Gene expression profiling of retrograde PAP-signaling and ABA-signaling mutants in response to ABA treatment

Chloroplast-nuclear retrograde signaling is viewed as a mechanism for inter-organelle communication. Here we show the SAL1-PAP (3-phosphoadenosine 5- phosphate) retrograde pathway functions more broadly in guard cells, interacting with abscisic acid (ABA) signaling at least in part via exoribonucleases. Unexpectedly, PAP bypasses the canonical signaling components ABA Insensitive 1 (ABI1) and Open Stomata 1 (OST1) by priming an alternative pathway that restores ABA-responsive gene expression, ROS bursts, ion channel function and stomatal closure in ost1-2. This alternative pathway up-regulates lowly expressed Calcium Dependent Protein Kinases (CDPKs) which have the capacity to activate the key slow anion channel SLAC1 in response to ABA-mediated and ost1-2 independent calcium release. The role of PAP in priming an alternative pathway to bypass components previously considered essential for stomatal closure demonstrates how a chloroplast signal can have broader roles as a secondary messenger to directly intersect with and tune hormone signaling.

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Pornsiriwong W, Estavillo GM, Chan KX, Tee EE, Ganguly D, Crisp PA, Phua SY, Zhao C, Qiu J, Park J, Yong MT, Nisar N, Yadav AK, Schwessinger B, Rathjen J, Cazzonelli CI, Wilson PB, Gilliham M, Chen ZH, Pogson BJ