Flowering and Flower Development in Plants

In the life cycle of a plant, flowering marks the transition from vegetative growth to reproductive development. The optimal flowering time of different plants is an important agricultural trait that ensures the yield and quality of their harvested products. In the case of ornamental plants, the period of time between anthesis and floral senescence facilitates pollination and ensures the seed production necessary for the survival of the species. In addition, the development of flower organs also plays a crucial role in the reproductive development of plants. Many plant species have evolved multiple ways of regulating flowering to improve their adaptability to endogenous factors and complex environmental conditions. The importance of flowering and flower development, as well as the more comprehensive understanding of this issue that we have developed in recent years, make a systematic summary of the progress of research related to this topic quite necessary.

Extensive physiological and molecular genetic analyses have revealed that plant flowering is largely regulated by six major floral regulatory pathways in Arabidopsis, namely, the vernalization, photoperiod, autonomous, gibberellin, thermosensory (ambient temperature perception), and age pathways [1,2,3,4,5]. These pathways are ultimately intertwined with several central transcription factors, including FLOWERING LOCUS T (FT), TERMINAL FLOWER1 (TFL1), LEAFY (LFY), and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1) [6]. Many plant species have demonstrated that FT promotes their flowering and TFL1 inhibits it [7,8,9,10]. An updated article on the role of transcription factors in the regulation of flowering highlights the involvement of PsFT, PsTFL1, and PsFD in the continuous flowering of tree peony (Paeonia × lemoinei ‘High Noon’) [11]. The authors of this article also discuss how this understanding of the molecular regulatory mechanisms underlying continuous flowering will be conducive to further investigation, and will be of great importance for the breeding of tree peonies and other perennial woody plants. In many species, the role of flowering genes in regulating plant flowering is highly conserved. A recent publication on the function of MADS-box family genes detailed two MADS-box genes—LsAG2 and LsDEF1, isolated from Lagerstroemia speciosa—that were ectopically expressed in Arabidopsis, and how those transgenic plants exhibited early flowering and floral organ aberrations, during which the expression of genes associated with flowering (e.g., AP1, LFY, and FT) were up-regulated [12].

Floral organs are the basis of plant classification and evolution [13]. Studies of the MADS-box family have shown that members of this family are extensively involved in the floral transition and floral organ morphogenesis [14,15]. However, there are still many members of the MADS-box gene family that have not yet been discovered, and the molecular regulatory mechanisms of flower organ development require further exploration. Qin et al. 2023 recently established the evolution and divergence patterns of MIKC-type MADS-box gene family members in six Rosaceae plants [13]. This is important for predicting functional regions of the MIKC-type MADS-box genes and will be extremely useful in the discovery of other genes that regulate key traits.

Flower development was first summarized using the ABC model [16], which classifies the genes involved in the differentiation of floral organs into three categories—A, B, and C—which individually or cooperatively regulate the development of stamens, pistils, petals, and the calyx. Following the cloning and functional identification of a large number of transcription factors related to flower development in both annual plants and perennials, flower development was then summarized using the ABCDE model [17]. The newly identified D-class genes are involved in the regulation of ovule development and formation, while E-class genes are considered to be independent from ABC genes, able to regulate the expression of ABC genes, and ultimately participate in the regulation of flower organs during the floral transition and flower development. It is well known that studies on the molecular mechanisms of flower development are well established in angiosperms, while research into gymnosperms is relatively scarce [18,19]. In this Special Issue, a new publication compares the transcriptomic profiles of male and female Pinus halepensis cones at different developmental stages in order to uncover the genes involved in their reproductive induction and development [20]. The authors demonstrate that the expression of the C-class genes MADS1 and DAL14 correlate with female cone development, whereas male cone development is associated with the expression of the B-class genes PhDAL11 and PhDAL13 and the C-class gene PhMADS2. These results can provide important reference values for further investigation into the developmental stages of reproduction in gymnosperms.

Floral longevity is an important functional trait that reflects the ornamental value of flowering plants. Numerous researchers have conducted studies on the flowering process, including the morphological structure of flowers, and their mineral elements, nutrients, and related genes, in order to elucidate the biological and morphological characteristics that regulate the flowering period of various plants [21,22]. Another cutting-edge article in this Issue depicts the relationship between the water balance and different floral structural traits in six tree peonies, and investigates the relevant mechanism that causes the discrepancies seen in their flower longevity [23]. It is particularly important to understand the morphological development and physiological characteristics of flower organs during different stages of flower development, which makes the development of effective scientific techniques for investigating these traits so significant. A recent publication performed experiments to explore the structure of petals, their nutrient content, hormones, and mineral elements, and the activity of antioxidant enzymes at different flowering stages of Styrax japonicus, in order to indicate which specific factors are important for flowering, and thus lay the foundation for extending the flowering period [24]. Similar to studies on the extension of flower longevity, research on the flower color of ornamental plants is also important for future selective breeding. A recently updated study was the first to elucidate the molecular mechanisms of flavonoid biosynthesis and flower coloring in Loropetalum chinense and Loropetalum chinense var. rubrum via flavonoid metabolomics, transcriptomics, and full-length sequencing [25]. The authors demonstrated that significant differences in flavonoid content were observed among four cultivars with similar genetic backgrounds, and that the expression of the flavonoid synthesis gene correlates to a high proportion of flavonoids in Loropetalum chinense var. rubrum.

It is well known that plant flowering is also affected by various environmental factors [26]. A new review summarizes our current knowledge on the role of environmental factors, such as drought stress, low-temperature stress, and high-temperature stress, in the regulation of annuals and perennials’ flowering time [27]. Recently, plant growth regulators have been widely used in agricultural production due to their significant ability to promote flowering and control the branching of fruit trees [28]. Of these regulators, the inhibitory effect of gibberellin (GA) on flowering has been demonstrated in fruit trees such as pear, plum, peach, and apple [29]. Conversely, the application of paclobutrazol (PBZ) can promote the flowering of plants [30]. A publication in this Issue records the results of spraying lemons with different concentrations of GA and PBZ in order to study the effects of these two plant growth regulators on the nutritional physiology and flower formation of lemons [29]. The authors discovered that the optimal PBZ concentration for promoting lemon flowering was 600 mg/L, and that the quality of the lemons was also improved accordingly. The physiological effect of GA on lemon flowering was the opposite to that of PBZ. The selection of appropriate concentrations of plant growth regulators is of great significance in controlling the flowering and fruit setting of fruit trees; it can also provide a theoretical basis for the high quality, high yield, and stable production of fruit trees.

Flowering and flower development are critical to the reproductive success of plants and still require further investigation for future plant breeding. The series of articles presented in this Special Issue clearly illustrate that there are both conservative and highly-specific ways to regulate the flowering and flower development of different plants. We look forward to future scientific advancements in the reproductive development of plants, which may be influenced by our understanding of these Special Issue articles, as well as to the development of eco-friendly and innovative strategies for the regulation of flowering and flower development.