Programmed senescence of plant organs

The senescence of plant organs associated with reproductive development has been studied extensively during the past century, and it has long been recognized that this type of death is internally programmed. The regulation of organ senescence as well as its biochemical and genetic determinants has been an historically rich area of research. Certain plant hormones have been implicated as regulators or modulators of organ senescence and many of the biochemical pathways associated with the senescence syndrome have been elucidated. The genetic basis of organ senescence has also been well established by the identification of mutations that impair the senescence program and recently, transgenic plants have been used to critically determine the role of specific enzymes and hormonal signals in mediating programmed senescence of plant organs. Here, we review the current understanding of the processes that regulate leaf, flower and fruit senescence, emphasizing the role that programmed organ senescence plays in the adaptive fitness of plants.     

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 .     

ORE9, an F-box protein that regulates leaf senescence in Arabidopsis

Senescence is a sequence of biochemical and physiological events that constitute the final stage of development. The identification of genes that alter senescence has practical value and is helpful in revealing pathways that influence senescence. However, the genetic mechanisms of senescence are largely unknown. The leaf of the oresara9 (ore9) mutant of Arabidopsis exhibits increased longevity during age-dependent natural senescence by delaying the onset of various senescence symptoms. It also displays delayed senescence symptoms during hormone-modulated senescence. Map-based cloning of ORE9 identified a 693-amino acid polypeptide containing an F-box motif and 18 leucine-rich repeats. The F-box motif of ORE9 interacts with ASK1 (Arabidopsis Skp1-like 1), a component of the plant SCF complex. These results suggest that ORE9 functions to limit leaf longevity by removing, through ubiquitin-dependent proteolysis, target proteins that are required to delay the leaf senescence program in Arabidopsis.     

Evidence for a genetic basis of aging in two wild vertebrate populations

Aging, or senescence, defined as a decline in physiological function with age, has long been a focus of research interest for evolutionary biologists. How has natural selection failed to remove genetic effects responsible for such reduced fitness among older individuals? Current evolutionary theory explains this phenomenon by showing that, as a result of the risk of death from environmental causes that individuals experience, the force of selection inevitably weakens with age. This in turn means that genetic mutations having detrimental effects that are only felt late in life might persist in a population. Although widely accepted, this theory rests on the assumption that there is genetic variation for aging in natural systems, or (equivalently), that genotype-by-age interactions (GxA) occur for fitness. To date, empirical support for this assumption has come almost entirely from laboratory studies on invertebrate systems, most notably Drosophila and C. elegans, whereas tests of genetic variation for aging are largely lacking from natural populations. By using data from two wild mammal populations, we perform quantitative genetic analyses of fitness and provide the first evidence for a genetic basis of senescence to come from a study in the natural environment. We find evidence that genetic differences among individuals cause variation in their rates of aging and that additive genetic variance for fitness increases with age, as predicted by the evolutionary theory of senescence.     

EXPERIMENTAL EVOLUTION OF SENESCENCE: AN ANALYSIS USING A “HETEROGENEITY” MORTALITY MODEL

A long-term laboratory selection experiment has produced replicated populations of fruit flies that differ in mean life span by more than twofold. An analysis of age-specific mortality rates indicated that differences in mean life span have been achieved principally by evolution of patterns of senescence. These results provide empirical confirmation that senescence can be modified within species by appropriate forms of natural selection, which is a fundamental prediction of theories regarding the genetic basis and evolution of senescence. Mortality data were fit to a model that accounts for the leveling off of cohort mortality rates at older ages, but that does not necessarily imply that very old individuals cease to senesce.     

Anthesis date mainly explained correlations between post-anthesis leaf senescence, grain yield, and grain protein concentration in a winter wheat population segregating for flowering time QTLs

The genetic variability of the duration of leaf senescence during grain filling has been shown to affect both carbon and nitrogen acquisition. In particular, maintaining green leaves during grain filling possibly leads to increased grain yield, but its associated effect on grain protein concentration has not been studied. The aim of this study was to dissect the genetic factors contributing to correlations observed at the phenotypic level between leaf senescence during grain filling, grain protein concentration, and grain yield in winter wheat. With this aim in view, an analysis of quantitative trait locus (QTL) co-locations for these traits was carried out on a doubled haploid mapping population grown in a large multienvironment trial network. Pleiotropic QTLs affecting leaf senescence and grain yield and/or grain protein concentration were identified on chromosomes 2D, 2A, and 7D. These were associated with QTLs for anthesis date, showing that the phenotypic correlations with leaf senescence were mainly explained by flowering time in this wheat population. Study of the allelic effects of these pleiotropic QTLs showed that delaying leaf senescence was associated with increased grain yield or grain protein concentration depending on the environments considered. It is proposed that this differential effect of delaying leaf senescence on grain yield and grain protein concentration might be related to the nitrogen availability during the post-anthesis period. It is concluded that the benefit of using leaf senescence as a selection criterion to improve grain protein concentration in wheat cultivars may be limited and would largely depend on the targeted environments, particularly on their nitrogen availability during the post-anthesis period.     

The Length of the Shortest Telomere as the Major Determinant of the Onset of Replicative Senescence

The absence of telomerase in many eukaryotes leads to the gradual shortening of telomeres, causing replicative senescence. In humans, this proliferation barrier constitutes a tumor suppressor mechanism and may be involved in cellular aging. Yet the heterogeneity of the senescence phenotype has hindered the understanding of its onset. Here we investigated the regulation of telomere length and its control of senescence heterogeneity. Because the length of the shortest telomeres can potentially regulate cell fate, we focus on their dynamics in Saccharomyces cerevisiae. We developed a stochastic model of telomere dynamics built on the protein-counting model, where an increasing number of protein-bound telomeric repeats shift telomeres into a nonextendable state by telomerase. Using numerical simulations, we found that the length of the shortest telomere is well separated from the length of the others, suggesting a prominent role in triggering senescence. We evaluated this possibility using classical genetic analyses of tetrads, combined with a quantitative and sensitive assay for senescence. In contrast to mitosis of telomerase-negative cells, which produces two cells with identical senescence onset, meiosis is able to segregate a determinant of senescence onset among the telomerase-negative spores. The frequency of such segregation is in accordance with this determinant being the length of the shortest telomere. Taken together, our results substantiate the length of the shortest telomere as being the key genetic marker determining senescence onset in S. cerevisiae.     

Natural variation in the regulation of leaf senescence and relation to other traits in Arabidopsis

Leaf senescence results in the recycling of nutrients, thereby providing resources required for growth and reproduction. In this study, the effect of day-length on leaf senescence in eight different Arabidopsis thaliana ecotypes was determined and the relationship between senescence and other morphological and life history traits was analysed. A significant variation in the start and extent of leaf senescence depending on the genetic background and the response to day-length was found. Whereas senescence of early flowering ecotypes was accelerated by long days, no effect of day-length on senescence could be found in late flowering Kas-1 plants. Senescence in the different ecotypes was associated with other traits, such as floral transition, the total number of fruits, the total number of leaves and the maximum chlorophyll content. Plants that bolted early also senesced early, produced fewer leaves, accumulated less chlorophyll, but produced more fruits. The present results indicate that senescence may be a key component in the trade-off between investment in photosynthetic capacity and reproduction. The relationship between senescence and other traits was maintained independent of whether differences in senescence were caused by genetic (ecotype) or environmental (day-length) variation, suggesting that genetic and environmental factors affect these traits through common regulatory pathways.     

Demographic variability and heterogeneity among individuals within and among clonal bacteria strains

Identifying what drives individual heterogeneity has been of long interest to ecologists, evolutionary biologists and biodemographers, because only such identification provides deeper understanding of ecological and evolutionary population dynamics. In natural populations one is challenged to accurately decompose the drivers of heterogeneity among individuals as genetically fixed or selectively neutral. Rather than working on wild populations we present here data from a simple bacterial system in the lab, Escherichia coli. Our system, based on cutting‐edge microfluidic techniques, provides high control over the genotype and the environment. It therefore allows to unambiguously decompose and quantify fixed genetic variability and dynamic stochastic variability among individuals. We show that within clonal individual variability (dynamic heterogeneity) in lifespan and lifetime reproduction is dominating at about 90–92%, over the 8–10% genetically (adaptive fixed) driven differences. The genetic differences among the clonal strains still lead to substantial variability in population growth rates (fitness), but, as well understood based on foundational work in population genetics, the within strain neutral variability slows adaptive change, by enhancing genetic drift, and lowering overall population growth. We also revealed a surprising diversity in senescence patterns among the clonal strains, which indicates diverse underlying cell‐intrinsic processes that shape these demographic patterns. Such diversity is surprising since all cells belong to the same bacteria species, E. coli, and still exhibit patterns such as classical senescence, non‐senescence, or negative senescence. We end by discussing whether similar levels of non‐genetic variability might be detected in other systems and close by stating the open questions how such heterogeneity is maintained, how it has evolved, and whether it is adaptive.     

Small non-coding RNAome changes during human chondrocyte senescence as potential epigenetic targets in age-related osteoarthritis

Chondrocyte senescence is a decisive component of age-related osteoarthritis, however, the function of small noncoding RNAs (sncRNAs) in chondrocyte senescence remains underexplored. Human hip joint cartilage chondrocytes were cultivated up to passage 4 to induce senescence. RNA samples were extracted and then analyzed using small RNA sequencing and qPCR. β-galactosidase staining was used to detect the effect of sncRNA on chondrocyte aging. Results of small RNA sequencing showed that 279 miRNAs, 136 snoRNAs, 30 snRNAs, 102 piRNAs, and 5 rasiRNAs were differentially expressed in senescent chondrocytes. The differential expression of 150 sncRNAs was further validated by qPCR. Transfection of sncRNAs and β-galactosidase staining were also performed to further revealed that hsa-miR-135b-5p, SNORA80B-201, and RNU5E-1-201 have the function to restrain chondrocyte senescence, while has-piR-019102 has the function to promote chondrocyte senescence. Our data suggest that sncRNAs have therapeutic potential as novel epigenetic targets in age-related osteoarthritis.     

Arabidopsis onset of leaf death mutants identify a regulatory pathway controlling leaf senescence

The onset of leaf senescence is controlled by leaf age and ethylene can promote leaf senescence within a specific age window. We exploited the interaction between leaf age and ethylene and isolated mutants with altered leaf senescence that are named as onset of leaf death (old) mutants. Early leaf senescence mutants representing three genetic loci were selected and their senescence syndromes were characterised using phenotypical, physiological and molecular markers. old1 is represented by three recessive alleles and displayed earlier senescence both in air and upon ethylene exposure. The etiolated old1 seedlings exhibited a hypersensitive triple response. old2 is a dominant trait and the mutant plants were indistinguishable from the wild-type when grown in air but showed an earlier senescence syndrome upon ethylene treatment. old3 is a semi-dominant trait and its earlier onset of senescence is independent of ethylene treatment. Analyses of the chlorophyll degradation, ion leakage and SAG expression showed that leaf senescence was advanced in ethylene-treated old2 plants and in both air-grown and ethylene-treated old1 and old3 plants. Epistatic analysis indicated that OLD1 might act downstream of OLD2 and upstream of OLD3 and mediate the interaction between leaf age and ethylene. A genetic model was proposed that links the three OLD genes and ethylene into a regulatory pathway controlling the onset of leaf senescence.     

Over-expression of Arabidopsis Bax inhibitor-1 delays methyl jasmonate-induced leaf senescence by suppressing the activation of MAP kinase 6

Methyl jasmonate (MeJA) is an important signalling molecule that has been reported to be able to promote plant senescence. The cell death suppressor Bax inhibitor-1 (BI1) has been found to suppress stress factor-mediated cell death in yeast and Arabidopsis. However, the effect and the genetic mechanism of Arabidopsis thaliana BI1 (AtBI1) on leaf senescence remain unclear. It was found here that the AtBI1 mutant, atbi1-2 (a gene knock-out), showed accelerated progression of MeJA-induced leaf senescence, while the AtBI1 complementation lines displayed similar symptoms as the WT during the senescence process. In addition, over-expression of the AtBI1 gene delayed the onset of MeJA-induced leaf senescence. Further analyses showed that during the process of MeJA-induced senescence, the activity of MPK6, a mitogen-activated protein kinase (MAPK), increased in WT plants, whereas it was significantly suppressed in AtBI1-overexpressing plants. Under the MeJA treatment, cytosolic calcium ([Ca(2+)](cyt)) functioned upstream of MPK6 activation and the elevation of [Ca(2+)](cyt) was reduced in AtBI1-overexpressing leaves. These results suggested a role of AtBI1 over-expression in delaying MeJA-induced leaf senescence by suppressing the [Ca(2+)](cyt)-dependent activation of MPK6, thus providing a new insight into the function and mechanism of AtBI1 in plant senescence.     

The role of sugars in integrating environmental signals during the regulation of leaf senescence

Although leaf senescence results in a loss of photosynthetic carbon fixation, the senescence-dependent release of nutrients, especially of nitrogen, is important for the growth of young leaves and for reproduction. Environmental regulation of senescence is therefore a vital factor in the carbon and nitrogen economy of plants. Leaf senescence is a highly plastic trait that is affected by a range of different environmental factors including light, nutrient supply, CO2 concentration, and abiotic and biotic stress. In this review, the focus is on the impact of environmental conditions on sugar accumulation and sugar signalling during senescence. By signalling a high availability of carbon relative to nitrogen in the old leaves, sugar accumulation can trigger leaf senescence. Sugar-induced senescence is therefore particularly important under low nitrogen availability and may also play a role in light signalling. Whether or not sugars are involved in regulating the senescence response of plants to elevated CO2 remains unresolved. Senescence can be delayed or accelerated in elevated CO2 and no clear relationship between sugar accumulation and senescence has been found. Plasticity in the response to environmental factors, such as daylength and sugar accumulation, varies between different Arabidopsis accessions. This natural variation can be exploited to analyse the genetic basis of the regulation of senescence and the consequences for growth and fecundity. Different evolutionary strategies, i.e. early senescence combined with a high reproductive effort or late senescence combined with a low reproductive effort, may be an important adaptation of Arabidopsis accessions to their natural habitat.     

Ageing in Plants: Conserved Strategies and Novel Pathways

Abstract: Ageing increases chaos and entropy and ultimately leads to the death of living organisms. Nevertheless, single gene mutations substantially alter lifespan, revealing that ageing is subject to genetic control. In higher plants, ageing is most obviously manifested by the senescence of leaves, and recent molecular genetic studies, in particular the isolation of Arabidopsis mutants with altered leaf senescence, have greatly advanced our understanding of ageing regulation in plants. This paper provides an overview of the identified genes and their respective molecular pathways. Hormones, metabolic flux, reactive oxygen species and protein degradation are prominent strategies employed by plants to control leaf senescence. Plants predominantly use similar ageing-regulating strategies as yeast and animals but have evolved different molecular pathways. The senescence window concept is proposed to describe the age-dependent actions of the regulatory genes. It is concluded that the similarities and differences in ageing between plants and other organisms are deeply rooted in the evolution of ageing and we hope to stimulate discussion and research in the fascinating field of leaf senescence.