ZMBP Project Group Ulrike Zentgraf
Leaf senescence of annual plants
Department of Genetics
University of Tübingen
Auf der Morgenstelle 32
Senescence is a key process that underpins many important agronomic traits including yield outcomes and pre- and post-harvest quality. Senescence is a highly regulated and organized process during which macromolecules of the mature green leaf are dismantled and remobilized for further use by the plant, either for growth or for deposition in developing fruits and seeds. Efficient senescence is essential for the overall success of the plant and its following generations; however, premature senescence, induced by stress, leads to reduced yield and quality of crop plants. Abiotic stress is estimated to be the primary cause of crop loss worldwide, with the potential to cause a reduction of more than 50% in the average yield of the main crops.
In general, selective activation of gene expression but also the decrease of certain RNAs and/or proteins initiate and regulate this process. The gene products are involved in the breakdown of macromolecules and the mobilization of nutrients out of the senescing tissues. The rapid loss of chlorophyll, the lowering of protein and RNA levels, activation of autophagy or the leakiness of the cell membranes are used as senescence markers. Despite the importance of the senescence processes in plants, our knowledge on the signals and regulatory mechanisms is still limited.
In order to characterize factors which are involved in the onset and regulation of senescence two main approaches are followed:
a) Involvement of reactive oxygen species, especially hydrogen peroxide, in regulating leaf senescence
b) Senescence-associated gene expression: Regulatory network controlling expression, activity, and degradation of the transcription factor WRKY53 in Arabidopsis leaves.
a) Free radicals are thought to play an essential role in senescence, especially those derived from oxygen. Increased levels of activated oxygen measured in senescing tissues could either occur through an enhanced production of activated species or a decline of the various defense systems that normally afford protection against oxidative injury. Both mechanisms seem to be realized in senescing tissues, so that investigating the regulation of these scavenging systems probably offers an opportunity to characterize factors and parameters regulating senescence. Especially hydrogen peroxide is discussed to be a signaling molecule in Arabidopsis thaliana leaf senescence. Intracellular hydrogen peroxide levels are controlled by the hydrogen peroxide scavenging enzyme catalase in concert with other scavenging and producing systems. The coordinated regulation of these hydrogen peroxide scavenging enzymes on the transcriptional and posttranscriptional level creates a distinct increase of hydrogen peroxide at the time point when the plants start to bolt and a coordinated senescence process of all rosette leaves should be induced (Zimmermann et al., 2006). Conversely, it was already shown that the senescence regulating transcription factor WRKY53 and its regulators as well as other senescence-associated transcription factors and SAGs can be induced by hydrogen peroxide (Miao et al., 2004) so that the hydrogen peroxide peak during bolting time most likely serves as a signal to induce senescence-associated gene transcription. Since transcriptional down-regulation of CAT2 appears to be the initial step to create this senescence-promoting signal, we identified G-box binding factor 1 (GBF1) as a negative regulator for CAT2 transcription. In gbf1 plants, CAT2 decrease in expression and activity and the thereby created H2O2 peak disappeared. Consequently, gbf1 plants showed a delayed senescence phenotype and an affected expression of SAGs.
The differential expression of genes according to the age of the leaves within one rosette was also analyzed using suppression subtractive hybridization (SSH). A transcription factor of the WRKY family, WRKY53, was isolated in this screen (Hinderhofer et al., 2001).
To understand the regulatory role of the senescence related WRKY53 factor, we identified targets of this transcription factor by a pull down assay using genomic DNA and recombinant WRKY53 protein. We isolated a number of candidate target genes including other transcription factors, also of the WRKY family, stress- and defence related genes, and senescence-associated genes (SAGs). WRKY53 protein could bind to these different promoters in vitro and in vivo and it could act either as transcriptional activator or transcriptional repressor depending on the sequences surrounding the W-boxes. WRKY53 can be induced by H2O2 and can regulate its own expression in a negative feed-back loop (Miao et al., 2004). Our results suggest that WRKY53 acts in a complex transcription factor signaling network regulating senescence specific gene expression and hydrogen peroxide might be involved in signal transduction.
WRKY53 is tightly regulated on different levels: i) activity, positively by phosphorylation (MEKK1) and negatively by blocking the DNA-binding (ESR/ESP), ii) gene expression which is regulated by at least four proteins that bind to the promoter of WRKY53 and iii) degradation of the protein by a HECT E3 ubiquitin ligase (Miao and Zentgraf, 2007, Miao et al., 2007, Miao et al., 2008, Miao and Zentgraf, 2010, for review see Zentgraf et al., 2010, Potschin et al., 2014).