The Arabidopsis Mitochondrial Protease FtSH4 Is Involved in Leaf Senescence via Regulation of WRKY-Dependent Salicylic Acid Accumulation and Signaling

Abstract

Mitochondria and autophagy play important roles in the networks that regulate plant leaf senescence and cell death. However, the molecular mechanisms underlying the interactions between mitochondrial signaling and autophagy are currently not well understood. This study characterized the function of the Arabidopsis (<i>Arabidopsis thaliana</i>) mitochondrial AAA-protease gene <i>FtSH4</i> in regulating autophagy and senescence, finding that FtSH4 mediates WRKY-dependent salicylic acid (SA) accumulation and signaling. Knockout of <i>FtSH4</i> in the <i>ftsh4-4</i> mutant resulted in severe leaf senescence, cell death, and high autophagy levels. The level of SA increased dramatically in the <i>ftsh4-4</i> mutant. Expression of <i>nahG</i> in the <i>ftsh4-4</i> mutant led to decreased SA levels and suppressed the leaf senescence and cell death phenotypes. The transcript levels of several SA synthesis and signaling genes, including <i>SALICYLIC ACID</i><i>INDUCTION DEFICIENT2</i> (<i>SID2</i>), <i>NON-RACE-SPECIFIC DISEASE RESISTANCE1</i> (<i>NDR1</i>), and <i>NONEXPRESSOR OF PATHOGENESIS-RELATED PROTEINS1</i> (<i>NPR1</i>), increased significantly in the <i>ftsh4-4</i> mutants compared with the wild type. Loss of function of <i>SID2</i>, <i>NDR1</i>, or <i>NPR1</i> in the <i>ftsh4-4</i> mutant reversed the <i>ftsh4-4</i> senescence and autophagy phenotypes. Furthermore, <i>ftsh4-4</i> mutants had elevated levels of transcripts of several <i>WRKY</i> genes, including <i>WRKY40</i>, <i>WRKY46</i>, <i>WRKY51</i>, <i>WRKY60</i>, <i>WRKY63</i>, and <i>WRKY75</i>; all of these WRKY proteins can bind to the promoter of <i>SID2</i> Loss of function of <i>WRKY75</i> in the <i>ftsh4-4</i> mutants decreased the levels of SA and reversed the senescence phenotype. Taken together, these results suggest that the mitochondrial ATP-dependent protease FtSH4 may regulate the expression of <i>WRKY</i> genes by modifying the level of reactive oxygen species and the WRKY transcription factors that control SA synthesis and signaling in autophagy and senescence.