between PGE receptor (EP) subtypes and HCO 3 Ϫ secretion, using specific EP1, EP2, and EP3 agonists, and found that the secretion was stimulated by EP agonists having a potent affinity for EP3 receptors. Using EP3 receptor knockout mice, we (21) also demonstrated the involvement of EP3 receptors in the acid-induced secretion of HCO 3 Ϫ in the duodenum. At that time, however, the possibility remained that the stimulatory action of PGE 2 is mediated by EP4 receptors, because specific EP4 agonists and antagonists were not available. It is now known that the increase in duodenal HCO 3 Ϫ secretion is mediated by the stimulation of adenylate cyclase (AC) activity and an elevation in intracellular cAMP levels (8,16,20) and that the activation of EP4 receptors results in an increase of intracellular cAMP via G s protein (3). Thus it is possible that EP4 receptors play a role in the regulation of duodenal HCO...

Pattern recognition receptors on the plant cell surface mediate the recognition of microbe/damage-associated molecular patterns (MAMPs/DAMPs) and activate downstream immune signaling. Autophosphorylation of signaling receptor-like kinases is a critical event for the activation of downstream responses but the function of each phosphorylation site in the regulation of immune signaling is not well understood. In this study, 41 Ser/Thr/Tyr and 15 Ser/Thr residues were identified as in vitro and in vivo autophosphorylation sites of Arabidopsis CERK1, which is essential for chitin signaling. Comprehensive analysis of transgenic plants expressing mutated CERK1 genes for each phosphorylation site in the cerk1-2 background indicated that the phosphorylation of T479 in the activation segment and Y428 located upstream of the catalytic loop is important for the activation of chitin-triggered defense responses. Contribution of the phosphorylation of T573 to the chitin responses was also suggested. In vitro evaluation of kinase activities of mutated kinase domains indicated that the phosphorylation of T479 and T573 is directly involved in the regulation of kinase activity of CERK1 but the phosphorylation of Y428 regulates chitin signaling independently of the regulation of kinase activity. These results indicated that the phosphorylation of specific residues in the kinase domain contributes to the regulation of downstream signaling either through the regulation of kinase activity or the different mechanisms, e.g. regulation of protein-protein interactions.