ABSTRACT
Jasmonates are essential phytohormones for plant development and survival. However, the molecular details of their signalling pathway remain largely unknown. The identification more than a decade ago of COI1 as an F-box protein suggested the existence of a repressor of jasmonate responses that is targeted by the SCF(COI1) complex for proteasome degradation in response to jasmonate. Here we report the identification of JASMONATE-INSENSITIVE 3 (JAI3) and a family of related proteins named JAZ (jasmonate ZIM-domain), in Arabidopsis thaliana. Our results demonstrate that JAI3 and other JAZs are direct targets of the SCF(COI1) E3 ubiquitin ligase and jasmonate treatment induces their proteasome degradation. Moreover, JAI3 negatively regulates the key transcriptional activator of jasmonate responses, MYC2. The JAZ family therefore represents the molecular link between the two previously known steps in the jasmonate pathway. Furthermore, we demonstrate the existence of a regulatory feed-back loop involving MYC2 and JAZ proteins, which provides a mechanistic explanation for the pulsed response to jasmonate and the subsequent desensitization of the cell.
Subject(s)
Arabidopsis Proteins/metabolism , Arabidopsis/drug effects , Arabidopsis/metabolism , Cyclopentanes/pharmacology , Multigene Family , Repressor Proteins/metabolism , Signal Transduction/drug effects , Arabidopsis/genetics , Arabidopsis Proteins/chemistry , Arabidopsis Proteins/genetics , Basic Helix-Loop-Helix Leucine Zipper Transcription Factors/genetics , Basic Helix-Loop-Helix Leucine Zipper Transcription Factors/metabolism , Feedback, Physiological , Gene Expression Regulation, Plant/drug effects , Oxylipins , Proteasome Endopeptidase Complex/metabolism , Protein Structure, Tertiary , Repressor Proteins/chemistry , Repressor Proteins/genetics , SKP Cullin F-Box Protein Ligases/metabolism , Transcriptional Activation/drug effectsABSTRACT
We isolated a full-length cDNA clone (StCDPK1) encoding a calcium-dependent protein kinase (CDPK) by screening a stolon tip cDNA library from potato plants (Solanum tuberosum L.). The predicted amino acid sequence of the cDNA reveals a high degree of similarity with other members of the CDPK family except in the N-terminal region. As described for other CDPKs, StCDPK1 has a putative N-terminal myristoylation sequence. A coupled transcription/translation system was used to demonstrate that this post-translational modification occurs in vitro. The behaviour of the myristoylated form of StCDPK1 during its purification on a phenyl-Sepharose column mimics that of the endogenous potato enzyme suggesting that this modification occurs in vivo. In addition, a possible palmitoylation site is present in StCDPK1. Southern blot analysis suggests that more than one CDPK isoform is present in potato plants. Northern blot analysis of steady-state mRNA levels for StCDPK1 in different tissues of potato plants shows that the transcript is differentially expressed in tuberizing stolons. The transcript appears in the early steps of tuber formation before the induction of other genes, such as Pin2 and patatin. This result parallels previous data on CDPK activity in potato plants which was highest at the beginning of tuberization. Our results suggest that StCDPK1 is developmentally regulated. The early and transient expression of this CDPK isoform in the tuberization process suggests that this kinase could trigger a cascade of phosphorylation events involved in tuber induction.
Subject(s)
Calcium-Binding Proteins/genetics , Plant Proteins , Protein Kinases/genetics , Solanum tuberosum/genetics , Amino Acid Sequence , Blotting, Northern , Blotting, Southern , Calcium-Binding Proteins/metabolism , Cloning, Molecular , DNA, Complementary/chemistry , DNA, Complementary/genetics , DNA, Plant/genetics , Gene Expression Regulation, Developmental , Gene Expression Regulation, Enzymologic , Gene Expression Regulation, Plant , Molecular Sequence Data , Myristic Acid/metabolism , Protein Kinases/metabolism , RNA, Messenger/genetics , RNA, Messenger/metabolism , RNA, Plant/genetics , RNA, Plant/metabolism , Sequence Alignment , Sequence Analysis, DNA , Sequence Homology, Amino Acid , Solanum tuberosum/enzymology , Solanum tuberosum/growth & developmentABSTRACT
Photosynthetic performance was monitored during two consecutive summers in four co-occurring evergreen Mediterranean tree species growing on a south-facing rocky slope. In response to midday water stress, the drought-avoiding species Pinus halepensis Mill. exhibited marked stomatal closure (g(s)) but no changes in stem water potential (Psi(s)), whereas the drought-tolerant species Quercus coccifera L., Q. ilex ssp. ballota (Desf.) Samp. and Juniperus phoenicea L. displayed declines in midday g(s) and Psi(s). The higher resistance to CO(2) influx in needles of P. halepensis compared with the other species did not result in either a proportional increase in non-radiative dissipation of excess energy or photo-inactivation of photosystem II (PSII). No significant differences were found among species either in the de-epoxidation state of the xanthophyll cycle (DPS) or in the pool of its components on a total chlorophyll basis (VAZ). Despite contrasting midday assimilation rates, the three drought-tolerant species all exhibited a pronounced drop in photochemical efficiency at midday that was characterized by a decrease in the excitation capture efficiency of the open PSII centers. Although photoinhibition was not fully reversed before dawn, it apparently did not result in cumulative photo-damage. Thus, the drought-avoiding and drought-tolerant species employed different mechanisms for coping with excess light during the midday depression in photosynthesis that involved contrasting midday photochemical efficiencies of PSII and different degrees of dynamic photoinhibition as a photo-protective mechanism. These behaviors may be related to the different mechanisms employed by drought-avoiding and drought-tolerant species to withstand water deficit.
