Androgenesis: Affecting the fate of the male gametophyte

Natural leaf senescence proceeds through an orderly program of events referred to, generally, as the 'senescence syndrome'. Leaf senescence consists of primarily, but not exclusively, a set of degradative and remobilization activities that salvage valuable nutrients by reallocation to the seeds or other viable parts of the plant. The program requires changes in gene expression and eventually culminates in death of the leaf or whole plant. Leaf/whole plant senescence has now been scrutinized extensively using molecular genetic approaches and a clearer picture of the events that comprise the developmental program is beginning to emerge. However, while understandings of the phenomenological aspects of the program have become apparent, the mechanistic aspects, particularly with regard to the processes required for induction and regulation of the program, are still far from clear. Molecular evidence suggests the process is complex in terms of the wide array of genes and activities expressed, and in terms of the overall regulation of progression of the events of the syndrome. This article attempts to review our current understanding of leaf senescence and includes a brief discussion of aspects of the process that require clarification if we are to more fully understand this complex developmental program.     

水稻叶片衰老相关基因的研究进展

水稻叶片的衰老是制约杂交稻产量提高的主要因素之一,有数据表明水稻籽粒灌浆所需营养物质的60%～80%来自叶片的光合作用,实践证明叶片每推迟1天衰老,产量可提高产1%左右.因此,对叶片衰老的形态、生理生化及其相关分子机理等进行研究具有重要的现实意义.近年来水稻叶片衰老的相关研究表明,叶片的衰老是一个受众多因素影响的复杂过程,在这个过程中叶片发生了巨大的形态与生理生化变化,而这些变化均离不开基因的调控作用.大量实验结果表明:在衰老过程中,叶片细胞有选择地启动或增强某些基因(叶片衰老相关基因)的表达,而关闭或减弱另一些基因(衰老下调基因)的表达,由此来调控叶片衰老的进程.目前研究者已在研究衰老突变体等相关的材料中发现了许多与水稻叶片衰老有关的基因.本文重点概述了近年来水稻叶片衰老相关基因的研究状况,并对未来研究方向等问题做了思考与探讨,以期能为开展进一步的研究工作提供参考.     

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.     

Natural developmental variations in leaf and plant senescence in Arabidopsis thaliana

Leaf senescence is a developmentally regulated process that contributes to nutrient redistribution during reproductive growth and finally leads to tissue death. Manipulating leaf senescence through breeding or genetic engineering may help to improve important agronomic traits, such as crop yield and the storage life of harvested organs. Here, we studied natural variations in the regulation of plant senescence among 16 Arabidopsis thaliana accessions. Chlorophyll content and the proportion of yellow leaves were used as indicator parameters to determine leaf and plant senescence respectively. Our study indicated significant genotype effects on the onset and development of senescence. We selected three late- and five early-senescence accessions for further physiological studies. The relationship between leaf and plant senescence was accession-dependent. There was a significant correlation between plant senescence and the total number of leaves, siliques and plant bolting age. We monitored expression of two senescence marker genes, SAG12 and WRKY53 , to evaluate progression of senescence. Our data revealed that chlorophyll content does not fully reflect leaf age, because even fully green leaves had already commenced senescence at the molecular level. Integrating senescence parameters, such as the proportion of senescent leaves, at the whole plant level provided a better indication of the molecular status of the plant than single leaf senescence parameters.     

Cellular senescence and cancer treatment

Cellular senescence, an irreversible cell-cycle arrest, reflects a safeguard program that limits the proliferative capacity of the cell exposed to endogenous or exogenous stress signals. A number of recent studies have clarified that an acutely inducible form of cellular senescence may act in response to oncogenic activation as a natural barrier to interrupt tumorigenesis at a premalignant level. Paralleling the increasing insights into premature senescence as a tumor suppressor mechanism, a growing line of evidence identifies cellular senescence as a critical effector program in response to DNA damaging chemotherapeutic agents. This review discusses molecular pathways to stress-induced senescence, the interference of a terminal arrest condition with clinical outcome, and the critical overlap between premature senescence and apoptosis as both tumor suppressive and drug-responsive cellular programs.     

植物叶片衰老的表观遗传调控

叶片是植物重要的光合器官, 它的衰老由外界环境刺激和内源发育信号所启动, 复杂的基因调控网络参与衰老过程的精确调控。最新研究表明, 植物通过对基因表达的重编程, 在表观遗传水平上调节着叶片衰老过程。该文简要介绍了表观遗传的分子机制, 在此基础上重点综述了组蛋白修饰、染色质重塑、DNA甲基化及小RNAs途径对叶片衰老调控的最新研究进展, 同时讨论了该领域存在的问题和未来研究方向。     

Molecular regulation of leaf senescence

Leaf senescence is a process of programmed cell death, which is induced in an age-dependent manner and by various environmental cues. The mechanisms that regulate the induction and progression of leaf senescence remain unclear because of their complexity. However, recent genetic and reverse-genetic approaches have identified key components of the regulation of leaf senescence and have revealed glimpses of the underlying molecular mechanisms.     

Epigenetic and small RNA regulation of senescence

Leaf senescence is regulated through a complex regulatory network triggered by internal and external signals for the reprogramming of gene expression. In plants, the major developmental phase transitions and stress responses are under epigenetic control. In this review, the underlying molecular mechanisms are briefly discussed and evidence is shown that epigenetic processes are also involved in the regulation of leaf senescence. Changes in the chromatin structure during senescence, differential histone modifications determining active and inactive sites at senescence-associated genes and DNA methylation are addressed. In addition, the role of small RNAs in senescence regulation is discussed.     

Identification of early senescence-associated genes in rice flag leaves

Leaf senescence is one of the key stages of plant leaf development. It is a highly complex but ordered process involving expression of large scale senescence associated genes, and its molecular mechanisms still remain unclear. By using suppression subtractive hybridization, 815 ESTs that are up-regulated at the onset of rice flag leaf senescence have been isolated. A total of 533 unigenes have been confirmed by macroarray detection and sequencing. 183 of these unigenes have GO annotations, involved in macromolecule metabolism, protein biosynthesis regulation, energy metabolism, gene expression regulations, detoxification, pathogenicity and stress, cytoskeleton organization and flower development. Another 121 unigenes co-localized with previously reported known stay-green QTLS. RT-PCR analysis on the other novel genes indicated that they can be up-regulated in natural early senescence and induced by hormone. Our results indicate that senescence is closely related to various metabolic pathways, thus providing new insight into the onset of leaf senescence mechanism. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

表观遗传调控植物叶片衰老的研究进展

植物叶片衰老是一个非常重要的发育过程,涉及大分子的有序分解从而将营养物质从叶片转移到其他器官,对植物的生存和适应至关重要。叶片衰老主要受植物的发育调控,但同时也受内部和外部环境因素的影响,涉及高度复杂的基因调控网络和多层级的调控。近年来的研究表明表观遗传是调控植物叶片衰老的一种重要调控方式。该研究综述了植物叶片衰老过程中的表观遗传调控机制,包括组蛋白修饰、DNA甲基化、ATP依赖的染色质重塑和非编码RNA介导的调控,并对该领域今后的发展方向进行了展望。     

SALT REGULATION AND LEAF SENESCENCE IN AGING LEAVES OF JAUMEA CARNOSA (LESS.) GRAY (ASTERACEAE), A SALT MARSH SPECIES EXPOSED TO NaCl STRESS

The effects of increasing salt stress on leaf senescence and salt regulation were investigated in the halophyte Jaumea carnosa in hydroponic culture experiments. The plants were grown in Hoagland's nutrient solution plus additional NaCl salt (0, 300, 400, 500 mm NaCl). Decreases in nucleic acids, protein, and chlorophyll were used as indicators of leaf senescence. The results indicated no definitive pattern of acceleration in leaf senescence with increasing salt stress. Salt regulation was also unaffected as leaves aged under increasing NaCl concentrations. The results are consistent with those of previous studies of the halophyte which indicated that the species was very tolerant of high NaCl concentrations.