Abiotic and Biotic Stresses and Changes in the Lignin Content and Composition in Plants

Lignin is a polymer of phenylpropanoid compounds formed through a complex biosynthesis route,represented by a metabolic grid for which most of the genes involved have been sequenced in several plants,mainly in the model-plants Arabidopsis thaliana and Populus.Plants are exposed to different stresses,which may change lignin content and composition.In many cases,particularly for plant-microbe interactions,this has been suggested as defence responses of plants to the stress.Thus,understanding how a stressor modulates expression of the genes related with lignin biosynthesis may allow us to develop study-models to increase our knowledge on the metabolic control of lignin deposition in the cell wall.This review focuses on recent literature reporting on the main types of abiotic and biotic stresses that alter the biosynthesis of lignin in plants.     

Tyrosine metabolic enzymes from insects and mammals：A comparative perspective

Differences in the metabolism of tyrosine between insects and mammals present an interesting example of molecular evolution. Both insects and mammals possess finetuned systems of enzymes to meet their specific demands for tyrosine metabolites; however, more homologous enzymes involved in tyrosine metabolism have emerged in many insect species. Without knowledge of modem genomics, one might suppose that mammals, which are generally more complex than insects and require tyrosine as a precur sor for important catecholamine neurotransmitters and for melanin, should possess more enzymes to control tyrosine metabolism. Therefore, the question of why insects actually possess more tyrosine metabolic enzymes is quite interesting. It has long been known that insects rely heavily on tyrosine metabolism for cuticle hardening and for innate immune responses, and these evolutionary constraints are likely the key answers to this question. In terms of melanogenesis, mammals also possess a high level of regulation; yet mam malian systems possess more mechanisms for detoxification whereas insects accelerate pathways like melanogenesis and therefore must bear increased oxidative pressure. Our research group has had the opportunity to characterize the structure and function of many key proteins involved in tyrosine metabolism from both insects and mammals. In this mini review we will give a brief overview of our research on tyrosine metabolic enzymes in the scope of an evolutionary perspective of mammals in comparison to insects.     

Putative Nitrogen Sensing Systems in Higher Plants

Nitrogen （N） metabolism is essential for the biosynthesis of vital biomolecules. N status thus exerts profound effects on plant growth and development, and must be closely monitored. In bacteria and fungi, a few sophisticated N sensing systems have been extensively studied. In animals, the ability to receive amino acid signals has evolved to become an integral part of the nervous coordination system. In this review, we will summarize recent developments in the search for putative N sensing systems in higher plants based on homologous systems in bacteria, fungi, and animals. Apparently, although plants have separated and diversified from other organisms during the evolution process, striking similarities can be found in their N sensing systems compared with those of their counterparts; however, our understanding of these systems is still incomplete. Significant modifications of the N sensing systems （including cross-talk with other signal transduction pathways） in higher plants may be a strategy of adaptation to their unique mode of life.     

A change of course： JIPB to focus on fundamental questions in plant sciences

Benefited from the speedy development of omic tools and the use of model plants, such as rice, Arabidopsis and Medicargo, plant biologists have recently made tremendous breakthroughs in understanding plant growth, development, and response to environmental changes. Many fundamental questions, such as flower initiation, light signal transduction, intercellular communication, genomic composition and epigenetic regulation, etc,, have been resolved during the past two decades, Several dedicated plant science journals, such as The Plant Cell, The Plant Journal and Plant ＆ Cell Biology contributed to this progress through the publication of seminal articles that allowed scientists in the community to communicate with each other, to share their expertise and experimental materials.     

Adenine Phosphoribosyl Transferase 1 is a Key Enzyme Catalyzing Cytokinin Conversion from Nucleobases to Nucleotides in Arabidopsis

In plants, the cytokinin metabolic processes, including cytokinin biosynthesis, interconversion, inactivation, and degradation, play critical roles in the regulation of cytokinin homeostasis and plant development. Purine meta- bolic enzymes have been implied to catalyze the cytokinin interconversion in previous works. In this study, we report that Adenine Phosphoribosyl Transferase 1 （APT1） is the causal gene of the high-dose cytokinin-resistant mutants. APT1 catalyzes the cytokinin conversion from free bases to nucleotides, and is functionally predominant among the five members of the Arabidopsis Adenine Phosphoribosyl Transferase family. Loss of APT1 activity in plants leads to excess accumulation of cytokinin bases, thus evoking myriad cytokinin-regulated responses, such as delayed leaf senescence, anthocyanin accumulation, and downstream gene expression. Thus, our study defines APT1 as a key metabolic enzyme participating in the cytokinin inactivation by phosphoribosylation.     

Hydrogen Peroxide in Plants： a Versatile Molecule of the Reactive Oxygen Species Network

Plants often face the challenge of severe environmental conditions, which include various biotic and abiotic stresses that exert adverse effects on plant growth and development. During evolution, plants have evolved complex regulatory mechanisms to adapt to various environmental stressors. One of the consequences of stress is an increase in the cellular concentration of reactive oxygen species （ROS）, which are subsequently converted to hydrogen peroxide （H2O2）. Even under normal conditions, higher plants produce ROS during metabolic processes. Excess concentrations of ROS result in oxidative damage to or the apoptotic death of cells. Development of an antioxidant defense system in plants protects them against oxidative stress damage. These ROS and, more particularly, H2O2, play versatile roles in normal plant physiological processes and in resistance to stresses. Recently, H2O2 has been regarded as a signaling molecule and regulator of the expression of some genes in cells. This review describes various aspects of H2O2 function, generation and scavenging, gene regulation and cross-links with other physiological molecules during plant growth, development and resistance responses.     

Metabolic Engineering of Tropane Alkaloid Biosynthesis in Plants

Over the past decade, the evolving commercial importance of so-called plant secondary metabolites has resulted in a great interest in secondary metabolism and, particularly, in the possibilities to enhance the yield of fine metabolites by means of genetic engineering. Plant alkaloids, which constitute one of the largest groups of natural products, provide many pharmacologically active compounds. Several genes in the tropane alkaloids biosynthesis pathways have been cloned, making the metabolic engineering of these alkaloids possible. The content of the target chemical scopolamine could be significantly increased by various approaches, such as introducing genes encoding the key biosynthetic enzymes or genes encoding regulatory proteins to overcome the specific rate-limiting steps. In addition, antisense genes have been used to block competitive pathways. These investigations have opened up new, promising perspectives for increased production in plants or plant cell culture. Recent achievements have been made in the metabolic engineering of plant tropane alkaloids and some new powerful strategies are reviewed in the present paper.     

植物苯丙烷代谢及其对重金属胁迫的响应研究进展

苯丙烷代谢途径是植物中最重要的次生代谢途径之一,在植物抵抗重金属胁迫中直接或间接发挥了抗氧化作用,并能够提高植物对重金属离子的吸收与胁迫耐性。本文就苯丙烷代谢途径核心反应与关键酶系进行了总结,同时分析了木质素、类黄酮及原花青素等关键代谢产物的生物合成过程及相关机制,并以此为基础探讨了苯丙烷代谢途径关键产物响应重金属胁迫的相关机制。此外,结合当前研究现状,就苯丙烷代谢参与植物防御重金属胁迫的相关研究提出展望,以期为重金属污染环境的植物修复提供理论依据。     

Cell‐wall invertases,key enzymes in the modulation of plant metabolism during defence responses

Most plant–pathogen interactions do not result in pathogenesis because of pre‐formed defensive plant barriers or pathogen‐triggered activation of effective plant immune responses. The mounting of defence reactions is accompanied by a profound modulation of plant metabolism. Common metabolic changes are the repression of photosynthesis, the increase in heterotrophic metabolism and the synthesis of secondary metabolites. This enhanced metabolic activity is accompanied by the reduced export of sucrose or enhanced import of hexoses at the site of infection, which is mediated by an induced activity of cell‐wall invertase (Cw‐Inv). Cw‐Inv cleaves sucrose, the major transport sugar in plants, irreversibly yielding glucose and fructose, which can be taken up by plant cells via hexose transporters. These hexose sugars not only function in metabolism, but also act as signalling molecules. The picture of Cw‐Inv regulation in plant–pathogen interactions has recently been broadened and is discussed in this review. An interesting emerging feature is the link between Cw‐Inv and the circadian clock and new modes of Cw‐Inv regulation at the post‐translational level.     

Trichome regulator SlMIXTA‐like directly manipulates primary metabolism in tomato fruit

Trichomes are storage compartments for specialized metabolites in many plant species. In trichome, plant primary metabolism is significantly changed, providing substrates for downstream secondary metabolism. However, little is known of how plants coordinate trichome formation and primary metabolism regulation. In this report, tomato (Solanum lycopersicum) trichome regulator SlMIXTA‐like is indicated as a metabolic regulation gene by mGWAS analysis. Overexpression of SlMIXTA‐like in tomato fruit enhances trichome formation. In addition, SlMIXTA‐like can directly bind to the promoter region of gene encoding 3‐deoxy‐7‐phosphoheptulonate synthase (SlDAHPS) to activate its expression. Induction of SlDAHPS expression enhances shikimate pathway activities and provides substrates for downstream secondary metabolism. Our data provide direct evidence that trichome regulator can directly manipulate primary metabolism, in which way plants can coordinate metabolic regulation and the formation of storage compartments for specialized metabolites. The newly identified SlMIXTA‐like can be used for future metabolic engineering.     

植物苯丙烷代谢途径

众所周知,固着生长的植物经常受到环境中各种生物和非生物胁迫的威胁。所以在漫长的进化过程中,植物必须将多样的环境信号整合到其发育过程中,以实现适应性形态的发生和代谢途径的精确调控,最终使植物完成整个生长周期。研究显示,苯丙烷代谢作为植物重要的次级代谢途径之一,其代谢产物,例如木质素、孢粉素、花青素和有机酸等,在调控植物适应性生长的过程中发挥着重要功能。特别是在药用植物中,苯丙烷代谢还与众多药用活性成分的合成息息相关,几乎所有包含苯丙烷骨架的天然药效成分均由苯丙烷代谢途径直接或间接合成,例如黄酮类、萜类和酚类等。此外,经苯丙烷代谢途径产生的一些次级代谢产物还能由植物根系外泌到周际土壤中,通过改变根系微生物的菌群生态,而影响植物生长和抵抗生物或非生物胁迫的能力。同时,苯丙烷代谢介导的这种植物-微生物互作也与药用植物的道地品质密不可分。本文综述了近年来植物苯丙烷代谢途径的最新研究进展,重点对该代谢途径中代谢产物的生理功能及表达调控机制进行了介绍,以期更深入地理解药用植物苯丙烷代谢与药材性状之间的潜在关系,旨在指导优良中草药的遗传育种,以进一步促进我国中医药事业的蓬勃发展。