Uridylation and the SKI complex orchestrate the Calvin cycle of photosynthesis through RNA surveillance of <i>TKL1</i> in Arabidopsis.

Abstract

RNA uridylation, catalyzed by terminal uridylyl transferases (TUTases), represents a conserved and widespread posttranscriptional RNA modification in eukaryotes that affects RNA metabolism. In plants, several TUTases, including HEN1 SUPPRESSOR 1 (HESO1) and UTP: RNA URIDYLYLTRANSFERASE (URT1), have been characterized through genetic and biochemical approaches. However, little is known about their physiological significance during plant development. Here, we show that HESO1 and URT1 act cooperatively with the cytoplasmic 3'-5' exoribonucleolytic machinery component SUPERKILLER 2 (SKI2) to regulate photosynthesis through RNA surveillance of the Calvin cycle gene <i>TRANSKETOLASE 1</i> (<i>TKL1</i>) in <i>Arabidopsis</i>. Simultaneous dysfunction of <i>HESO1</i>, <i>URT1</i>, and <i>SKI2</i> resulted in leaf etiolation and reduced photosynthetic efficiency. In addition, we detected massive illegitimate short interfering RNAs (siRNAs) from the <i>TKL1</i> locus in <i>heso1 urt1 ski2</i>, accompanied by reduced <i>TKL1/2</i> expression and attenuated TKL activities. Consequently, the metabolic analysis revealed that the abundance of many Calvin cycle intermediates is dramatically disturbed in <i>heso1 urt1 ski2</i>. Importantly, all these molecular and physiological defects were largely rescued by the loss-of-function mutation in <i>RNA-DEPENDENT RNA POLYMERASE 6</i> (<i>RDR6</i>), demonstrating illegitimate siRNA-mediated <i>TKL</i> silencing. Taken together, our results suggest that HESO1- and URT1-mediated RNA uridylation connects to the cytoplasmic RNA degradation pathway for RNA surveillance, which is crucial for <i>TKL</i> expression and photosynthesis in <i>Arabidopsis</i>.