Importance of pre‐anthesis anther sink strength for maintenance of grain number during reproductive stage water stress in wheat

Reproductive stage water stress leads to spikelet sterility in wheat. Whereas drought stress at anthesis affects mainly grain size, stress at the young microspore stage of pollen development is characterized by abortion of pollen development and reduction in grain number. We identified genetic variability for drought tolerance at the reproductive stage. Drought‐tolerant wheat germplasm is able to maintain carbohydrate accumulation in the reproductive organs throughout the stress treatment. Starch depletion in the ovary of drought‐sensitive wheat is reversible upon re‐watering and cross‐pollination experiments indicate that the ovary is more resilient than the anther. The effect on anthers and pollen fertility is irreversible, suggesting that pollen sterility is the main cause of grain loss during drought conditions in wheat. The difference in storage carbohydrate accumulation in drought‐sensitive and drought‐tolerant wheat is correlated with differences in sugar profiles, cell wall invertase gene expression and expression of fructan biosynthesis genes in anther and ovary (sucrose : sucrose 1‐fructosyl‐transferase, 1‐SST; sucrose : fructan 6‐fructosyl‐transferase, 6‐SFT). Our results indicate that the ability to control and maintain sink strength and carbohydrate supply to anthers may be the key to maintaining pollen fertility and grain number in wheat and this mechanism may also provide protection against other abiotic stresses.     

雄性不育在作物杂种优势中的应用途径分析

雄性不育是植物雄性细胞或生殖器官丧失生理机能的现象,该现象的利用大大提高了杂交种生产的效率。植物雄性不育包含细胞质雄性不育、不受环境影响的核雄性不育、光温敏型雄性不育及化学诱导的雄性不育。这些不育类型也已经被以三系或二系的方式应用于很多作物的杂交种生产中。综述了雄性不育各个途径的研究进展及其在作物杂种优势中的应用。     

Drought-induced male sterility in rice: Changes in carbohydrate levels and enzyme activities associated with the inhibition of starch accumulation in pollen

Male reproductive development of rice (Oryza sativa L.) is very sensitive to drought. A brief, transitory episode of water stress during meiosis in pollen mother cells of rice grown under controlled environmental conditions induced pollen sterility. Anthers containing sterile pollen were smaller, thinner, and often deformed compared to normal anthers of well-watered plants. Only about 20% of the fully developed florets in stressed plants produced grains, compared to 90% in well-watered controls. Water stress treatments after meiosis were progressively less damaging. Levels of starch and sugars and activities of key enzymes involved in sucrose cleavage and starch synthesis were analyzed in anthers collected at various developmental stages from plants briefly stressed during meiosis and then re-watered. Normal starch accumulation during pollen development was strongly inhibited in stress-affected anthers. During the period of stress, both reducing and non-reducing sugars accumulated in anthers. After the relief of stress, reducing sugar levels fell somewhat below those in controls, but levels of non-reducing sugars remained higher than in controls. Activities of acid invertase and soluble starch synthase in stressed anthers were lower than in controls at comparable stages throughout development, during as well as after stress. Stress had no immediate effect on ADP-glucose pyrophosphorylase activity, but had an inhibitory aftereffect throughout post-stress development. Sucrose synthase activity, which was, relatively speaking, much lower than acid invertase activity, was only slightly suppressed by stress. The results show that it is unlikely that pollen sterility, or the attendant inhibition of starch accumulation, in water-stressed rice plants are caused by carbohydrate starvation per se. Instead, an impairment of enzymes of sugar metabolism and starch synthesis may be among the potential causes of this failure.     

Relationship Between Male Fertility and Endogenous Phytohormones in Photoperiod-Sensitive Genic Male-Sterile Rice

A spontaneous male sterile rice plant (Oryza sativa L. cv. Nongken 58S) "Photoperiod-sensitive Genic Male-sterile Rice has been found to be male sterile under long day cycles (LD) and fertile in short day cycles (SD). The period from secondary rachis-branch and spikelet primodia to pollen mother cell formation in the process of panical development was the photoperiod-sensitive stage for fertility alternation. The phenotype of this mutant was reported to be controlled by two pairs of recessive alleles. The research on relationship between the fertility alternation and phytohormone action in this mutant have been performed by Chinese scientists since 1985. In order to study the mechanism of fertility alternation in Nongken 58S, endogenous IAA, ABA, GA1 and GA4 in apical leaves and reproductive organs in different development stages under LD and SD conditions have been quantiatvely and qualitatively identified by GC-MS-SIM method. It was found that endogenous IAA in apical leaves at the stage of pistil and stamen primodia formation and in panicles at pollen mother cell stage of Nongken 58S with LD condition was deficient comparing with those in SD. Endogenous ABA level in panicles at pollen mother cell stage, in spikelets at uninucleate stage and in anthers at anthesis stage of Nongken 58S-LD were lower than those in SD. ABA levels in corresponding organs and developmental stages of wild type of rice, "Nongken 58" were always higher in LD treatment than those in SD. Endogenous IAA, GA1 and GA4 levels in anthers at anthesis stasge of "Nongken 58"-LD were increased obviously. Thus it appeared that "Nongken 58" possess stronger resistance to LD stress than Nongken 58S. It is concluded that IAA deficiency of reproductive organs at early developmental stage, ABA decrease implying poor resistance to LD stress and reduction of GAs in late developmental stages were the factors causing the anther sterility in Nongken 58S-LD.     

Effect of Water Deficit on Sporogenesis in Wheat (Triticum aestivum L.)

Plants of Triticum aestivum L., cv. Gabo, were grown in a glasshousefor 4 weeks and then transferred to a controlled environmentwith 20±1 °C temperature and 16 h photoperiod. Theywere subjected to water deficit by withholding the water supplyduring various stages of floral development, including thoseimmediately before meiosis and all stages until just after anthesis. The proportion of apparently normal florets which produced grainwas reduced when water deficit occurred during and immediatelyafter meiosis in the generative tissues. The effect of thisreduced grain set on total grain yield was partially compensatedby an increase in the weight of the remaining grains. Cross-pollinationbetween stressed and well-watered plants showed that grain setwas reduced as a direct consequence of the induction of malesterility by water stress, whereas female fertility was unaffected.A large proportion of the anthers on water-stressed plants weresmall and shrivelled, did not dehisce normally and containedpollen which was devoid of normal cytoplasmic constituents andshowed no staining reaction with triphenyl tetrazolium chloride.This effect on male fertility was not a result of desiccationof the sporogenous tissue, but an indirect outcome of the decreasein water potential elsewhere in the plant. Water stress, Triticum aestivum L., wheat, pollen, sporogenesis, grain set, male sterility     

The impact of environmental stress on male reproductive development in plants: biological processes and molecular mechanisms

In plants, male reproductive development is extremely sensitive to adverse climatic environments and (a)biotic stress. Upon exposure to stress, male gametophytic organs often show morphological, structural and metabolic alterations that typically lead to meiotic defects or premature spore abortion and male reproductive sterility. Depending on the type of stress involved (e.g. heat, cold, drought) and the duration of stress exposure, the underlying cellular defect is highly variable and either involves cytoskeletal alterations, tapetal irregularities, altered sugar utilization, aberrations in auxin metabolism, accumulation of reactive oxygen species (ROS; oxidative stress) or the ectopic induction of programmed cell death (PCD). In this review, we present the critically stress‐sensitive stages of male sporogenesis (meiosis) and male gametogenesis (microspore development), and discuss the corresponding biological processes involved and the resulting alterations in male reproduction. In addition, this review also provides insights into the molecular and/or hormonal regulation of the environmental stress sensitivity of male reproduction and outlines putative interaction(s) between the different processes involved.     

Down‐regulation of OsSPX1 caused semi‐male sterility,resulting in reduction of grain yield in rice

OsSPX1, a rice SPX domain gene, involved in the phosphate (Pi)‐sensing mechanism plays an essential role in the Pi‐signalling network through interaction with OsPHR2. In this study, we focused on the potential function of OsSPX1 during rice reproductive phase. Based on investigation of OsSPX1 antisense and sense transgenic rice lines in the paddy fields, we discovered that the down‐regulation of OsSPX1 caused reduction of seed‐setting rate and filled grain number. Through examination of anthers and pollens of the transgenic and wild‐type plants by microscopy, we found that the antisense of OsSPX1 gene led to semi‐male sterility, with lacking of mature pollen grains and phenotypes with a disordered surface of anthers and pollens. We further conducted rice whole‐genome GeneChip analysis to elucidate the possible molecular mechanism underlying why the down‐regulation of OsSPX1 caused deficiencies in anthers and pollens and lower seed‐setting rate in rice. The down‐regulation of OsSPX1 significantly affected expression of genes involved in carbohydrate metabolism and sugar transport, anther development, cell cycle, etc. These genes may be related to pollen fertility and male gametophyte development. Our study demonstrated that down‐regulation of OsSPX1 disrupted rice normal anther and pollen development by affecting carbohydrate metabolism and sugar transport, leading to semi‐male sterility, and ultimately resulted in low seed‐setting rate and grain yield.     

Early signs of disruption of wheat anther development associated with the induction of male sterility by meiotic-stage water deficit

 Water deficit during meiosis in microspore mother cells of wheat (Triticum aestivum L.) induces male sterility, which reduces grain yield. In plants stressed during meiosis and then re-watered, division of microspore mother cells seems to proceed normally, but subsequent pollen development is arrested. Stress-affected anthers generally lack starch. We employed light microscopy in conjunction with histochemistry to compare the developmental anatomy of water-stress-affected and normal anthers. The earliest effects of stress, detectable between meiosis and young microspore stages, were the degeneration of meiocytes, loss of orientation of the reproductive cells, and abnormal vacuolization of tapetal cells. Other effects observed during subsequent developmental stages were deposition of starch in the connective tissue where it is normally not present, hypertrophy of the middle layer or endothecial cells, and deposition of sporopollenin-like substances in the anther loculus. The resulting pollen grains lacked both starch and intine. These results suggest that abnormal degeneration of the tapetum in water-stressed anthers coupled with a loss of orientation of the reproductive cells could be part of early events leading to abortion of microspores. Received: 19 July 1996 / Revision accepted: 6 November 1996     

Heat Shock Stress Causes Stage-specific Male Sterility in Arabidopsis thaliana

Arabidopsis during flower development, floral organs such as sepals, petals, stamens, and carpels developed normally. However, the development of pollen inside the anther was disrupted in a stage-specific manner, with floral stage 9 primordia failing to produce any pollen grains. Morphological analyses suggested that heat shock causes a failure of separation of pollen mother cells followed by microspore differentiation and/or inhibition of male meiotic processes. Heat shock also caused sterility in floral stage 12 flowers but the sterility was due to the failure of pollen release from the pollen sacs. Received 12 December 2000/ Accepted in revised form 4 April 2001     

Translating auxin responses into ovules,seeds and yield: Insight from Arabidopsis and the cereals

Grain production in cereal crops depends on the stable formation of male and female gametes in the flower.In most angiosperms, the female gamete is produced from a germline located deep within the ovary, protected by several layers of maternal tissue, including the ovary wall,ovule integuments and nucellus. In the field, germline formation and floret fertility are major determinants of yield potential, contributing to traits such as seed number,weight and size. As such, stimuli affecting the timing and duration of reproductive phases, as well as the viability,size and number of cells within reproductive organs can significantly impact yield. One key stimulant is the phytohormone auxin, which influences growth and morphogenesis of female tissues during gynoecium development, gametophyte formation, and endosperm cellularization. In this review we consider the role of the auxin signaling pathway during ovule and seed development, first in the context of Arabidopsis and then in the cereals. We summarize the gene families involved and highlight distinct expression patterns that suggest a range of roles in reproductive cell specification and fate. This is discussed in terms of seed production and how targeted modification of different tissues might facilitate improvements.     

Low Water Potential Disrupts Carbohydrate Metabolism in Maize (Zea mays L.) Ovaries

Water deficit during pollination increases the frequency of kernel abortion in maize (Zea mays L.). Much of the kernel loss is attributable to lack of current photosynthate, but a large number of kernels fail to develop on water-deficient plants even when assimilate supply is increased. We examined the possibility that assimilate utilization by developing ovaries might be impaired at low water potential ([Psi]w). Plants were grown in the greenhouse in 20-L pots containing 22 kg of amended soil. Water was withheld on the first day silks emerged, and plants were hand-pollinated 4 d later when leaf [Psi]w decreased to approximately - 1.8 MPa and silk [Psi]w was approximately -1.0 MPa. Plants were rehydrated 2 d after pollination. The brief water deficit inhibited ovary growth (dry matter accumulation) and decreased kernel number per ear by 60%, compared to controls. Inhibition of ovary growth was associated with a decrease in the level of reducing sugars, depletion of starch, a 75-fold increase in sucrose concentration (dry weight basis), and inhibition of acid invertase (EC 3.2.1.26) activity. These results indicate that water deficits during pollination disrupt carbohydrate metabolism in maize ovaries. They suggest that acid invertase activity is important for establishing and maintaining reproductive sink strength during pollination and early kernel development.     

被子植物有性生殖过程中的细胞程序死亡

细胞程序死亡是植物发育过程中的一种普遍现象。早就认识到高等植物生殖器官中一些细胞的死亡对植物有性生殖具有重要作用。这些细胞的死亡过程与动物组织中的细胞程序死亡基本相同。但植物体内诱导生殖细胞程序死亡的信号及其传导系统则显示出其特点 ,有些还表现出雌、雄性细胞的相互作用。探索植物生殖过程中的细胞程序死亡现象将有利于澄清植物生殖过程中的一些机理问题 ,使过去的细胞学研究结果深入到分子水平进行探讨     

Patterns of male sterility within and among populations of the distylous shrub Erythroxylum havanense (erythroxylaceae)

Distyly, a reproductive system characterized by the presence of long-styled (thrum). and short-styled (pin) individuals within a population, has been repeatedly used as a model for the study of the evolution of the reproductive systems in plants. Erythroxylum havanense is a distylous species in which most thrum plants fail to develop a fertile androecium, thus behaving as male-sterile or partially male-sterile plants. Short-styled (thrum) individuals have an increased performance as female parents, thereby compensating for their loss of male fitness. Previous studies of populations within close proximity to each other suggest that E. havanense may be involved in a process of gender specialization in which, unlike other heterostylous species, thrum plants are specializing as females and pins (long-styled) as males. In this paper we describe more general patterns of male sterility, one of the first steps in the evolution of gender specialization, among populations of the distylous shrub Erythroxylum havanense. Pollen germination differed among populations (range 0.52 ± 0.03 to 0.06 ± 0.04), and between morphs. Pollen from pin plants was almost two times (1.89) as fertile as that from thrums (0.36 ± 0.03 and 0.19 ± 0.03, pin and thrums respectively). Thrums were significantly more male sterile in four out of five populations. The population where differences between the floral morphs were not apparent showed the lowest levels of pollen fertility. Accordingly, our results indicate that populations of E. havanense show marked differences in pollen fertility and higher male sterility associated with the thrum morph. We hypothesize that differences between morphs could be explained if restorers of male sterility are linked to the distyly haplotype, while differences in genes associated with male sterility could explain the variation among populations. Overall, the prevalence of thrum-biased male sterility across populations suggests that E. havanense is subject to a process of gender specialization.     

Influence of UV-B radiation on the reproductive function of Hordeum vulgare L. plants

Irradiation of barley seedlings by ultraviolet-B radiation influences the growth and development of the sexual elements of the spike, accelerating differentiation of the sporogenous tissue of the anther and the male gametophyte, a phenomenon which is accompanied by an increase in the asynchronicity of microgametogenesis and in heterogeny of the pollen grains, and in an increase in the sterility of the pollen grains. High ultraviolet doses tend to reduce the level of sterility of the anther due to intensification of haplontic cell selection. The influence of ultraviolet irradiation on the reproductive system of the plant is expressed in genotoxic (through damage to the DNA of meristemic cells) and photo-inductive (through acceleration of efflorescence and differentiation of gametophytes) effects.     

High-temperature stress in crops: male sterility,yield loss and potential remedy approaches

Global food security is one of the utmost essential challenges in the 21st century in providing enough food for the growing population while coping with the already stressed environment. High temperature (HT) is one of the main factors affecting plant growth, development and reproduction and causes male sterility in plants. In male reproductive tissues, metabolic changes induced by HT involve carbohydrates, lipids, hormones, epigenetics and reactive oxygen species, leading to male sterility and ultimately reducing yield. Understanding the mechanism and genes involved in these pathways during the HT stress response will provide a new path to improve crops by using molecular breeding and biotechnological approaches. Moreover, this review provides insight into male sterility and integrates this with suggested strategies to enhance crop tolerance under HT stress conditions at the reproductive stage.     

The "drought-inducible" histone H1s of tobacco play no role in male sterility linked to alterations in H1 variants

Tobacco ( Nicotiana tabacum L.) has two major H1 variants (H1A and H1B), which account for over 80% of chromatin linker histones, and four minor variants: H1C, H1D, H1E and H1F. We have shown previously [M. Prymakowska-Bosak et al. (1999) Plant Cell 11:2317-2329] that reversal of the natural proportion of major to minor H1 variants in transgenic tobacco plants results in a characteristic male-sterility phenotype identical to that occurring in many plant species subjected to water deficit at the time of male meiosis. It has been proposed by others that the drought-induced arrest of male gametophyte development is linked to decreased sugar delivery to reproductive tissues. Within the family of angiosperm H1s there is a well-defined class of minor H1 variants named "drought inducible" because some of its members have been shown to be induced by water deficit. We have identified and cloned the tobacco H1C gene, which, based on sequence similarity, represents a "drought-inducible" minor H1 variant. Analysis of the un-translated mRNA and promoter regions of H1C suggests a regulation by sucrose concentration. Antisense silencing of H1C and its close homologue H1D in plants that do not express H1A and H1B does not affect the characteristic H1A(-)/ H1B(-) male-sterility phenotype. Silencing of H1C and H1D also has no effect on growth and development of plants. Our findings demonstrate that H1C and H1D are dispensable for normal growth and development of tobacco, and that the compensatory up-regulation of "drought-inducible" H1s observed in H1A(-)/ H1B(-) plants is not the direct cause of male sterility linked to alterations in H1 variants.     

Temperature stress interferes with male reproductive system development in clementine (Citrus clementina Hort. ex. Tan.)

Male gametophyte development is a critical phase of the plant life cycle due to its high sensitivity to environmental stresses. The rise in the average global temperature, often accompanied by extreme fluctuations, has an important impact on biological processes. Among those, male gametophytes are particularly sensitive to temperature stress during flower bud development and anthesis. Male gametophyte development was extensively studied in several plant species, but little information is available about the effects of temperature stress on male gametophyte development in the genus Citrus. We evaluated the effects of cold and hot temperatures during microsporogenesis and microgametogenesis on one of the most economically valuable citrus species, the “Comune” clementine (Citrus clementina Hort. ex. Tan.). The effect of constant temperature on the androecium was evaluated by a time course histological analysis performed on the anthers and by monitoring in vitro pollen germination. The results revealed how suboptimal hot and cold temperatures induce drastic alterations on the morphology of the tapetal cells, microspores and mature pollen grains. Shifting from the optimal temperature affected the timing of starch depletion in the anther walls, such as epidermis, endothecium and middle layer, influencing the pollen germination rate and pollen tube growth. To the best of our knowledge this is the first study attempting to assess how temperature stress affects male reproductive development in citrus. A better understanding of the mechanisms underlining male sterility will provide novel insights to elucidate the physiology of this agronomical important quality trait.     

Roles of microRNAs in mammalian reproduction: from the commitment of germ cells to peri‐implantation embryos

MicroRNAs (miRNAs) are active regulators of numerous biological and physiological processes including most of the events of mammalian reproduction. Understanding the biological functions of miRNAs in the context of mammalian reproduction will allow a better and comparative understanding of fertility and sterility in male and female mammals. Herein, we summarize recent progress in miRNA‐mediated regulation of mammalian reproduction and highlight the significance of miRNAs in different aspects of mammalian reproduction including the biogenesis of germ cells, the functionality of reproductive organs, and the development of early embryos. Furthermore, we focus on the gene expression regulatory feedback loops involving hormones and miRNA expression to increase our understanding of germ cell commitment and the functioning of reproductive organs. Finally, we discuss the influence of miRNAs on male and female reproductive failure, and provide perspectives for future studies on this topic.