Senescence In Arabidopsis Research Articles

Comparative transcriptome analysis reveals significant differences in gene expression and signalling pathways between developmental and dark/starvation‐induced senescence in Arabidopsis

An analysis of changes in global gene expression patterns during developmental leaf senescence in Arabidopsis has identified more than 800 genes that show a reproducible increase in transcript abundance. This extensive change illustrates the dramatic alterations in cell metabolism that underpin the developmental transition from a photosynthetically active leaf to a senescing organ which functions as a source of mobilizable nutrients. Comparison of changes in gene expression patterns during natural leaf senescence with those identified, when senescence is artificially induced in leaves induced to senesce by darkness or during sucrose starvation-induced senescence in cell suspension cultures, has shown not only similarities but also considerable differences. The data suggest that alternative pathways for essential metabolic processes such as nitrogen mobilization are used in different senescent systems. Gene expression patterns in the senescent cell suspension cultures are more similar to those for dark-induced senescence and this may be a consequence of sugar starvation in both tissues. Gene expression analysis in senescing leaves of plant lines defective in signalling pathways involving salicylic acid (SA), jasmonic acid (JA) and ethylene has shown that these three pathways are all required for expression of many genes during developmental senescence. The JA/ethylene pathways also appear to operate in regulating gene expression in dark-induced and cell suspension senescence whereas the SA pathway is not involved. The importance of the SA pathway in the senescence process is illustrated by the discovery that developmental leaf senescence, but not dark-induced senescence, is delayed in plants defective in the SA pathway.

Targets of the WRKY53 transcription factor and its role during leaf senescence in Arabidopsis

Targets of the WRKY53 transcription factor and its role during leaf senescence in Arabidopsis

Molecular analysis of programmed cell death during senescence in Arabidopsis thaliana and Brassica oleracea: cloning broccoli LSD1, Bax inhibitor and serine palmitoyltransferase homologues.

This study was undertaken to characterize the programmed cell death (PCD) processes that occur during detached and natural on-plant senescence and correlate them with the expression of putative regulatory genes that may be involved in the process. DNA fragmentation and TUNEL analysis of broccoli florets showed that DNA was processed into fragments of approximately 180 bp after 48 h of harvest-induced tissue senescence. Characteristic laddering patterns were also visible in Arabidopsis leaves undergoing natural on-plant senescence and during detached senescence. Several recently isolated plant proteins have been assigned a PCD role, for example, the zinc finger containing protein, LSD1 (lesion simulating disease); Bax inhibitor (BI); and serine palmitoyltransferase (SPT), an enzyme in the sphingolipid signalling pathway. Two cDNAs encoding each of these proteins were isolated from broccoli (BoBI-1, BoBI-2, BoLSD1, BoLSD2, BoSPT1, BoSPT2), and the mRNAs increased during harvest-induced senescence in floret tissue. Expression of the Arabidopsis homologues (AtBI-1, AtLSD1, AtSPT1) were also characterized during detached leaf senescence in Arabidopsis leaves. AtBI-1 expression was constitutively expressed during detached senescence, AtLSD1 expression remained constitutively low, and AtSPT1 expression increased during detached senescence.

SAG2 and SAG12 protein expression in senescing Arabidopsis plants

During leaf senescence, nutrients are remobilized from the senescing tissues to the growing parts of the plant. Many senescence‐associated genes (SAGs) were identified based on the induction of their transcripts. However, little is known about the protein expression of the corresponding genes. We have raised antibodies against two Arabidopsis SAGs, SAG2 and SAG12, which encode putative cysteine proteases. The SAG2 antibodies recognized a 29‐kDa protein that was abundant in senescing leaves, but was also present at low levels in green tissues. SAG12 antibodies labelled a 38‐kDa protein present only in senescent leaves. The protein expression of these SAGs parallels their mRNA expression patterns, indicating that control of SAG2 and SAG12 is at the level of transcription or transcript stability. In addition, we found that SAGs are induced during stem senescence with delayed kinetics of their expression relative to leaf expression, suggesting that age‐dependent factor(s) regulating the onset of senescence in Arabidopsis may act in tissue‐dependent manner.

Overexpression of a kinase-deficient form of the EDR1 gene enhances powdery mildew resistance and ethylene-induced senescence in Arabidopsis.

The EDR1 gene of Arabidopsis has previously been reported to encode a Raf-like mitogen-activated protein kinase kinase (MAPKK) kinase, and to function as a negative regulator of disease resistance. A phylogenetic analysis of plant and animal protein kinases revealed, however, that plant Raf-like kinases are more closely related to animal mixed lineage kinases (MLKs) than Raf-like kinases, and are deeply divergent from both classes of animal kinases, making inferences of substrate specificity questionable. We, therefore, assayed the kinase activity of recombinant EDR1 protein in vitro. The EDR1 kinase domain displayed autophosphorylation activity and phosphorylated the common MAP kinase substrate myelin basic protein. The EDR1 kinase domain also phosphorylated a kinase-deficient EDR1 protein, indicating that EDR1 autophosphorylation can occur via an intermolecular mechanism. Overexpression of a kinase-deficient full-length EDR1 gene (35S::dnEDR1) in wild-type Arabidopsis plants caused a dominant negative phenotype, conferring resistance to powdery mildew (Erysiphe cichoracearum) and enhancing ethylene-induced senescence. RNA-gel blot analyses showed that the 35S::dnEDR1 transgene was highly transcribed in transgenic plants. Western blot analysis, however, revealed that neither the wild-type nor mutant EDR1 protein could be detected in these lines, indicating that the dominant negative phenotype may be caused by a translational inhibition mechanism rather than by a protein level effect. Overexpression of orthologous dnEDR1 constructs may provide a novel strategy for controlling powdery mildew disease in crops.

A gene encoding an acyl hydrolase is involved in leaf senescence in Arabidopsis.

SAG101, a leaf senescence-associated gene, was cloned from an Arabidopsis leaf senescence enhancer trap line and functionally characterized. Reporter gene and RNA gel blot analyses revealed that SAG101 was not expressed until the onset of senescence in leaves. A recombinant SAG101 fusion protein overexpressed in Escherichia coli displayed acyl hydrolase activity. Antisense RNA interference in transgenic plants delayed the onset of leaf senescence for approximately 4 days. Chemically induced overexpression of SAG101 caused precocious senescence in both attached and detached leaves of transgenic Arabidopsis plants. These data suggest that SAG101 plays a significant role in leaf senescence.

Mutation of the Matrix Metalloproteinase At2-MMP Inhibits Growth and Causes Late Flowering and Early Senescence in Arabidopsis

This study characterizes the expression and functional significance of the member of the matrix metalloproteinase (MMP) family At2-MMP from Arabidopsis. By transcript analysis, expression of At2-MMP was found in leaves and roots of juvenile Arabidopsis and leaves, roots, and inflorescences of mature flowering plants showing strong increase of transcript abundance with aging. Cell specificity of expression of At2-MMP was studied by in situ hybridizations in leaves and flowers of Arabidopsis. In leaves, the gene was expressed in the phloem, in developing xylem elements, epidermal cells, and neighboring mesophyll cell layers. In flowers, signals were localized in pistils, ovules, and receptacles. In an Arabidopsis mutant (at2-mmp-1) carrying a tDNA insertion in At2-MMP, neither germination nor development of plants was modified in comparison to the wild type in the juvenile rosette stage. Starting with the onset of shoots, growth of roots, leaves, and shoots was inhibited compared with the wild type, and the plants were characterized by late flowering. Besides the flowering, at2-mmp-1 plants showed fast degradation of chlorophyll in leaves and early senescence. These results demonstrate the involvement of At2-MMP in plant growth, morphogenesis, and development with particular relevance for flowering and senescence.

Identification of BFN1, a bifunctional nuclease induced during leaf and stem senescence in Arabidopsis.

Nuclease I enzymes are responsible for the degradation of RNA and single-stranded DNA during several plant growth and developmental processes, including senescence. However, in the case of senescence the corresponding genes have not been reported. We describe the identification and characterization of BFN1 of Arabidopsis, and demonstrate that it is a senescence-associated nuclease I gene. BFN1 nuclease shows high similarity to the sequence of a barley nuclease induced during germination and a zinnia (Zinnia elegans) nuclease induced during xylogenesis. In transgenic plants overexpressing the BFN1 cDNA, a nuclease activity of about 38 kD was detected on both RNase and DNase activity gels. Levels of BFN1 mRNA were extremely low or undetectable in roots, leaves, and stems. In contrast, relatively high BFN1 mRNA levels were detected in flowers and during leaf and stem senescence. BFN1 nuclease activity was also induced during leaf and stem senescence. The strong response of the BFN1 gene to senescence indicated that it would be an excellent tool with which to study the mechanisms of senescence induction, as well as the role of the BFN1 enzyme in senescence using reverse genetic approaches in Arabidopsis.

Antisense suppression of phospholipase D alpha retards abscisic acid- and ethylene-promoted senescence of postharvest Arabidopsis leaves.

Membrane disruption has been proposed to be a key event in plant senescence, and phospholipase D (PLD; EC 3.1.4.4) has been thought to play an important role in membrane deterioration. We recently cloned and biochemically characterized three different PLDs from Arabidopsis. In this study, we investigated the role of the most prevalent phospholipid-hydrolyzing enzyme, PLD alpha, in membrane degradation and senescence in Arabidopsis. The expression of PLD alpha was suppressed by introducing a PLD alpha antisense cDNA fragment into Arabidopsis. When incubated with abscisic acid and ethylene, leaves detached from the PLD alpha-deficient transgenic plants showed a slower rate of senescence than did those from wild-type and transgenic control plants. The retardation of senescence was demonstrated by delayed leaf yellowing, lower ion leakage, greater photosynthetic activity, and higher content of chlorophyll and phospholipids in the PLD alpha antisense leaves than in those of the wild type. Treatment of detached leaves with abscisic acid and ethylene stimulated PLD alpha expression, as indicated by increases in PLD alpha mRNA, protein, and activity. In the absence of abscisic acid and ethylene, however, detached leaves from the PLD alpha-deficient and wild-type plants showed a similar rate of senescence. In addition, the suppression of PLD alpha did not alter natural plant growth and development. These data suggest that PLD alpha is an important mediator in phytohormone-promoted senescence in detached leaves but is not a direct promoter of natural senescence. The physiological relevance of these findings is discussed.

Identification of three genetic loci controlling leaf senescence in Arabidopsis thaliana.

Four mutants that show the delayed leaf senescence phenotype were isolated from Arabidopsis thaliana. Genetic analyses revealed that they are all monogenic recessive mutations and fall into three complementation groups, identifying three genetic loci controlling leaf senescence in Arabidopsis. Mutations in these loci cause delay in all senescence parameters examined, including chlorophyll content, photochemical efficiency of photosystem II, relative amount of the large subunit of Rubisco, and RNase and peroxidase activity. Delay of the senescence symptoms was observed during both age-dependent in planta senescence and dark-induced artificial senescence in all of the mutant plants. The results indicate that the three genes defined by the mutations are key genetic elements controlling functional leaf senescence and provide decisive genetic evidence that leaf senescence is a genetically programmed phenomenon controlled by several monogenic loci in Arabidopsis. The results further suggest that the three genes function at a common step of age-dependent and dark-induced senescence processes. It is further shown that one of the mutations is allelic to ein2-1, an ethylene-insensitive mutation, confirming the role of ethylene signal transduction pathway in leaf senescence of Arabidopsis.

Identification of three genetic loci controlling leaf senescence in Arabidopsis thaliana

SummaryFour mutants that show the delayed leaf senescence phenotype were isolated from Arabidopsis thaliana. Genetic analyses revealed that they are all monogenic recessive mutations and fall into three complementation groups, identifying three genetic loci controlling leaf senescence in Arabidopsis. Mutations in these loci cause delay in all senescence parameters examined, including chlorophyll content, photochemical efficiency of photosystem II, relative amount of the large subunit of Rubisco, and RNase and peroxidase activity. Delay of the senescence symptoms was observed during both age‐dependent in planta senescence and dark‐induced artificial senescence in all of the mutant plants. The results indicate that the three genes defined by the mutations are key genetic elements controlling functional leaf senescence and provide decisive genetic evidence that leaf senescence is a genetically programmed phenomenon controlled by several monogenic loci in Arabidopsis. The results further suggest that the three genes function at a common step of age‐dependent and dark‐induced senescence processes. It is further shown that one of the mutations is allelic to ein2‐1, an ethylene‐insensitive mutation, confirming the role of ethylene signal transduction pathway in leaf senescence of Arabidopsis.