Alternative splicing expands the repertoire of dominant JAZ repressors of jasmonate signaling

Abstract

SUMMARYJasmonates (JAs) are fatty acid-derived signaling compounds that control diverse aspects of plant growth, development and immunity. The F-box protein COI1 functions both as a receptor for jasmonoyl-L-isoleucine (JA-Ile) and as the component of an E3-ubiquitin ligase complex (SCF COI1 ) that targets JAZ transcriptional regulators for degradation. A key feature of JAZ proteins is the C-terminal Jas motif that mediates the JA-Iledependent interaction with COI1. Here, we show that most JAZ genes from evolutionarily diverse plants contain a conserved intron that splits the Jas motif into 20 N-terminal and seven C-terminal (X 5 PY) amino acid submotifs. In most members of the Arabidopsis JAZ family, alternative splicing events involving retention of this intron generate proteins that are truncated before the X 5 PY sequence. In vitro pull-down and yeast twohybrid assays indicate that these splice variants have reduced capacity to form stable complexes with COI1 in the presence of the bioactive stereoisomer of the hormone (3R,7S)-JA-Ile. cDNA overexpression studies showed that some, but not all, truncated splice variants are dominant repressors of JA signaling. We also show that strong constitutive expression of an intron-containing JAZ10 genomic clone is sufficient to repress JA responses. These findings provide evidence for functional differences between JAZ isoforms, and establish a direct link between the alternative splicing of JAZ pre-mRNA and the dominant repression of JA signal output. We propose that production of dominant JAZ repressors by alternative splicing reduces the negative consequences associated with inappropriate or hyperactivation of the JA response pathway.

A bHLH-Type Transcription Factor, ABA-INDUCIBLE BHLH-TYPE TRANSCRIPTION FACTOR/JA-ASSOCIATED MYC2-LIKE1, Acts as a Repressor to Negatively Regulate Jasmonate Signaling inArabidopsis

“…Although overexpression of undegradable JAZ10.4 caused JAinsensitive root growth and male sterile phenotypes (Chung and Howe, 2009), overexpression of a JAZ10 genomic fragment caused only a JA-insensitive root growth phenotype (Chung et al, 2010). These results suggest that the mechanisms that promote JA responses in flower organs are different from those in other organs.…”

“…Molecular mechanisms for attenuating JA signaling were uncovered by identification of JAZ proteins, which negatively regulate JA responses by binding to transcriptional activators, such as MYC2 (Chini et al, 2007;Thines et al, 2007). The existence of degradation-resistant JAZs, including JAZ splice variants, further supports the importance of negative regulation of JA signaling by JAZ proteins (Chung and Howe, 2009;Chung et al, 2010;Shyu et al, 2012). However, JAM1 CRES-T plants exhibit JA-insensitive phenotypes, even when the expression of most of JAZ genes, including JAZ8, was reduced (Figures 2 and 7B) (see Supplemental Figure 17 online).…”

“…However, because downregulation of the expression of early JA-responsive genes initiates while the JA-Ile level remains high (Koo et al, 2009(Koo et al, , 2011, additional mechanisms may act to attenuate transcription. For example, stable JAZ proteins produced by alternative splicing desensitize JA responsiveness in cells containing high JA-Ile levels (Yan et al, 2007;Chung and Howe, 2009;Chung et al, 2010). Overexpression of an alternatively spliced form of JAZ10, which is resistant to COI1-mediated degradation, considerably suppressed JA responses.…”

ABA signalling is fine-tuned by antagonistic HAB1 variants

“…Alternative splicing (AS) regulates not only transcript levels but also transcript isoforms, giving rise to proteins that differ in subcellular localization, stability and function [23][24][25] . More than 61% of intron-containing Arabidopsis genes undergo AS under normal conditions 26 .…”

“…Thus, AS plays a fundamental role in plant development and stress adaptation 27,28 . We are beginning to understand the potential control of AS over hormone signalling 23,25 . However, the role of AS in regulating ABA signalling and plant stress adaptations remains elusive.…”