Sexual Plant Reproduction: a Fertile and Flourishing Field

Since the very beginning of plant science, sexual plant reproduction （SPR） has proved an attractive and enduring topic for generations of botanists. With the rapid development of modern technology, a significant acceleration has occurred in our understanding on the developmental mechanisms of plant reproductive processes, particularly the evolution of double fertilisation, signalling in pollen tube orientation, molecular characterisation of plant gametes, maternal to zygotic transitions and parental gene involvement in early embryogenesis. This is reflected not only by several recent high-ranking research papers, but also by the frequent conferences and workshops on these topics. These include the 2008 XXth International Congress on SPR in Brasilia and ＂Frontiers in SPR Ⅲ＂ in Tucson, as well as ＂Cell- Cell Communication in Plant Reproduction＂ held in 2009 in Bath. The continuing efforts from around the world indicate that SPR is still a fertile and flourishing field, with great expectations for the coming decade.

Techniques of cell type-specific transcriptome analysis and applications in researches of sexual plant reproduction

In higher plants, specific cell differentiation and fate decision are controlled by differential gene expression.Cell type-specific transcriptome analysis has become an important tool for investigating cell regulatory mechanisms. Inrecent years, many different techniques have been developed for the isolation of specific cells and the subsequenttranscriptome analysis, and considerable data are available regarding the transcriptional profiles of some specific cells.These cell type-specific transcriptome analyses hold significant promise for elucidating the gene expression linked tocellular identities and functions, and are extraordinarily important for research in functional genomics and systemsbiology aimed toward basic understanding of molecular networks and pathway interactions. Moreover, to reveal thecritical mechanisms about sexual plant reproduction, the gamete and embryo cells have long been treated as goodsubjects for cell-specific transcriptome analysis, and there has been important progress in recent decades. In this review,we summarize current technologies in cell type-specific transcriptome analysis and review the applications of thesetechnologies in research into the mechanisms of sexual reproduction in higher plants.

Oxidative Stress and Role of Natural Plant Derived Antioxidants in Animal Reproduction

The experimental knowledge on the role of oxidative stress,and beneficial and detrimental effects of plant derived antioxidants in male and female animal reproduction are reviewed in this article.Free radical-induced oxidative stress in animal reproduction causes great loss to livestock industry.Antioxidant therapy has been implicated to be effective in preventing diseases resulted from oxidative stress.Considering the advantages of lower side effects of natural antioxidants than those of synthetic antioxidants,plants or their extracts have been extensively utilized in animals.Although many advances have been gained on application of plant derived antioxidants in alleviating oxidative stress,debatable issues still exist.Because many opposite effects were observed even using plant extracts containing similar bioactive substances in the same animal species.Therefore,plant derived antioxidants,like free radicals,are ＂double-edged swords＂ in animal reproduction,representing that they may exhibit beneficial or detrimental effects in animal reproduction,including spermatogenesis,semen functions,estrous cycles,ovulation,ovary functions,endometrium,embryo development,and pregnancy.Besides dose-dependent manner as an explanation of plant extracts＇ dual function,future studies are needed to investigate the mechanism of double-edged actions of plant derived antioxidants in different animal reproduction systems.

Effects of nectar-robbing on plant reproduction and evolution

The relationship between plant and pollinator is considered as the mutualism because plant benefits from the pollinator’s transport of male gametes and pollinator benefits from plant’s reward.Nectar robbers are frequently described as cheaters in the plant-pollinator mutualism,because it is assumed that they obtain a reward(nectar)without providing a service(pollination).Nectar robbers are birds,insects,or other flower visitors that remove nectar from flowers through a hole pierced or bitten in the corolla.Nectar robbing repre-sents a complex relationship between animals and plants.Whether plants benefit from the relationship is always a con-troversial issue in earlier studies.This paper is a review of the recent literatures on nectar robbing and attempts to acquire an expanded understanding of the ecological and evolutionary roles that robbers play.Understanding the effects of nectar robbers on the plants that they visited and other flower visi-tors is especially important when one considers the high rates of robbing that a plant population may experience and the high percentage of all flower visitors that nectar robbers make to some species.There are two standpoints in explaining why animals forage on flowers and steal nectar in an illegitimate behavior.One is that animals can only get food in illegitimate way because of the mismatch of the morphologies of animals’mouthparts and floral structure.The other point of view argues that nectar robbing is a relatively more efficient,thus more energy-saving way for animals to get nectar from flowers.This is probably associated with the difficulty of changing attitudes that have been held for a long time.In the case of positive effect,the bodies of nectar robbers frequently touch the sex organs of plants during their visiting to the flowers and causing pollination.The neutral effect,nectar robbers’behavior may destruct the corollas of flowers,but they neither touch the sex organs nor destroy the ovules.Their behavior does not affect the fruit sets or seed sets of the hosting plant.Besides the direct impacts on plants,nectar robbers may also have an indirect effect on the behavior of the legitimate pollinators.Under some circumstances,the change in pollinator behavior could result in improved repro-ductive fitness of plants through increased pollen flow and out-crossing.

Floral trait variation across individual plants within a population enhances defense capability to nectar robbing

Floral trait variation may help pollinators and nectar robbers identify their target plants and,thus,lead to differential selection pressure for defense capability against floral antagonists.However,the effect of floral trait variation among individuals within a population on multi-dimensional plant-animal interactions has been little explored.We investigated floral trait variation,pollination,and nectar robbing among individual plants in a population of the bumble bee-pollinated plant,Caryopteris divaricata,from which flowers are also robbed by bumble bees with varying intensity across individuals.We measured the variation in corolla tube length,nectar volume and sugar concentration among individual plants,and evaluated whether the variation were recognized by pollinators and robbers.We investigated the in-fluence of nectar robbing on legitimate visitation and seed production per fruit.We found that the primary nectar robber(Bombus nobilis)preferred to forage on plants with long-tubed flowers,which produced less nectar and had lower sugar concentration compared to those with shorter corolla tubes.Individuals with shorter corolla tubes had comparatively lower nectar robbing intensity but higher visitation by legitimate visitors(mainly B.picipes)and higher seed production.Nectar robbing signifi-cantly reduced seed production because it decreased pollinator visits.However,neither pollination nor seed production differed between plants with long and short corolla tubes when nectar robbers were excluded.This finding suggests that floral trait variation might not be driven by pollinators.Such variation among individual plants thus allows legitimate visitors and nectar robbers to segregate niches and enhances population defense against nectar robbing in unpredictable conditions.

Development of Plant Embryology in China

The development of plant embryology in China can be roughly divided into three stages: (1) the initial stage, (2) the stage of establishing plant embryology as a branch of plant sciences, and (3) the stage when plant embryology evolves into plant reproductive biology with its vigorous development. It is in the third stage that research work in this field in China has developed rapidly and many of the significant achievements obtained are described in more details in this review. Researches of experimental embryology are not included in this paper.

Elaborating the Functional Roles of a Leucine-Rich Repeat Protein from Arabidopsis thaliana

Plants, just like any other living organism, naturally get attacked by various pathogenic microorganisms such as bacteria, fungi and viruses. However, unlike animals that utilize their specialized circulatory macrophage system to protect themselves, plants instead use a multi-layered complex system termed the plant innate immunity, which recognizes pathogens and transducing downstream defense responses. They have developed a unique type of trans-membrane receptors or R proteins, which extracellularly, are capable of recognizing pathogen-associated molecular patterns (PAMP) such as flagellin and chitin, while intracellularly, they activate their harbored nucleotide cyclases (NCs) such as adenylyl cyclases (ACs), to generate second messenger molecules such as 3’,5’-cyclic adenosine monophosphate (cAMP), which then propagates and magnifies the defense response. To date, only a single R protein from Arabidopsis thaliana (AtLRR) has been shown to possess AC activity as well as having the ability to defend plants against infection by biotrophic and hemi-biotrophic pathogens. Therefore, in order to further broaden information around the functional roles of this protein (AtLRR), we explored it further, using an array of web-based tools or bioinformatics. These included structural analysis, anatomical expression analysis, developmental expression analysis, co-expression analysis, functional enrichment analysis, stimulus-specific expression analysis and promoter analysis. Findings from structural analysis showed that AtLRR is a multi-domain, trans-membrane molecule that is multi-functional, and thus consistent with all known R-proteins. Findings from anatomical and developmental expression analyses showed that AtLRR is mostly expressed in pollen grains and flowers, senescing leaves as well as during the development of seeds, shoots, roots, seedlings, leaves, flowers, and siliques, linking it to the three key plant physiological processes of reproduction, defense and development respectively. Lastly, findings from co-expression, functional enrichment, stimulus-specific expression and promoter analyses, showed that AtLRR is mostly co-expressed with several other proteins linked to disease resistance, plant reproduction and plant development. Activities and functions of such protein are also commonly regulated by cAMP via a common W-box promoter. So, all in all, our study managed to establish that besides being strongly involved in disease resistance against biotrophic and hemi-biotrophic pathogens, AtLRR also plays key roles in plant development (seed, shoot, root, seedling, leaf, and silique development) and reproduction (flowering, and pollen tube growth and re-orientation), whereby it effects its functions via a W-box or WRKY transcription factor, TTGACY, mediated by cAMP.

Germination Biology and Occurrence of Polyembryony in Two Forms of Cats Claw Creeper Vine, Dolichandra unguis-cati (Bignoniaceae): Implications for Its Invasiveness and Management

Cat’s claw creeper vine, Dolichandra unguis-cati (L.) Lohmann (syn. Macfadyena unguis-cati (L.) Gentry), is a major environmental weed in Australia. Two forms (“long” and “short” pod) of the weed occur in Australia. This investigation aimed to evaluate and compare germination behavior and occurrence of polyembryony (production of multiple seedlings from a single seed) in the two forms of the weed. Seeds were germinated in growth chambers set to 10/20°C, 15/25°C, 20/30°C, 30/45°C and 25°C, representing ambient temperature conditions of the region. Germination and polyembryony were monitored over a period of 12 weeks. For all the treatments in this study, seeds from the short pod form exhibited significantly higher germination rates and higher occurrence of polyembryony than those from the long pod form. Seeds from the long pod form did not germinate at the lowest temperature of 10/20°C;in contrast, those of the short pod form germinated under this condition, albeit at a lower rate. Results from this study could explain why the short pod form of D. unguis-cati is the more widely distributed form in Australia, while the long pod form is confined to a few localities. The results have implication in predicting future ranges of both forms of the invasive D. unguis-cati, as well as inform management decisions for control of the weed.

Seasonal change mediates the shift between resource and pollen limitation in Hedysarum scoparium(Fabaceae)

Theory suggests that with sufficient environmental variation, pollen limitation might be observed at some places or times, and resource limitation at others, but there are no empirical data about the effect of seasonal change on the variation of pollen limitation and resource limitation within a flowering season. In this study, we examined pollen and resource limi- tation by comparing fruit set and seed production in natural- and hand-pollinated Hedysarum scoparium flowers in the middle reaches of the Hexi Corridor region, China, in 2010. We also described a role for the first substantial autumn rainfall in mediating a shift between pollen and resource limitation in H. scoparium, but did not analyze this experimentally Our results indicated that H. scoparium was resource limited at peak flowering during the summer, and was pollen limited at peak flowering during the autumn. The seasonal change （summer to autumn） mediated the shift between pollen and resource limitations in H. scoparium. The shift timing depended on the date of the first autumn rainfall in 2010. Changes in the first substantial rainfall in autumn may affect fruiting of H. scoparium, thus affecting population persistence of this species and development/structure of the local ecosystem if such conditions persist.

RLKs orchestrate the signaling in plant male-female interaction

Different from animals, sessile plants are equipped with a large receptor-like kinase(RLK) superfamily. RLKs are a family of single trans-membrane proteins with divergent N-terminal extracellular domains capped by a signal peptide and C-terminal intracellular kinase. Researches in the last two decades have uncovered an increasing number of RLKs that regulate plant development, stress response and sexual reproduction, highlighting a dominant role of RLK signaling in cell-to-cell communications. Sexual reproduction in flowering plants is featured by interactions between the male gametophyte and the female tissues to facilitate sperm delivery and fertilization. Emerging evidences suggest that RLKs regulate almost every aspect of plant reproductive process, especially during pollination. Therefore, in this review we will focus mainly on the function and signaling of RLKs in plant male-female interaction and discuss the future prospects on these topics.

Lipid Metabolism: Critical Roles in Male Fertility and Other Aspects of Reproductive Development in Plants

Fatty acids and their derivatives are essential building blocks for anther cuticle and pollen wall formation.Disruption of lipid metabolism during anther and pollen development often leads to genic male sterility(GMS).To date,many lipid metabolism-related GMS genes that are involved in the formation of anther cuticle,pollen wall,and subcellular organelle membranes in anther wall layers have been identified and characterized.In this review,we summarize recent progress on characterizing lipid metabolism-related genes and their roles in male fertility and other aspects of reproductive development in plants.On the basis of cloned GMS genes controlling biosynthesis and transport of anther cutin,wax,sporopollenin,and tryphine\r\Arabidopsis,rice,and maize as well as other plant species,updated lipid metabolic networks underlying anther cuticle development and pollen wall formation were proposed.Through bioinformatics analysis of anther RNA-sequencing datasets from three maize inbred lines(Oh43,W23,and B73),a total of 125 novel lipid metabolism-related genes putatively involved in male fertility in maize were deduced.More,we discuss the pathways regulating lipid metabolism-related GMS genes at the transcriptional and post-transcriptional levels.Finally,we highlight recent findings on lipid metabolism-related genes and their roles in other aspects of plant reproductive development.A comprehensive understanding of lipid metabolism,genes involved,and their roles in plant reproductive development will facilitate the application of lipid metabolism-related genes in gene editing,haploid and callus induction,molecular breeding and hybrid seed production in crops.

Nitric Oxide： A Multitasked Signaling Gas in Plants

Nitric oxide （NO） is a gaseous reactive oxygen species （ROS） that has evolved as a signaling hormone in many physiological processes in animals. In plants it has been demonstrated to be a crucial regulator of development, acting as a signaling molecule present at each step of the plant life cycle. NO has also been implicated as a signal in biotic and abiotic responses of plants to the environment. Remarkably, despite this plethora of effects and functional relationships, the fundamental knowledge of NO production, sensing, and transduction in plants remains largely unknown or inadequately characterized. In this review we cover the current understanding of NO production, perception, and action in different physiological sce- narios. We especially address the issues of enzymatic and chemical generation of NO in plants, NO sensing and downstream signaling, namely the putative cGMP and Ca^2＋ pathways, ion-channel activity modulation, gene expression regulation, and the interface with other ROS, which can have a profound effect on both NO accumulation and function. We also focus on the importance of NO in cell-cell communication during developmental processes and sexual reproduction, namely in pollen tube guidance and embryo sac fertilization, pathogen defense, and responses to abiotic stress.

Analyses of Sexual Reproductive Success in Transgenic and/or Mutant Plants

The pistil, the female reproductive organ of plants, is a key player in the success of sexual plant reproduction. Ultimately, the production of fruits and seeds depends on the proper pistil development and function. Therefore, the identification and characterization of pistil expressed genes is essential for a better understanding and manipulation of the plant reproduction process. For studying the function of pistil expressed genes, transgenic and/or mutant plants for the genes of interest are used. The present article provides a review of methods already exploited to analyze sexual reproductive success. We intend to supply useful information and to guide future experiments in the study of genes affecting pistil development and function.

Roles of Pectin Methylesterases in Pollen-Tube Growth

Elongation of the pollen tube in pistil is essential for delivering sperms into the female gametophyte in sexual plant reproduction. Recently, a group of cell wall enzymes, pectin methylesterases （PMEs）, have been identified as playing an important role in this process. This article reviews the new understanding of the roles of PMEs in regulating pollen tube growth.