Regulatory Genes and Intracellular Signaling

Grow Younger Blood

Longevity Health and Wellness Protocol

Get Instant Access

In order to understand senescence fully, it will be necessary to understand the regulation of the genes that participate in senescence, and this picture is beginning to emerge. In addition, these regulatory elements will be very useful in genetic engineering applications. For example, the isolation of the promoter for SAG12 has been extremely useful (Chapter 6).

Given that senescence is an active process involving induction as well as down-regulation of gene expression (Section II above), it is particularly significant that transcription factors are implicated in the senescence process. The mRNAs from microarray experiments have indicated that there are at least 402 distinct transcription factor genes of which 43 are induced during senescence and the WRKY proteins seem particularly important (Chen etal., 2002). WRKY proteins constitute a large family of plant-specific transcription factors in Arabidopsis, and all family members contain the WRKY domain, a 60-amino-acid domain with the conserved WRKYGQK motif at the N-terminal end, together with a novel zinc-finger motif (Eulgem et al, 2000). Two WRKY genes, AtWRKY6 and WRKY53, appear to be involved in the regulation of senescence. WRKY53 is expressed very early in leaf senescence but decreases again at a late stage (Hinderhofer and Zentgraf, 2001). This indicates that WRKY53 might play a regulatory role in the early events of leaf senescence. Significantly, several external and internal signals involved in triggering senescence influence the expression of AtWRKY6 (Robatzek and Somssich, 2001). AtWRKY6 may be a mediator for senescence, as its expression is not only strongly induced during leaf senescence, but is also associated with early to intermediate senescence stages.

A MADS domain-containing factor appears to relate to Arabidopsis flower senescence (Fang and Fernandez, 2002). A KN1 (see above) may be involved in maintaining regenerative growth (Hake et al., 1995).

The tomato SENU5 is a senescence-up regulated gene (John et al., 1997) and encodes a protein that belongs to the NAC domain family. The NAC domain family proteins include Petunia No Apical Meristem (NAM), Arabidopsis NAP (a target of the homeotic AP3/PI proteins), and GRAB (a protein interacting with Gemini virus RepA protein). Thus, this protein family appears to have regulatory roles in plant growth and differentiation (Xie et al., 1999). It is conceivable that the senescence-up regulated SENU5 gene belonging to this family may have a regulatory role in senescence.

Some senescence-related promoters have been found. For example, the mannopine synthase (mas) promoter is active in tobacco flower senescence but apparently not leaf senescence (Ursin and Shewmaker, 1993). The promoter for the senescence-related gene SAG12 encoding a cysteine protease in Arabidopsis has been isolated and characterized (Gan and Amasino, 1995; Noh and Amasino, 1999). In addition, the promoter for the gene oprl, a senescence-related gene that codes for the enzyme 12-oxo-phytodienoic acid-10,11-reductase, has been identified (He and Gan, 2001). Of particular interest is the report (Chen et al., 2002) that the promoter regions for 23 genes that are induced during leaf senescence contain the WRKY binding site. Using enhancer-trap lines in Arabidopsis, 125 senescence-associated promoters have been tagged, and these have revealed that the activity of the senescence-associated promoters may differ among different tissues (He et al., 2001).

Some information, albeit limited, is starting to emerge on the intracellular signaling pathway that induces senescence. A senescence-associated receptor-like kinase (SARK) gene is expressed exclusively during senescence in bean leaves, especially prior to the chlorophyll loss (Hajouj et al., 2000). Light and cytokinin delay SARK gene expression as they do senescence, but darkness and ethylene advance the initial appearance of the gene. Thus, SARK may play a role in the regulation of leaf senescence.

Using Arabidopsis as a model system, Nam et al. have undertaken a systematic genetic screening to identify the regulatory genes of senescence. The orel, ore3, and ore9 mutations were initially isolated based on delayed yellowing of leaf during in planta senescence or upon dark-induced senescence of detached leaves (Oh et al., 1997; Woo et al., 2002). The ore stands for oresara which means "long-living" in Korean. In addition to extending

Figure 5-1. Delayed senescence symptoms of the oresara mutants during in planta and dark-induced senescence. Chlorophyll content (upper) as a marker of catabolic activity and RNase activity (lower) as an indicator of anabolic activity were examined at several developmental ages of leaves in planta (left) or at the given times after incubating detached leaves in darkness (right) using the third and fourth foliar leaves of wild type (Col-0), oresaral, oresara3, and oresara9. Data were obtained from 12 independent leaves. Shown are relative values as percentage of the initial point value. The vertical bars denote standard deviations (Oh et al., 1997).

Figure 5-1. Delayed senescence symptoms of the oresara mutants during in planta and dark-induced senescence. Chlorophyll content (upper) as a marker of catabolic activity and RNase activity (lower) as an indicator of anabolic activity were examined at several developmental ages of leaves in planta (left) or at the given times after incubating detached leaves in darkness (right) using the third and fourth foliar leaves of wild type (Col-0), oresaral, oresara3, and oresara9. Data were obtained from 12 independent leaves. Shown are relative values as percentage of the initial point value. The vertical bars denote standard deviations (Oh et al., 1997).

leaf longevity, the ore mutations delayed the decrease of anabolic activities, including chlorophyll content, the photochemical efficiency of the photosystem II, and the relative amount of the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) as well as delaying increase in catabolic activities such as RNase and peroxidase activities during in planta senescence and dark-induced senescence (Fig. 5-1). All of the mutations are inherited as monogenic recessive traits and comprise three complementation groups.

The existence of ore1, ore3, and ore9 provides clear genetic evidence that leaf senescence in Arabidopsis is a genetically controlled event involving several monogenic genetic elements. Since these mutations affect a wide variety of senescence symptoms, these genes may be senescence regulatory genes. All of the mutations are recessive, suggesting that the genes defined by these mutations are positive regulators of senescence. The result also suggested that senescence could proceed through multiple pathways, since senescence is only delayed but not blocked in all of the mutants although there is a possibility that all of these mutations are weak alleles. Studies on expression of senescence-associated genes in the ore mutants will be needed to further define their role in controlling senescence. In addition, double mutant analysis will be required to elucidate the interaction among these genes in controlling senescence.

In addition to the delayed-senescence phenotype of the mutants in age and dark-induced senescence, these mutations delayed the responses to several phytohormones such as ethylene, abscisic acid (ABA), and methyl jasmonate (MeJA) (Woo, Park, and Nam, unpublished data). This suggests that ORE1, ORE3, and ORE9 may function at a common step of senescence affected by these factors. The ORE1, ORE3, and ORE9 genes may be required for proper progression of leaf senescence induced by the phytohormones, as well as age and darkness (Fig. 5-1). In fact, ore3, also designated as ein2-34, is an allele of ein2 (ethylene-insensitive2) mutations that are known to affect seedling responses to various hormones (see Section IVC). In contrast, the other mutants do not have a defect in general hormone responses but in hormone-induced senescence.

One of the most interesting glimpses of the regulatory cascade for senescence and findings to come is offered by ore9 (Woo et al., 2001). The ore9 gene encodes a protein containing an F-box motif and 18 leucine-rich repeats. The F-box motif of ORE9 protein interacts with ASK1, which implicates ubiquitination in the senescence process. It was thus suggested that ORE9 functions to limit leaf longevity by removing, through ubiquitin-dependent proteol-ysis, target proteins that are required to delay the leaf senescence program in Arabidopsis (Woo et al., 2001). This view is consistent with a report that proteolysis by the N-end rule pathway, one of the ubiquitin pathways, appears to be a mechanism involved in regulation of leaf senescence in Arabidopsis. The delayed-leaf-senescence 1 (dls1) mutant showed a delay of leaf senescence symptoms (Yoshida et al., 2002a). The mutant is due to a defect in arginyl tRNA:protein transferase (R-transferase), a component of the N-end rule proteolytic pathway that transfers arginine to the amino terminus of proteins with amino terminal glutamyl or aspartyl residues, thereby targeting the proteins for ubiquitin-dependent prote-olysis. Like ORE9, DLS1 was suggested to have a role in senescence by degrading target proteins that negatively regulate leaf senescence.

Another interesting insight into the regulation of senescence comes from ore4-1. This mutation inhibits leaf senescence, yellowing, and death, and reduces the leaf growth rate (Woo et al., 2002). Interestingly, the mutation delayed natural leaf senescence but not hormone or dark-induced senescence. The ore4-1 mutant has a partial lesion in the chloroplast function including the function of photosystem I, which resulted from reduced expression of the plastid ribosomal protein small subunit 17 (PRPS17) gene. It is conceivable that the delayed leaf senescence phenotype observed in the ore4-1 mutant is due to reduced metabolic rate, since the chloroplasts, a major energy source for plant growth through photosynthesis, are only partially functioning in the mutant. This interpretation is consistent with findings that metabolic rate is one of the key mechanisms involved in animal aging, although further evidence is needed.

The onset of leaf senescence was reported to be associated with a complex formation between ATBP1/ATBP2 and telomeric DNA (Zentgraf et al., 2000). This opens up the possibility that telomeric structure could be involved in post-mitotic senescence in plants.

Was this article helpful?

0 0
The Latest Anti Aging Treatments

The Latest Anti Aging Treatments

Are You Striving To Look And Feel Youthful? Wish You Could Add 20 Years To Your Life? Discover the Secrets to a Longer, Healthier Life With This Fantastic Anti-Aging Resource. You might be feeling and looking great now, but have you ever thought about what youll feel and look like several years from now? Have you ever considered that the choices you make today directly influence how well you age?

Get My Free Ebook


Post a comment