Unisexual flower initiation in the monoecious Quercus suber L.: a molecular approach

Genetic and epigenetic control of dormancy transitions throughout the year in the monoecious cork oak

Bud dormancy plays a vital role in flowering regulation and fruit production, being highly regulated by endogenous and environmental cues. Deployment of epigenetic modifications and differential gene expression control bud dormancy/break cycles. Information on how these genetic and epigenetic mechanisms are regulated throughout the year is still scarce for temperate trees such as Quercus suber. Here, the expression levels of CENTRORADIALIS-LIKE (CENL) and DORMANCY-ASSOCIATED PROTEIN 1 (QsDYL1) during seasonal cycles of bud development, suggesting that QsCENL may be implicated in growth cessation in Q. suber and that QsDYL1 is a good dormancy marker. As gene expression can be regulated by the activity of chromatin modifiers, we have analysed the expression of these genes and the deposition of epigenetic marks in dormant versus non-dormant bud meristems. The DNA methyl transferases CHROMOMEHTYLASE 3 (QsCMT3) and METHYLTRANSFERASE 1 (QsMET1) were more expressed in the transition between dormancy to bud swelling. QsCMT3 was also highly expressed during the late stages of active bud formation. Conversely, the HISTONE ACETYLTRANSFERASE 1 (QsHAC1) was up-regulated during growth cessation and dormancy when compared to bud swelling. These results indicate that epigenetic regulation is implicated in how bud development progresses in Q. suber, which can be observed in the different profile deposition of the repressive and active marks, 5mC and H3K18Ac/H3K4me, respectively. The identification of bud-specific genetic and epigenetic profiling opens new possibilities to predict the relative rate of dormancy/growth of the bud stages, providing tools to understand how trees respond to the current challenges posed by climate change.

How Climate Change May Impact Plant Reproduction and Fitness by Altering the Temporal Separation of Male and Female Flowering

The temporal separation of male and female flowering-known as dichogamy-is a widespread adaptation across the plant kingdom that increases reproductive success and enhances plant fitness. Differences in timing between male and female flowering can be highly sensitive to environmental variation-and with widespread evidence of shifts in seasonal timing of flowering (i.e., phenology) due to anthropogenic warming-climate change may alter the sequences of male and female flowering for a diversity of taxa around the globe. However, we currently lack a broad understanding of both the extent to which climate change may alter patterns of dichogamy and the potential implications of these shifts for plant reproduction. Here I present evidence that links variation in dichogamy to variation in temperature for a variety of plant taxa. I synthesize the limited number of studies that have investigated shifts in dichogamy specifically in the context of climate change, and detail the physiological, genetic, and developmental factors that control the relative timing of male and female flowering. The literature indicates that dichogamy is highly plastic and sensitive to temperature variation. Plasticity in dichogamy is observed across species with different sexual systems and growth habits, and in both female-first and male-first flowering taxa, but at present, no clear patterns of dichogamy shifts related to these associated traits are discernible. Together, these lines of evidence suggest that sequences of male and female flowering are likely to shift with climate change. However, more research is needed to better understand and predict the ecological consequences of shifting patterns of dichogamy in the context of global change.

Reproduction alternation in trees: testing the resource depletion hypothesis using experimental fruit removal in Quercus ilex

The keystones of resource budget models to explain mast seeding are that fruit production depletes tree stored resources, which become subsequently limiting to flower production the following year. These two hypotheses have, however, rarely been tested in forest trees. Using a fruit removal experiment, we tested whether preventing fruit development would increase nutrient and carbohydrates storage and modify allocation to reproduction and vegetative growth the following year. We removed all the fruits from nine adult Quercus ilex L. trees shortly after fruit set and compared, with nine control trees, the concentrations of nitrogen (N), phosphorus (P), zinc (Zn), potassium (K) and starch in leaves, twigs and trunk before, during and after the development of female flowers and fruits. The following year, we measured the production of vegetative and reproductive organs as well as their location on the new spring shoots. Fruit removal prevented the depletion of N and Zn in leaves during fruit growth. It also modified the seasonal dynamics in Zn, K and starch in twigs, but had no effect on reserves stored in the trunk. Fruit removal increased the production of female flowers and leaves the following year, and decreased the production of male flowers. Our results show that resource depletion operates differently for male and female flowering, because the timing of organ formation and the positioning of flowers in shoot architecture differ between male and female flowers. Our results suggest that N and Zn availability constrain flower production in Q. ilex, but also that other regulatory pathways might be involved. They strongly encourage further experiments manipulating fruit development over multiple years to describe the causal relationships between variations in resource storage and/or uptake, and male and female flower production in masting species.

Population structure in Quercus suber L. revealed by nuclear microsatellite markers

Quercus suber L. is a sclerophyllous tree species native to the western Mediterranean, a region that is considered highly vulnerable to increased temperatures and severe dry conditions due to environmental changes. Understanding the population structure and demographics of Q. suber is essential in order to anticipate whether populations at greater risk and the species as a whole have the genetic background and reproductive dynamics to enable rapid adaptation. The genetic diversity of Q. suber has been subject to different studies using both chloroplast and nuclear data, but population structure patterns remain unclear. Here, we perform genetic analyses on Q. suber using 13 nuclear microsatellite markers, and analysed 17 distinct locations across the entire range of the species. Structure analyses revealed that Q. suber may contain three major genetic clusters that likely result from isolation in refugia combined with posterior admixture and putative introgression from other Quercus species. Our results show a more complex structure scenario than previously inferred for Q. suber using nuclear markers and suggest that different southern populations contain high levels of genetic variation that may contribute to the resilience of Q. suber in a context of environmental change and adaptive pressure.

The Dynamics of Flower Development in Castanea sativa Mill

The sweet chestnut tree (Castanea sativa Mill.) is one of the most significant Mediterranean tree species, being an important natural resource for the wood and fruit industries. It is a monoecious species, presenting unisexual male catkins and bisexual catkins, with the latter having distinct male and female flowers. Despite the importance of the sweet chestnut tree, little is known regarding the molecular mechanisms involved in the determination of sexual organ identity. Thus, the study of how the different flowers of C. sativa develop is fundamental to understand the reproductive success of this species and the impact of flower phenology on its productivity. In this study, a C. sativa de novo transcriptome was assembled and the homologous genes to those of the ABCDE model for floral organ identity were identified. Expression analysis showed that the C. sativa B- and C-class genes are differentially expressed in the male flowers and female flowers. Yeast two-hybrid analysis also suggested that changes in the canonical ABCDE protein-protein interactions may underlie the mechanisms necessary to the development of separate male and female flowers, as reported for the monoecious Fagaceae Quercus suber. The results here depicted constitute a step towards the understanding of the molecular mechanisms involved in unisexual flower development in C. sativa, also suggesting that the ABCDE model for flower organ identity may be molecularly conserved in the predominantly monoecious Fagaceae family.