Plant organ senescence – regulation by manifold pathways

Senescence is the final stage of plant ontogeny before death. Senescence may occur naturally because of age or may be induced by various endogenous and exogenous factors. Despite its destructive character, senescence is a precisely controlled process that follows a well‐defined order. It is often inseparable from programmed cell death (PCD), and a correlation between these processes has been confirmed during the senescence of leaves and petals. Despite suggestions that senescence and PCD are two separate processes, with PCD occurring after senescence, cell death responsible for senescence is accompanied by numerous changes at the cytological, physiological and molecular levels, similar to other types of PCD. Independent of the plant organ analysed, these changes are focused on initiating the processes of cellular structural degradation via fluctuations in phytohormone levels and the activation of specific genes. Cellular structural degradation is genetically programmed and dependent on autophagy. Phytohormones/plant regulators are heavily involved in regulating the senescence of plant organs and can either promote [ethylene, abscisic acid (ABA), jasmonic acid (JA), and polyamines (PAs)] or inhibit [cytokinins (CKs)] this process. Auxins and carbohydrates have been assigned a dual role in the regulation of senescence, and can both inhibit and stimulate the senescence process. In this review, we introduce the basic pathways that regulate senescence in plants and identify mechanisms involved in controlling senescence in ephemeral plant organs. Moreover, we demonstrate a universal nature of this process in different plant organs; despite this process occurring in organs that have completely different functions, it is very similar. Progress in this area is providing opportunities to revisit how, when and which way senescence is coordinated or decoupled by plant regulators in different organs and will provide a powerful tool for plant physiology research.     

Caspase-like proteases and their role in programmed cell death in plants

The investigations performed over recent few years have proved the existence of caspase-like proteases in plants. Three groups of caspase-like proteases: metacaspases, legumain family proteases (VPEs) and saspases have been identified and characterized in plants so far. A considerable amount of evidence supports the role of these enzymes in programmed cell death (PCD) occurring during plant development, their organ senescence as well as hypersensitive response (HR) after pathogen attack. Current knowledge of these enzyme molecular and biochemical structures is summarized in the paper. The homology of caspase-like proteases to animal caspases has been also indicated. Some future perspectives of research concerning the signal pathway during PCD, the regulation of activity and mode of action of these proteases are presented in the article.     

Plant autophagy puts the brakes on cell death by controlling salicylic acid signaling

It has long been recognized that autophagy in plants is important for nutrient recycling and plays a critical role in the ability of plants to adapt to environmental extremes such as nutrient deprivation. Recent reverse genetic studies, however, hint at other roles for autophagy, showing that autophagy defects in higher plants result in early senescence and excessive immunity-related programmed cell death (PCD), irrespective of nutrient conditions. Until now, the mechanisms by which cells die in the absence of autophagy were unclear. In our study, using biochemical, pharmacological and genetic approaches, we reveal that excessive salicylic acid (SA) signaling is a major factor in autophagy-defective plant-dependent cell death and that the SA signal can induce autophagy. These findings suggest a novel physiological function for plant autophagy that operates via a negative feedback loop to modulate proper SA signaling.     

Plant and animal transglutaminases: do similar functions imply similar structures?

In plants the post-translational modification of proteins by polyamines catalysed by transglutaminases has been studied since 1987; it was identified by the production of glutamyl-polyamine derivatives, biochemical features, recognition by animal antibodies and modification of typical animal substrates. Transglutaminases are widespread in all plant organs and cell compartments studied until now, chloroplast being the most studied. Substrates are: photosynthetic complexes and Rubisco in chloroplasts, cytoskeleton and cell wall proteins. Roles either specific of plants or in common with animals are related to photosynthesis, fertilisation, stresses, senescence and programmed cell death, showing that the catalytic function is conserved across the kingdoms. AtPng1p, the first plant transglutaminase sequenced shows undetectable sequence homology to the animal enzymes, except for the catalytic triad. It is, however, endowed with a calcium-dependent activity that allowed us to build a three-dimensional model adopting as a template the animal tranglutaminase 2.     

Flower bud opening and senescence in roses (Rosa hybrida L.)

The flower is the most significant and beautiful part of plants. Flowers are very useful organs in plant developmental phenomenon. During flower bud opening, various events takes place in a well defined sequence, representing all aspects of plant development, such as cell division, cellular differentiation, cell elongation or expansion and a wide spectrum of gene expression. The complexity of flower bud opening illustrates that various biological mechanisms are involved at different stages. Senescence represents the ultimate stage of floral development and results in wilting or abscission of whole flower or flower parts. Senescence is an active process and governed by a well defined cell death program. Once a flower bud opens, the programmed senescence of petal allows the removal of a metabolically active tissue. In leaves, this process can be reversed, but in floral tissue it cannot, indicating that a highly controlled genetic program for cell death is operating. The termination of a flower involves at least two, sometimes overlapping, mechanisms. In one, the perianth abscises before the majority of its cells initiate a cell death program. Abscission may occur before or during the mobilization of food reserves to other parts of the plant. Alternatively, the petals may be more persistent, so that cell deterioration and food remobilization occur while the petals are still part of the flower. The overall pattern of floral opening varies widely between plant genera, therefore, a number of senescence parameters have been used to group plants into somewhat arbitrary categories. Opening and senescence of rose flower is still an unsolved jigsaw in the world of floriculture industry and the mechanism behind the onset of the very early events in the sequence still remains to be elucidated. Hence, for advancing the knowledge on the pertinent aspect of bud opening and senescence the literature has been cited under this review.     

高等植物的PCD研究进展(一)

植物细胞程序性死亡(programmed cell death,PCD)已成为当前生物学的研究热点之一。植物PCD普遍存在于植物器官和个体生长发育过程及与环境相互作用过程中,具有重要的生物学意义。在高等植物生长发育过程中,根冠细胞、导管细胞、绒毡层细胞、胚乳细胞、胚柄细胞、糊粉细胞、大孢子细胞、助细胞和反足细胞等细胞在一定程度上均发生了PCD。另外,衰老也涉及PCD。本文综述了最近几年来与发育有关的PCD研究进展,主要包括高等植物细胞死亡的形式、起因及其PCD的形态、生化特征及高等植物营养器官(根、茎和叶)和生殖器官(花、果实和种子)在其生长发育过程中的PCD。文章最后还对植物PCD的进化和生物学意义作了进一步的讨论。 Abstract：Plant programmed cell death(PCD),the details of which are becoming a focus of intensive research in biology, is a ubiquitous phenomenon and plays an improtant biological role in the develpoment of organs and whole organisms and in interactions with the environment.During higher plant development,root cap cells,tracheary elements(TEs),tapetalcells,endosperm cells,suspensor cells,aleurone cells,megaspore cells,help cells and antipodal cells,etc.undergo PCD to some degree.In addition,senescence also involves PCD.This paper mainly reviewed PCD research progress in higher plant development in recent years,including forms and causes of cell death and PCD morphological and biochemical features in higher plants;PCD in development of nutritive organs(root ,stem and leaf) and reproductive organs(flower ,fruit and seed),evolution and biological rloes of plant PCD were further discussed in the paper.     

Markers for hypersensitive response and senescence show distinct patterns of expression

Controlled cellular suicide is an important process that can be observed in various organs during plant development. From the generation of proper sexual organs in monoecious plants to the hypersensitive response (HR) that occurs during incompatible pathogen interactions, programmed cell death (PCD) can be readily observed. Although several biochemical and morphological parameters have been described for various types of cell death in plants, the relationships existing between those different types of PCD events remain unclear. In this work, we set out to examine if two early molecular markers of HR cell death (HIN1 and HSR203J) as well as a senescence marker (SAG12) are coordinately induced during these processes. Our result indicates that although there is evidence of some cross-talk between both cell death pathways, spatial and temporal characteristics of activation for these markers during hypersensitive response and senescence are distinct. These observations indicate that these markers are relatively specific for different cell death programs. Interestingly, they also revealed that a senescence-like process seems to be triggered at the periphery of the HR necrotic lesion. This suggests that cells committed to die during the HR might release a signal able to induce senescence in the neighboring cells. This phenomenon could correspond to the establishment of a second barrier against pathogens. Lastly, we used those cell death markers to better characterize cell death induced by copper and we showed that this abiotic induced cell death presents similarities with HR cell death.     

Plant proteolytic enzymes: possible roles during programmed cell death

Proteolytic enzymes are known to be associated with developmentally programmed cell death during organ senescence and tracheary element differentiation. Recent evidence also links proteinases with some types of pathogen- and stress-induced cell suicide. The precise roles of proteinases in these and other plant programmed cell death processes are not understood, however. To provide a framework for consideration of the importance of proteinases during plant cell suicide, characteristics of the best-known proteinases from plants including subtilisin-type and papain-type enzymes, phytepsins, metalloproteinases and the 26S proteasome are summarized. Examples of serine, cysteine, aspartic, metallo- and threonine proteinases linked to animal programmed cell death are cited and the potential for plant proteinases to act as mediators of signal transduction and as effectors of programmed cell death is discussed.     

Melatonin: A master regulator of plant development and stress responses

Melatonin is a pleiotropic molecule with multiple functions in plants. Since the discovery of melatonin in plants, numerous studies have provided insight into the biosynthesis, catabolism, and physiological and biochemical functions of this important molecule. Here, we describe the biosynthesis of melatonin from tryptophan, as well as its various degradation pathways in plants. The identification of a putative melatonin receptor in plants has led to the hypothesis that melatonin is a hormone involved in regulating plant growth,aerial organ development, root morphology, and the floral transition. The universal antioxidant activity of melatonin and its role in preserving chlorophyll might explain its anti-senescence capacity in aging leaves. An impressive amount of research has focused on the role of melatonin in modulating postharvest fruit ripening by regulating the expression of ethylene-related genes.Recent evidence also indicated that melatonin functions in the plant's response to biotic stress,cooperating with other phytohormones and wellknown molecules such as reactive oxygen species and nitric oxide. Finally, great progress has been made towards understanding how melatonin alleviates the effects of various abiotic stresses, including salt, drought, extreme temperature, and heavy metal stress. Given its diverse roles, we propose that melatonin is a master regulator in plants.     

植物细胞程序性死亡中的类胱天蛋白酶研究进展

胱天蛋白酶(caspases)在动物细胞程序性死亡(programmed cell death,PCD)的起始、执行以及信号转导阶段起着关键作用,目前在植物中也发现有类胱天蛋白酶(caspase-like proteases,CLPs)的存在,并确认液泡加工酶(VPEs)、metacaspases和丝氨酸内肽酶(sapases)具有CLPs的作用,并证实CLPs参与植物的生长发育、抗病性及胁迫诱导的细胞程序性死亡等.本文对植物CLPs活性、生化结构及生理作用等方面的研究进展进行综述,并与动物caspases进行比较,为今后CLPs活性调节、作用方式及其在植物细胞程序性死亡中的作用等方面的研究提供参考.     

In vitro chili pepper biotechnology

Summary Chili pepper is an important horticultural crop that can surely benefit from plant biotechnology. However, although it is a Solanaceous member, developments in plant cell, tissue, and organ culture, as well as on plant genetic transformation, have lagged far behind those achieved for other members of the same family, such as tobacco (Nicotiana tabacum), tomato (Lycopersicon esculentum), and potato (Solanum tuberosum), species frequently used as model systems because of their facility to regenerate organs and eventually whole plants in vitro, and also for their ability to be genetically engineered by the currently available transformation methods. Capsicum members have been shown to be recalcitrant to differentiation and plant regeneration under in vitro conditions, which in turn makes it very difficult or inefficient to apply recombinant DNA technologies via genetic transformation aimed at genetic improvement against pests and diseases. Some approaches, however, have made possible the regeneration of chili pepper plants from in vitro-cultured cells, tissues, and organs through organogenesis or embryogenesis. Anther culture has been successfully applied to obtain haploid and doubledhaploid plants. Organogenic systems have been used for in vitro micropropagation as well as for genetic transformation. Application of both tissue culture and genetic transformation techniques have led to the development of chili pepper plants more resistant to at least one type of virus. Cell and tissue cultures have been applied successfully to the selection of variant cells exhibiting increased resistance to abiotic stresses, but no plants exhibiting the selected traits have been regenerated. Production of capsaicinoids, the hot principle of chili pepper fruits, by cells and callus tissues has been another area of intense research. The advances, limitations, and applications of chili pepper biotechnology are discussed.     

Molecular approaches for designing heat tolerant wheat

Global warming is causing changes in temperature rapidly for over two decades. The increased temperature during reproductive phase of plant growth has emerged as a serious problem all over the world. Constant or transitory high temperatures may affect the plant growth and development which may lead to diverse morphological, physiological and biochemical changes in plants ultimately decrease in yield. Genetic approaches leading to improved thermo-tolerance can mitigate the reduction in yield. In this backdrop, several indirect traits or parameters have been developed for identification of heat tolerant plants/lines. The traits like stay green/delayed senescence are reported to contribute toward capability of plants to tolerate heat stress. In addition, understanding of biochemical and molecular basis of thermo-tolerance in combination with genetic approaches like identification and mapping of heat tolerant QTLs will not only assist conventional breeders to develop heat tolerant cultivars but also help molecular biologists to clone and characterize genes associated with heat tolerance, which could be used in genetically modified heat tolerant plants. Therefore, overviews of different strategies for developing heat tolerant wheat are discussed in this review.     

Programmed Cell Death in Relation to Petal Senescence in Ornamental Plants

Cell death is a common event in all types of plant organisms. Understanding the phenomenon of programmed cell death (PCD) is an important area of research for plant scientists because of its role in senescence and the post-harvest quality of ornamentals, fruits, and vegetables. In the present paper, PCD in relation to petal senescence in ornamental plants is reviewed. Morphological, anatomical, physiological,and biochemical changes that are related to PCD in petals, such as water content, sink-source relationships,hormones, genes, and signal transduction pathways, are discussed. Several approaches to improving the quality of post-harvest ornamentals are reviewed and some prospects for future research are given.     

An evolutionary perspective on the 'death hormone'hypothesis in plants

Many plants die after first reproduction, as if committing suicide, a phenomenon known as 'monocarpic senescence'. The process is different from the ageing seen in most animals. A 'death hormone'has been proposed to explain the phenomenon, both for plants and also separately for animals. This paper considers for plants whether and how such a death hormone could have evolved.
Monocarpic senescence has sometimes been attributed to starvation of the vegetative parts for photosynthate or mineral nutrients, by nutrient diversion to developing fruit. In this case, no evolutionary problem arises. In other cases, the phenomenon seems to be one of programmed senescence, since details of the process conflict with an explanation in terms of nutrient diversion, since there is apparently a hormonal senescence signal, and since, e.g., the same type of senescence occurs even in male plants that do not bear fruit.
A range of evolutionary mechanisms are considered, by which a death hormone could have evolved: species selection, kin selection, direct Darwinian selection, pleiotropy, functional correlation, orthogenesis, genetic drift and founder effects. The two most likely are identified as: (1) kin selection, in which the mother plant dies, and thus facilitates establishment of her offspring, and (2) selection for efficiency of assimilate mobilisation. Neither of these explanations seems completely satisfactory.     

Transglutaminases: widespread cross-linking enzymes in plants

BACKGROUND: Transglutaminases have been studied in plants since 1987 in investigations aimed at interpreting some of the molecular mechanisms by which polyamines affect growth and differentiation. Transglutaminases are a widely distributed enzyme family catalysing a myriad of biological reactions in animals. In plants, the post-translational modification of proteins by polyamines forming inter- or intra-molecular cross-links has been the main transglutaminase reaction studied. CHARACTERISTICS OF PLANT TRANSGLUTAMINASES: The few plant transglutaminases sequenced so far have little sequence homology with the best-known animal enzymes, except for the catalytic triad; however, they share a possible structural homology. Proofs of their catalytic activity are: (a) their ability to produce glutamyl-polyamine derivatives; (b) their recognition by animal transglutaminase antibodies; and (c) biochemical features such as calcium-dependency, etc. However, many of their fundamental biochemical and physiological properties still remain elusive. TRANSGLUTAMINASE ACTIVITY IS UBIQUITOUS: It has been detected in algae and in angiosperms in different organs and sub-cellular compartments, chloroplasts being the best-studied organelles. POSSIBLE ROLES: Possible roles concern the structural modification of specific protein substrates. In chloroplasts, transglutaminases appear to stabilize the photosynthetic complexes and Rubisco, being regulated by light and other factors, and possibly exerting a positive effect on photosynthesis and photo-protection. In the cytosol, they modify cytoskeletal proteins. Preliminary reports suggest an involvement in the cell wall construction/organization. Other roles appear to be related to fertilization, abiotic and biotic stresses, senescence and programmed cell death, including the hypersensitive reaction. CONCLUSIONS: The widespread occurrence of transglutaminases activity in all organs and cell compartments studied suggests a relevance for their still incompletely defined physiological roles. At present, it is not possible to classify this enzyme family in plants owing to the scarcity of information on genes encoding them.     

Programmed Cell Death in Relation to Petal Senescence in Ornamental Plants

Cell death is a common event in all types of plant organisms. Understanding the phenomenon of programmed cell death (PCD) is an important area of research for plant scientists because of its role in senescence and the post-harvest quality of ornamentals, fruits, and vegetables. In the present paper, PCD in relation to petal senescence in ornamental plants is reviewed. Morphological, anatomical, physiological,and biochemical changes that are related to PCD in petals, such as water content, sink-source relationships,hormones, genes, and signal transduction pathways, are discussed, Several approaches to improving the quality of post-harvest ornamentals are reviewed and some prospects for future research are given.