Focus on Flowering and Reproduction

Flowering plants (angiosperms) emerged on our planet approximately 140 to 160 million years ago and today represent about 90% of the more than 350,000 known plant species (Paton et al., 2008). Flowers are their reproductive structures, which produce fruits containing one to many seeds. Usually flowers are both male and female, and they are often brightly colored to attract animal pollinators, but every possible variation exists from unisexual flowers to inconspicuous flowers that use wind for pollination to flowers that produce seeds without fertilization. During sexual reproduction, pollen produced in anthers, themale flower organ, is transferred to the stigma of the pistil, the female reproductive organ. In compatible conditions, the pollen then germinates and pollen tubes grow in female tissues, eventually delivering two sperm cells to an ovule for double fertilization, a process characteristic of angiosperms (Dresselhaus et al., 2016). After gamete fusion, the fertilized egg cell develops into the embryo, while the fertilized central cell forms the endosperm, the two major components of the seed. The transition of vegetative meristems to the formation of flowers, the subsequent developmental programs that result in a range of flower morphologies, and the diverse pollination and seed development mechanisms have fascinated scientists for hundreds of years. Although the discoveries of processes such as flower induction in vegetativemeristems (Sachs, 1863), photoperiodism (Garner and Allard, 1920), and the double fertilization event (Nawaschin, 1898) occurred at least a century ago, our understanding of the underlying molecular mechanisms of these processes is relatively recent, beginning, for example, with the formulation of the ABC model of flower organ development (Coen and Meyerowitz, 1991), the discovery of the flowering regulator CONSTANS (Putterill et al., 1995), and the identification of the first selfincompatibility gene (McClure et al., 1989). Since then tremendous progress in understanding flowering and reproduction has been made; the last 10 years have seen another boost of further advances inspiring us to organize a Focus Issue representing the first issue about the topic in Plant Physiology. In many plant species, the timing of flowering is critical for reproductive success, and thus the evolution of flowering-control systems that optimize reproductive success provide a selective advantage. For example, flowering must occur early enough in the growing season to enable proper seed development, but premature flowering when a plant is small will limit the amount of seed that can be produced. Also, in outcrossing species, synchronous flowering enables cross-pollination. In many plant species, systems have evolved to perceive the seasonal cues of changing daylength and temperature and to translate that perception into flowering. An Update by Shim et al. (2017) discusses the immense progress in understanding the molecular details of how changes in daylength lead to the induction of flowering. The daylength response system has common features in all flowering plants, indicating an ancient evolutionary origin. In contrast, vernalization systems, which evolved to enable flowering only after plants experienced exposure to the prolonged cold of winter, evolved more recently and, as discussed in the Update by Bouché et al. (2017), are molecularly distinct in different groups of plants. In Arabidopsis (Arabidopsis thaliana), many of the biochemical details of how vernalization results in competence to flower are being uncovered as illustrated in an upcoming research article (D.-H. Kim and S. Sung, unpublished data). Also, temperature fluctuations other than exposure towinter cold influence flowering; the role of this so-called ambient temperature response in petunia (Petunia hybrida) will be discussed in an upcoming research article (H.A.C.F. Leeggangers, H. Nijveen, J.N. Bigas, H.W.M. Hilhorst, and R.G.H. Immink, unpublished data). In some species, a specific developmental state must be reached before plants can respond to seasonal cues; Hyun et al. (2017) discuss in an Update recent advances in our understanding of how such systems operate. Within a species, there is often substantial natural variation in flowering responses. In his Update, Blackman (2017) presents several examples of such variation in a range of species and discusses the possible adaptive value of such variation. In two research articles, Woods et al. (2017) and Bettgenhaeuser et al. (2017) explore the genetic basis of the substantial natural variation in flowering responses in the model grass Brachypodium distachyon. Finally, as discussed in an Update by McGarry et al. (2017), our knowledge of the molecular basis of how flowering is regulated has led to genetic engineering strategies to advance breeding programs and research efforts in crops such as tree species inwhich long generation timeswould otherwise greatly slow progress. An Update by Thomson et al. (2017) revisits the ABC model of floral organ identity indicating that while it has experienced a number of modifications over the past 25 years, the model has been largely validated and essentially applies across angiosperms. Moreover, studies in recent years have yielded new insights into the specificity of the different floral organ identity factor complexes and into the elaborate downstream pathways that execute the ABC program and lead to the formation of the reproductive organs. The genetic consequence and stereotypical cell division pattern during microsporogenesis, formation of www.plantphysiol.org/cgi/doi/10.1104/pp.16.01867