Family life at close quarters: communication and constraint in angiosperm seed development

The formation of viable angiosperm seeds involves the co-ordinated growth and development of three genetically distinct organisms, the maternally derived seed coat and the zygotic embryo and endosperm. The physical relationships of these tissues are initially established during the specification and differentiation of the female gametophyte within the tissues of the developing ovule. The molecular programmes implicated in both ovule and seed development involve elements of globally important pathways (such as auxin signalling), as well as ovule- and seed-specific pathways. Recurrent themes, such as the precisely controlled death of specific cell types and the regulation of cell–cell communication and nutrition by the selective establishment of symplastic and apoplastic barriers, appear to play key roles in both pre- and post-fertilization seed development. Much of post-fertilization seed growth occurs during a key developmental window shortly after fertilization and involves the dramatic expansion of the young endosperm, constrained by surrounding maternal tissues. The complex tissue-specific regulation of carbohydrate metabolism in specific seed compartments has been shown to provide a driving force for this early seed expansion. The embryo, which is arguably the most important component of the seed, appears to be only minimally involved in early seed development. Given the evolutionary and agronomic importance of angiosperm seeds, the complex combination of communication pathways which co-ordinate their growth and development remains remarkably poorly understood.     

Structural aspects of ovule and seed development and nonrandom abortion inMelilotus officinalis (Fabaceae)

Summary Only one ovule matures into a seed inMelilotus officinalis. Although eight ovules form within an ovary, only the basal ovule develops into a mature seed, whereas the other ovules degenerate. The investigation of ovule and seed structure at different developmental stages and a comparison of quantitative characters of differently fated ovules within an ovary were undertaken by light, phase contrast, and fluorescence microscopy. In this species, campylotropous ovules develop simultaneously on marginal placentae in an apocarpous unilocular gynoecium. Megasporo- and megagametogenesis proceed normally and are completed in bud. The maturation of the Polygonum type embryo sac takes place after the flower opens. Shortly before fertilization, synergids show signs of degeneration in all ovules. At this stage, neither the structure nor the sizes of ovules within one ovary differ significantly. In spite of this, only the basal ovule develops into a seed. Rarely, one of the upper-situated ovules or the basal and another ovule mature into seeds. Seed enlargement is insignificant until the stage when globular embryo and nuclear endosperm are formed. At the seed-filling stage, other ovules have collapsed and the seed gradually comes to occupy the total volume of the pod. The fruit-to-seed length ratio decreases considerably during seed ripening. At fertilization, ovary length is four times greater than ovule length. In the mature state, the fruit and seed lengths are approximately equal. Seed size and weight diminish with an increase in seed number within a pod, although pod size remains constant. It is assumed that nonrandom abortion of young seeds inM. officinalis is under maternal control and is not related to structural abnormalities in ovule development or with limitation in pollen. We suppose that evolution of this species may have proceeded in the direction of a decrease in seed number and an increase in its sizes, which may play an important role in seed dispersal and seedling establishment.     

Cytological analysis of ginseng carpel development

Panax ginseng Meyer, commonly known as ginseng, is considered one of the most important herbs with pharmaceutical values due to the presence of ginsenosides and is cultivated for its highly valued root for medicinal purposes. Recently, it has been recognized that ginseng fruit contains high contents of triterpene such as ginsenoside Re as pharmaceutical compounds. However, it is unclear how carpel, the female reproductive tissue of flowers, is formed during the three-year-old growth before fruit is formed in ginseng plants. Here, we report P. ginseng carpel development at the cytological level, starting from the initial stage of ovule development to seed development. The carpel of P. ginseng is composed of two free stigmas, two free styles, and one epigynous bilocular ovary containing one ovule in each locule. Based on our cytological study, we propose that the female reproductive development in P. ginseng can be classified into seven stages: early phase of ovule development, megasporogenesis, megagametogenesis, pre-fertilization, fertilization, post-fertilization, and seed development. We also describe the correlation of the female and male gametophyte development and compare morphological differences in carpel development between ginseng and other higher plants. One unique feature for ginseng seed development is that it takes 40 days for the embryo to develop to the early torpedo stage and that the embryo is small relative to the seed size, which could be a feature of taxonomic importance. This study will provide an integral tool for the study of the reproductive development and breeding of P. ginseng.     

ATP binding cassette transporters ABCG1 and ABCG16 affect reproductive development via auxin signalling in Arabidopsis

Angiosperm reproductive development is a complex event that includes floral organ development, male and female gametophyte formation and interaction between the male and female reproductive organs for successful fertilization. Previous studies have revealed the redundant function of ATP binding cassette subfamily G (ABCG) transporters ABCG1 and ABCG16 in pollen development, but whether they are involved in other reproductive processes is unknown. Here we show that ABCG1 and ABCG16 were not only expressed in anthers and stamen filaments but also enriched in pistil tissues, including the stigma, style, transmitting tract and ovule. We further demonstrated that pistil‐expressed ABCG1 and ABCG16 promoted rapid pollen tube growth through their effects on auxin distribution and auxin flow in the pistil. Moreover, disrupted auxin homeostasis in stamen filaments was associated with defective filament elongation. Our work reveals the key functions of ABCG1 and ABCG16 in reproductive development and provides clues for identifying ABCG1 and ABCG16 substrates in Arabidopsis.     

microRNA159‐targeted SlGAMYB transcription factors are required for fruit set in tomato

The transition from flowering to fruit production, namely fruit set, is crucial to ensure successful sexual plant reproduction. Although studies have described the importance of hormones (i.e. auxin and gibberellins) in controlling fruit set after pollination and fertilization, the role of microRNA‐based regulation during ovary development and fruit set is still poorly understood. Here we show that the microRNA159/GAMYB1 and ‐2 pathway (the miR159/GAMYB1/2 module) is crucial for tomato ovule development and fruit set. MiR159 and SlGAMYBs were expressed in preanthesis ovaries, mainly in meristematic tissues, including developing ovules. SlMIR159‐overexpressing tomato cv. Micro‐Tom plants exhibited precocious fruit initiation and obligatory parthenocarpy, without modifying fruit shape. Histological analysis showed abnormal ovule development in such plants, which led to the formation of seedless fruits. SlGAMYB1/2 silencing in SlMIR159‐overexpressing plants resulted in misregulation of pathways associated with ovule and female gametophyte development and auxin signalling, including AINTEGUMENTA‐like genes and the miR167/SlARF8a module. Similarly to SlMIR159‐overexpressing plants, SlGAMYB1 was downregulated in ovaries of parthenocarpic mutants with altered responses to gibberellins and auxin. SlGAMYBs likely contribute to fruit initiation by modulating auxin and gibberellin responses, rather than their levels, during ovule and ovary development. Altogether, our results unveil a novel function for the miR159‐targeted SlGAMYBs in regulating an agronomically important trait, namely fruit set.     

Seed abortion in the sexual counterpart of <Emphasis Type="Italic">Brachiaria brizantha</Emphasis> apomicts (Poaceae)

Gametophytic organization, fertilization and reproductive success are described for the fertile diploid Brachiaria brizantha accession BRA-002747 which is being raised for use in Brachiaria breeding programs, as well as to understand and control of apomixis in this genus. The current paper reports on reproductive biology and analysis of seed set in field experiments during three consecutive years. Unsuccessful seed production in this plant is believed to correlate with early inbreeding depression, based on the reproductive features analyzed. Caryopsis development was observed using differential interference contrast microscopy with seed set determined by the number of self- and open-pollinated pistils that fully developed into viable seeds. Developing and mature female and male gametophytes were observed in the context of flower phenology, morphology and anthesis patterns. Pollen viability was determined by acetocarmine staining and by observation of germination in vivo, which was also used to observe pollen tube/pistil interaction. Although normal development was observed in floral structures, anthesis and gametophytes, seed set was low, with 2 and 6% in self- and open-pollination, respectively, producing seed. Variations observed in the female organs, such as the presence of a hermaphrodite flower in 50% of the inferior floscules and the presence of multiple embryo sacs of the Polygonum type within the same ovule in 15% of the pistils, are not related to low fertility. The majority of pollen grains are viable, in spite of the reduced number of pollen tubes within the style and ovary carpel, and a developing caryopsis was observed in 70% of self-pollinated pistils, indicating successful double fertilization from 2 days after anthesis (DAA). Nevertheless, abortion gradually increased from 2 until 7 DAA and remains elevated until 12 DAA, when caryopsis maturity is achieved. These data confirm low seed set in this accession and indicate that low fertility is not a consequence of abnormalities, either in the floral or gamete structures, or pollen tube rejection, but most likely a consequence of inbreeding depression.     

EFFECT OF OVULE POSITION ON SEED PRODUCTION,SEED WEIGHT,AND PROGENY PERFORMANCE IN PHASEOLUS COCCINEUS L. (LEGUMINOSAE)

We examined the effect of ovule position within the ovary on the probability of seed maturation, on seed weight, and on progeny performance in the outbreeding legume Phaseolus coccineus. Ovaries of P. coccineus possess six linearly arranged ovules (ovule position one = stylar end). We found that in both 1987 and 1988, ovule position had a significant effect on the probability of seed maturation under field conditions. In 1987, ovule positions one. two, and three had a higher probability of maturing seeds than the three most basal ovule positions. In 1988, the probability of producing a mature seed in ovule position one was more similar to the three most basal ovule positions than to ovule positions two and three. The position of the ovule in the ovary had no significant effect on seed weight in 1987, but it had a significant effect in 1988. Overall, seeds from ovule positions one, two, and three tended to produce heavier seeds than the three most basal ovule positions. The effects of ovule position on progeny performance were determined in a greenhouse and a field study. In the greenhouse study, we found no significant overall effect of the position of the ovule that produced the seed on progeny performance. In the field study, we did find a significant ovule position effect on several measures of reproductive performance as well as an overall effect on reproductive performance. In addition, we found a significant interaction between ovule position and number of seeds per fruit. Progeny from the stylar end of the fruit outperformed the progeny from the peduncular end in fruits containing many seeds, whereas there were no significant differences between progeny produced in the stylar and peduncular ends of fruits containing few seeds. Causes of position effects are unknown but hypotheses abound.     

Ovary and Gametophyte Development Are Coordinately Regulated by Auxin and Ethylene following Pollination

The differentiation and development of ovules in orchid flowers are pollination dependent. To define the developmental signals and timing of critical events associated with ovule differentiation, we have examined factors that regulate the initial events in megasporogenesis and female gametophyte development and characterized its progression toward maturity and fertilization. Two days after pollination, ovary wall epidermal cells begin to elongate and form hair cells; this is the earliest visible morphological change, and it occurs at least 3 days prior to pollen germination, indicating that signals associated with pollination itself trigger these early events. The effects of inhibitors of ethylene biosynthesis on early morphological changes indicated that ethylene, in the presence of auxin, is required to initiate ovary development and, indirectly, subsequent ovule differentiation. Surprisingly, pollen germination and growth were also strongly inhibited by inhibitors of ethylene biosynthesis, indicating that male gametophyte development is also regulated by ethylene. Detailed characterization of the development of both the female and male gametophyte in pollinated orchid flowers indicated that pollen tubes entered the ovary and grew along the ovary wall for 10 to 35 days, at which time growth was arrested. Approximately 40 days after pollination, coincident with ovule differentiation as indicated by the presence of a single archesporial cell, the direction of pollen tube growth became redirected toward the ovule, suggesting a chemical signaling between the developing ovule and male gametophyte. Taken together, these results indicate that both auxin and ethylene contribute to the regulation of both ovary and ovule development and to the coordination of development of male and female gametophytes.     

Genetic Evidence for a Long-Range Activity That Directs Pollen Tube Guidance in Arabidopsis

The fertilization process of plants is governed by different kinds of cell-cell interactions. In higher plants, these interactions are required both for recognition of the pollen grain by the female reproductive system and to direct the growth of the pollen tube inside the ovary. Despite many years of study, the signaling mechanisms that guide the pollen tube toward its target, the ovule, are largely unknown. Two distinct types of principles, mechanical and chemotropic, have been suggested to account for the directed growth of the pollen tube. The first of these two types of models implies that the guidance of the pollen tube depends on the architecture and chemical properties of the female reproductive tissues, whereas the latter suggests that the ovule provides a signal for the target-directed growth of the pollen tube. To examine such a role for the ovules, we analyzed the growth path of pollen tubes in mutants defective in ovule development in Arabidopsis. The results presented here provide unique in vivo evidence for an ovule-derived, long-range activity controlling pollen tube guidance. A morphological comparison of the ovule mutants used in this study indicates that within the ovule, the haploid embryo sac plays an important role in this long-range signaling process.     

Reproductive patterns within racemes in protandrous Aconitum gymnandrum (Ranunculaceae): potential mechanism and among-family variation

The adaptive significance and mechanism of patterns in floral sex allocation and female success within inflorescences has attracted attention recently, whereas few studies have examined genetic variation of intra-inflorescence pattern. The purpose of this study is to investigate patterns of reproduction within racemes in protandrous Aconitum gymnandrum Maxim., and illuminate potential mechanisms and genetic variation of such patterns. Data from pot experiment on 40 maternal families were collected in field. Anther number, pollen:ovule ratio and seed germination rate increased from bottom to top flowers within racemes, but other traits, such as gynoecium mass, carpel number, sepal galea height and seed production decreased significantly with flowering sequence. Variation in floral sex allocation within racemes in A. gymnandrum fitted entirely the prediction of protandry, which was not a result of architectural effect. Such selected pattern may result from a variety of factors influencing the mating environment, such as pollinator directionality, display size and flower longevity. Decline of female success within racemes in A. gymnandrum also resulted from male-biased allocation selected by variation in the mating environment, not resource competition or pollen limitation. Moreover, there was genetic variation for most reproductive traits and the position effect, as evinced by significant variation among families.     

Polar auxin transport together with AINTEGUMENTA and REVOLUTA coordinate early Arabidopsis gynoecium development

In flowering plants the gynoecium is the female reproductive structure and the site of oogenesis, fertilization, and maturation of the embryo and the seed. Proper development of the gynoecium requires that the early gynoecial primordium be partitioned into distinct spatial domains with divergent fates. Regulated transport of the phytohormone auxin previously has been shown to play a role in the patterning of spatial domains along the apical-basal axis of the gynoecium. Here we establish a role for auxin transport in patterning along the medio-lateral axis of the gynoecial ovary. We demonstrate that auxin transport is required for the development of the medial ovary domain that contains the carpel margin meristem, a vital female reproductive structure. Disruptions in auxin transport enhance the medial domain defects observed in aintegumenta and revoluta mutant genotypes. AINTEGUMENTA and REVOLUTA are likely to function in parallel and partially overlapping pathways required for medial domain development. Our data indicate that different ovary domains are differentially sensitive to the reduction of polar auxin transport and the loss of AINTEGUMENTA and REVOLUTA activity. We suggest that an auxin-mediated positional cue is important for the differential specification of the medial and lateral ovary domains.     

Variation in ovule and seed size and associated size-number trade-offs in angiosperms

Unlike pollen and seed size, the extent and causes of variation in ovule size remain unexplored. Based on 45 angiosperm species, we assessed whether intra- and interspecific variation in ovule size is consistent with cost minimization during ovule production or allows maternal plants to dominate conflict with their seeds concerning resource investment. Despite considerable intraspecific variation in ovule volume (mean CV = 0.356), ovule production by few species was subject to a size-number trade-off. Among the sampled species, ovule volume varied two orders of magnitude, whereas seed volume varied four orders of magnitude. Ovule volume varied positively among species with flower mass and negatively with ovule number. Tenuinucellate ovules were generally larger that crassinucellate ovules, and species with apical placentation (which mostly have uniovulate ovaries) had smaller ovules than those with other placentation types. Seed volume varied positively among species with fruit mass and seed development time, but negatively with seed number. Seeds grew a median 93-fold larger than the ovules from which they originated. Our results provide equivocal evidence that selection minimizes ovule size to allow efficient resource allocation after fertilization, but stronger evidence that ovule size affords maternal plants an advantage in parent-offspring conflict.     

Adaptive Shifts in the Regulation System of Seed Yield in Japanese Species of Epimedium (Berberidaceae)

Abstract Total seed yield per plant in one season was differentiated neither between Epimedium diphyllum and the E. grandiflorum complex ( E. grandiflorum and E. sempervirens ), nor between these two groups and one of their hybrid derived species, E. trifoliatobinatum . Total ovules per plant and seed-set rate per capsule ( SR ) did not vary greatly between these species, and seed weight ( SW ) was almost the same between them. The number of flowers per inflorescence ( F ) was also constant. However, the remaining reproductive component characters, ovule number per ovary ( O ) and inflorescence number per plant ( I ), were differentiated between the three taxa. These two characters were negatively correlated and a trade-off relationship occurred under the constant total seed yield (= O × F × I × SR × SW ). Ovule number per ovary was highly correlated with spur length of the flower. During the course of the hybrid speciation of E. trifoliatobinatum , selection pressure by pollinators on intermediate spur length seems to have favored plants with an intermediate ovule number. On the other hand, this selection pressure counteracted the increase of the inflorescence number under the trade-off. The resultant seed yield of E. trifoliatobinatum did not differ from that of the parental species, but the pattern of ovule allocation to ovaries (capsules) was altered.     

The relationship of the female reproductive success of Eucalyptus globulus to the endogenous properties of the flower

Low capsule and seed set is a major factor limiting seed production in Eucalyptus globulus seed orchards. Controlled pollination studies showed that the reproductive success (number of seeds produced per flower pollinated) was primarily determined by the female. We aimed to identify the factors contributing to the differences in reproductive success between female genotypes in terms of the physical and anatomical properties of the flower. We studied pairs of genotypes of high and low reproductive success from each of three races (Furneaux Group, Strzelecki Ranges and Western Otways) growing in a seed orchard. Controlled pollinations were performed on six females and along with flower physical measurements, pollen tube growth and seed set were assessed. Overall tree reproductive success was positively correlated with flower size, ovule numbers, style size, cross-sectional area of conductive tissue within the style (all of which were inter-correlated) and the proportion of pollen tubes reaching the bottom of the style. Significant positive correlations of reproductive success and flower physical properties between different ramets of the same genotypes across seasons suggests a genetic basis to the variation observed. The majority of pollen tube attrition occurred within the first millimetre of the cut style and appeared to be associated with differences in style physiology. When examined as pairs within races the difference in reproductive success for the Western Otways pair was simply explained by differences in flower size and the number of ovules per flower. Physical features did not differ significantly for the Strzelecki Ranges pair, but the proportion of pollen tubes reaching the bottom of the style was lower in the less reproductively successful genotype, suggesting an endogenous physiological constraint to pollen tube growth. The difference in reproductive success between the females from the Furneaux Group was associated with a combination of these factors.     

Effects of population size, seed predation and plant size on male and female reproductive success in Vincetoxicum hirundinaria (Asclepiadaceae)

Here we present the results of a study on the effects of population size, seed predation, and plant size on male and female reproductive success in ten Vincetoxicum hirundinaria populations in southwestern Finland. We investigated male reproductive success by recording pollinia removal, and studying pollen quality and quantity. Pod and seed production, and the number of ovules per ovary were counted to estimate female reproductive success. Both pollen quality and pollinia removal were higher in small V. hirundinaria populations compared to large populations. In contrast, the quantity of pollen was higher in large populations than in small populations. Pod initiation was higher in large populations. However, large populations had higher abortion rates and proportionally fewer intact pods. Plant size did not affect male reproductive success whereas pod and seed production correlated positively with plant size, although only in large populations. Population size had no clear impact on seed predation intensities. Variation was, however, observed among populations. Larger plant individuals suffered from an increased risk of being attacked only in two populations. This correlation was not clearly attributed to plant population size. The results of this study suggest that reproductive success in V. hirundinaria may be highly variable and this variation is to some extent explained by population size. Further, population size affected male and female reproductive success differently.     

Variation in Floral Sex Allocation and Reproductive Success in Sequentially Flowering Inflorescence of Corydalis remota var. lineariloba (Fumariaceae)

In hermaphroditic plants, female reproductive success often varies among different positions within an inflorescence.However, few studies have evaluated the relative importance of underlying causes such as pollen limitation, resource limitation or architectural effect, and few have compared male allocation. During a 2-year investigation, we found that female reproductive success of an acropetally flowering species, Corydalis remota Fisch. ex Maxim. var. Iineariloba Maxim. was significantly lower in the upper late developing flowers when compared with the lower early flowers. Supplementation with outcross pollen did not improve female reproductive success of the upper flowers, while removal of the lower developing fruits significantly increased female reproductive success of the upper flowers in both years, evidencing resource limitation of the upper flowers. Female production in upper flowers was greatly improved by simultaneous pollen supplementation of the upper flowers and removal of the lower fruits, suggesting that, when resources are abundant, pollen may limit the female reproductive success of the upper flowers. The less seed mass in the upper flowers didn't increase in all treatments due to architecture. In the upper flowers, ovule production was significantly lower and the pollen : ovule ratio was significantly higher. These results suggest that male-biased sex allocation in the upper flowers may lead to increased male reproductive success, whereas the lower flowers have higher female reproductive success.     

Effect of ovule position within the pod on the probability of seed production in Bauhinia ungulata (Fabaceae)

BACKGROUND AND AIMS: It has been claimed that ovules linearly ordered within a fruit differ in their probabilities of reaching maturity. This was investigated by studying the effect the position of an ovule within the pod has on seed abortion and seed production in Bauhinia ungulata. METHODS: Fruits collected during the dry seasons of 1999, 2000 and 2001 were opened, and the number, position and status of each ovule within the fruit were recorded. A GLM model was used to assess the effects of population, tree identity and ovule position within the pod on ovule fertilization, seed abortion, seed damage and seed maturation in two populations of B. ungulata. KEY RESULTS: Nearly 30% of the ovules were not fertilized in 1999; this percentage dropped to 5% the following two years. Seed abortion (50%) and seed damage (15%) were the same every year during the study period. Only 15% of the initial ovules developed into mature seeds in 1999; this value increased to 35% in 2000 and 2001. However, seed survivorship was dependent on the position of the ovule within the pod; non-fertilized and early aborted ovules were found more often near the basal end of the ovary. The frequency of seed damage was not affected by position. Mature seeds were found mainly in the stylar half of fruits, where ovules are likely to be fertilized by fast pollen tubes. CONCLUSIONS: The pattern of seed production in B. ungulata is non-random but is dependent upon the position of the ovule within the pod. The results suggest that the seeds produced within a fruit might differ in their vigour.