A comparative flower and fruit anatomical study of Quercus acutissima, a biennial-fruiting oak from the Cerris group (Fagaceae)

A seasonal strategy for pollen tube growth and ovule development to overcome winter in Japanese stone oak (Lithocarpus edulis)

Delayed fertilization is commonly observed in many acorn-producing Fagaceae trees, yet its underlying mechanisms and adaptive significance remain poorly understood. In recent years, a new hypothesis has been proposed suggesting that the nearly year-long delay in fertilization is driven by an overwintering strategy, wherein female gametophyte development is delayed, and pollen tube growth is arrested before winter. This mechanism allows ovules to be fertilized and seeds to develop during more favorable seasons while avoiding adverse winter conditions. However, empirical evidence for this overwintering strategy has been limited. To address this, we observed the seasonal progression of pollen tube growth and ovule development in Lithocarpus edulis, a species with spring and autumn flowering seasons. Monthly observations of pistillate flowers from both seasons were conducted using microtome techniques and scanning confocal microscopy. Our findings revealed that pollen tubes were arrested at the style joining site, and ovules remained immature in both spring and autumn flowers prior to winter. Following winter, pollen tube regrowth and ovule maturation were synchronized in the subsequent spring, regardless of the flowering season. These results support the hypothesis that ovule development is delayed, leading to delayed fertilization, until after winter. This study highlights the importance of temporally coordinating fertilization phenology with flowering and fruiting phenology in seasonal environments to avoid unfavorable winter conditions.

Seasonal gene expression signatures of delayed fertilization in Fagaceae

In the family Fagaceae, fertilization is delayed by several weeks to 1 year after pollination, leading to 1- or 2-year fruiting species depending on whether fruiting occurs in the same or the next year after flowering. To investigate physiological responses underlying the regulation of delayed fertilization, we monitored seasonal changes in genome-wide gene expression in tissues including leaves and buds over 2 years under natural conditions in one- (Quercus glauca) and 2-year fruiting species (Lithocarpus edulis). Genes associated with metabolic changes in response to winter cold, photosynthesis and cell proliferation, which are essential for survival and growth, showed highly conserved seasonal expression profiles between species. However, seasonal expression profiles diverged between species in genes associated with pollination, an important process contributing to the origin and maintenance of the reproductive barrier between plant species. By comparing seasonal progression of ovule development and gene expression in pistillate flowers, we revealed that ovules started developing after winter in the 2-year fruiting species, which could be linked to the activation of genes involved in fertilization and female gametophyte development after winter. These findings suggest that the 2-year fruiting species may have evolved a requirement of winter cold to prevent fertilization before winter and facilitate fertilization and embryo development in the following spring when temperature rises. This study offers new possibilities to explore the evolution of reproductive strategies in Fagaceae.

Characterization of some Fagaceae kernels nutritional composition for potential use as novel food ingredients

The present study attempts to characterise Fagaceae kernels as a promising source of nutritional compounds for potential use as novel food ingredients. Thus, the proximate and mineral composition of some kernels (beech achene-BA, sessile oak acorn-SOA, turkey oak acorn-TOA, and red oak acorn-ROA), total phenolic content, individual polyphenols, and cytotoxicity of their aqueous extracts, respectively, the fatty acid composition of kernel oils were investigated using physicochemical and analytical techniques. Results revealed that BAK is rich in lipid and protein, OAKs in carbohydrates. All tested kernels contain high oleic-linoleic acid oils. BAK is abundant in phenolic acids, OAKs in hydrolysable tannins. Only BA and SOA kernels exert cytotoxicity against human fibroblasts. In all kernels, macroelements are dominated by K and microelements by Cu, Mn, and Fe. In conclusion, BA and OA kernels could be alternatively used as protein-rich, respectively, starch-rich ingredients in food.

Possible molecular mechanisms of persistent pollen tube growth without de novo transcription

The vegetative cell nucleus proceeds ahead of a pair of sperm cells located beneath the pollen tube tip during germination. The tip-localized vegetative nucleus had been considered to play a pivotal role in the control of directional pollen tube growth and double fertilization. However, we recently reported the female-targeting behavior of pollen tubes from mutant plants, of which the vegetative nucleus and sperm nuclei were artificially immotile. We showed that the apical region of the mutant pollen tubes became physiologically enucleated after the first callose plug formation, indicating the autonomously growing nature of pollen tubes without the vegetative nucleus and sperm cells. Thus, in this study, we further analyzed another Arabidopsis thaliana mutant producing physiologically enucleated pollen tubes and discussed the mechanism by which a pollen tube can grow without de novo transcription from the vegetative nucleus. We propose several possible molecular mechanisms for persistent pollen tube growth, such as the contribution of transcripts before and immediately after germination and the use of persistent transcripts, which may be important for a competitive race among pollen tubes.

Development of Quercus acutissima (Fagaceae) pollen tubes inside pistils during the sexual reproduction process

Extensive histology of pistillate flowers revealed two pollen tube arresting sites (the style-joining and micropyle) within the pistil of Quercus acutissima during the postpollination-prezygotic stage, which reflects a unique female and male gametophyte recognition/selection mechanism. Sexual reproduction is among the most delicate and essential stages in plant life cycles and involves a series of precise interactions between pistils and male gametophytes. Quercus is a woody genus that dominates Northern Hemisphere forests and is notorious for interspecific hybridization, but its sexual reproduction is poorly understood, especially its pollen tube (PT) growth dynamics within pistils. This study used microtome techniques and scanning electron microscopy to observe the postpollination-prezygotic process in the biennially fruiting oak Quercus acutissima. Many pollen grains germinated at anthesis instantly, and PTs penetrated stigmatic surfaces and elongated through the stylar transmitting tissue, then arrested at style-joining for about 12-13 months. Few PTs resumed growth along the compitum in the upper ovarian locule wall in the subsequent April, concurrent with the rapid growth of rudimentary ovules. PTs arrived in the micropyle, and upper septum during megaspore mother cell meiosis, then arrested again for 7-10 days waiting for the embryo sac maturation. Fertilization occurred one week later. Our study shows a clear female dominant crosstalk growth pattern between PT and the ovule. The intermittent PT growth might reflect a unique male gametophyte recognition/selection mechanism to avoid self-pollination and enhance PT competition while increasing interspecific hybridization.

Delayed fertilization facilitates flowering time diversity in Fagaceae

Fagaceae includes typical masting species that exhibit highly synchronized and fluctuating acorn production. Fagaceae shows an interesting feature in that fertilization is delayed by several weeks to more than 1 year after pollination. Although delayed fertilization was recorded over a century ago, the evolutionary advantage of delayed fertilization is still poorly understood. Here, we present a new hypothesis that delayed fertilization facilitates temporal niche differentiation via non-overlapping flowering times among species. Comparing flowering and fruiting times in 228 species from five genera in Fagaceae, we first show that there is a close association between a wider spread of flowering times and the likelihood of a 2-year fruiting habit in which there is a long delay from pollination to fertilization. To study the coevolution of flowering time and delayed fertilization, we developed a mathematical model that incorporates the effects of competition for pollinators, seed predator satiation and unfavourable season for reproduction on fitness. The model shows that delayed fertilization facilitates the diversification of flowering time in a population, which is advantageous for animal-pollinated trees that compete over pollinators. Our new hypothesis about the coevolution of delayed fertilization and flowering time will provide new insight into the evolution of masting. This article is part of the theme issue 'The ecology and evolution of synchronized seed production in plants'.

Fruit development of Lithocarpus (Fagaceae) and the role of heterochrony in their evolution

Stone oaks, or Lithocarpus species of Fagaceae are ecologically important canopy trees in the tropical and subtropical forests over East Asia, and the fruits of which are important food sources for insects and vertebrates there. The great fruit morphological variation of this genus represents two fruit types, acorn and enclosed receptacle fruit types. However, the evolutionary mechanisms of differentiation into these two fruit types with contrasting morphology remain a puzzle. To reveal the morphogenetic properties of two fruit types, we observed tissue differentiation and development among 20 Lithocarpus species from fruit set to maturity. Unlike in fruits of Quercus, the endocarp differentiation in Lithocarpus fruits occurred later than exocarp and mesocarp. Cupules provided further protection of developing seeds, particularly of acorn-type fruits. Fruits of Lithocarpus and Quercus acorns share similar insect predators. At fruit set, both acorn and enclosed receptacle types were largely identical, with similar tissue morphology and the sequence of differentiation. The distinct difference between two fruit types at maturity came from varied rates and degrees of development between the pericarp and receptacle tissues. We found that heterochrony between two tissues could create substantially divergent ecological strategies for protection and dispersal of their seeds, which is essential for the evolution of two fruit types.

Characterization of the cork oak transcriptome dynamics during acorn development

BACKGROUND

Cork oak (Quercus suber L.) has a natural distribution across western Mediterranean regions and is a keystone forest tree species in these ecosystems. The fruiting phase is especially critical for its regeneration but the molecular mechanisms underlying the biochemical and physiological changes during cork oak acorn development are poorly understood. In this study, the transcriptome of the cork oak acorn, including the seed, was characterized in five stages of development, from early development to acorn maturation, to identify the dominant processes in each stage and reveal transcripts with important functions in gene expression regulation and response to water.

RESULTS

A total of 80,357 expressed sequence tags (ESTs) were de novo assembled from RNA-Seq libraries representative of the several acorn developmental stages. Approximately 7.6 % of the total number of transcripts present in Q. suber transcriptome was identified as acorn specific. The analysis of expression profiles during development returned 2,285 differentially expressed (DE) transcripts, which were clustered into six groups. The stage of development corresponding to the mature acorn exhibited an expression profile markedly different from other stages. Approximately 22 % of the DE transcripts putatively code for transcription factors (TF) or transcriptional regulators, and were found almost equally distributed among the several expression profile clusters, highlighting their major roles in controlling the whole developmental process. On the other hand, carbohydrate metabolism, the biological pathway most represented during acorn development, was especially prevalent in mid to late stages as evidenced by enrichment analysis. We further show that genes related to response to water, water deprivation and transport were mostly represented during the early (S2) and the last stage (S8) of acorn development, when tolerance to water desiccation is possibly critical for acorn viability.

CONCLUSIONS

To our knowledge this work represents the first report of acorn development transcriptomics in oaks. The obtained results provide novel insights into the developmental biology of cork oak acorns, highlighting transcripts putatively involved in the regulation of the gene expression program and in specific processes likely essential for adaptation. It is expected that this knowledge can be transferred to other oak species of great ecological value.

Managed bumblebees outperform honeybees in increasing peach fruit set in China: different limiting processes with different pollinators

Peach Prunus persica (L.) Batsch is self-compatible and largely self-fertile, but under greenhouse conditions pollinators must be introduced to achieve good fruit set and quality. Because little work has been done to assess the effectiveness of different pollinators on peach trees under greenhouse conditions, we studied ‘Okubo’ peach in greenhouse tunnels near Beijing between 2012 and 2014. We measured pollen deposition, pollen-tube growth rates, ovary development, and initial fruit set after the flowers were visited by either of two managed pollinators: bumblebees, Bombus patagiatus Nylander, and honeybees, Apis mellifera L. The results show that B. patagiatus is more effective than A. mellifera as a pollinator of peach in greenhouses because of differences in two processes. First, B. patagiatus deposits more pollen grains on peach stigmas than A. mellifera, both during a single visit and during a whole day of open pollination. Second, there are differences in the fertilization performance of the pollen deposited. Half of the flowers visited by B. patagiatus are fertilized 9–11 days after bee visits, while for flowers visited by A. mellifera, half are fertilized 13–15 days after bee visits. Consequently, fruit development is also accelerated by bumblebees, showing that the different pollinators have not only different pollination efficiency, but also influence the subsequent time course of fertilization and fruit set. Flowers visited by B. patagiatus show faster ovary growth and ultimately these flowers produce more fruit. Our work shows that pollinators may influence fruit production beyond the amount of pollen delivered. We show that managed indigenous bumblebees significantly outperform introduced honeybees in increasing peach initial fruit set under greenhouse conditions.

Pistillate flower development and pollen tube growth mode during the delayed fertilization stage in Corylus heterophylla Fisch

Unlike most angiosperms, in which fertilization occurs within several days after pollination, fertilization in hazel (Corylus Spp.) is delayed by two to three and a half months. However, the female inflorescences or young fruits are too hard or lignified to be dissected according to regular paraffin sectioning technique. So, what the nature of development during the extended progamic phases of hazel remains unknown. The female inflorescence development and pollen tube growth mode during the delayed fertilization stage in hazel were investigated by improved paraffin sectioning and aniline blue staining of pollen tubes. The results showed ovaries and ovules of hazel were invisible at the time of blooming. Early ovary and ovule primordium began to form from 15 to 20 days after blooming, respectively. Integument and mature embryo sacs differentiated from the nucellus on 40th and 55th day after blooming, respectively. Pollen tubes were retarded in the bottom of the style or the pollen tube cavity (PTC, a specifical lignified cavity structure at the bottom of style for pollen tube to rest during progamic phase) for about 26 days. Then, the pollen tubes were observed to leave the PTC and began to enter the ovary. After that, a single pollen tube passed through the vicinity of the micropyle. Finally, pollen tubes turned a corner and penetrated the embryo sac through the tissue of the chalaza instead of micropyle on 52 and 55 days after blooming, respectively. The results of more in-depth information will be beneficial to better understanding of the delayed fertilization process in hazel.