Research progress on flower development of Juglandaceae plants

Juglandaceae plants are monoecious and dioecious, and they are major woody oil crops and nuts. Juglans and Carya fruits are highly valuable economically; however in practice, the ratio of male to female flowers on the tree is out of balance, with an excessive number of male flowers and a deficiency of female flowers. This phenomenon could significantly reduce the tree's economic yield and benefit. The effects of the physical characteristics of male and female flower growth, location, and endogenous hormones on flower bud differentiation, as well as the function of genes linked to floral induction and a flower development model, were reviewed in this paper. Most Juglandaceae plants have unisexual blooms, which typically open once a year. Nonetheless, a few early-bearing Juglans regia varieties can produce a large number of secondary flowers and fruits in a unique environment that allows for late-autumn blossoming. The female flower bud differentiation process can be separated into four stages: the emergence stage of the female flower primordium, the formation stage of the involucre and perianth, and the formation stage of the pistil primordium. The flower bud differentiation process can take up to a year. The undifferentiated stage of the male flower, the differentiation stage of the male inflorescence, the differentiation stage of the male flower primordium, the differentiation stage of the stamen primordium, and the formation stage of the anther and pollen grain are the main divisions of the male flower differentiation period. Furthermore, it is possible to determine the stage of flower bud development by looking at the outward morphological traits of the buds, which is useful for managing blooming and floral induction. Numerous plant hormones, including CTK, GA, ABA, IAA, and PAs, significantly impact the differentiation of flower buds. GA promotes the development of male flowers, and spraying proper concentration of PAs can increase the amount of female flower buds. The balance of the endogenous hormones may be more significant for the flower bud differentiation in Juglandaceae plants, evenif a single exogenous plant growth regulator may control the quantity of male and female flower buds to some extent. It is well known that the material foundation for flower bud differentiation is carbon and mineral nutrition. During the physiological differentiation stage, the soluble sugar concentration rises first in female flower buds. The potassium fertilizer applied during production aids in the differentiation of flower buds, and girdling, branch ringing and root cutting can all be used to control C/N and induce flowering. Plants have developed sexual and reproductive allocation mechanisms over their lengthy evolutionary history. There is conjecture that male flower buds situated at the base of walnut plants, which differentiate early, would prefer obtaining greater nutritional resources. Consequently, there would be significantly more male flower buds than female ones. The LFY, FT, CO, and AP1 in the flowering pathway may have favorable effects on the female flower formation in Juglandaceae plants, and the MADSbox family also has a significant impact on flower development, according to research on the flower development of the model plant Arabidopsis thaliana. Up to 77 members comprise the MADS-box family of J. regia, and some MADS-box genes are strongly expressed in floral organs. Following their heterologous transformation into A. thaliana, the JrAG, CiMADS9, and CcAGL24 underwent a considerable alteration in their floral phenotype. Genes that are homologous to MADS-box have a variety of regulatory roles. For instance, functional distinctions exist between the homologous genes CcAGL24a and CcAGL24b in Carya cathayensis when controlling carpel formation. A. thaliana plants that overexpress CcAGL24b have longer carpels, larger sepals, and depression, whereas plants that overexpress CcAGL24a have shorter, thicker carpels. The lncRNAs are a type of epigenetics that regulate plant growth and development but do not encode proteins. Analysis was made using the J. regia female flower bud and leaf bud development miRNA-seq database. It was shown that female flower buds had larger levels of differentially expressed miRNAs than leaf buds. It was discovered that the target gene of miR156/ 157 was the flower-promoting gene SPL. The current lack of a complete understanding of the genetic system of Juglandaceae plants restricts the study of linked genes’functional properties. We will keep working to find a solution for the genetic transformation system in the future. When combined with additional biological techniques, it should be possible to precisely induce more female flowers, uncover the essential genes for determining the sex of flowers, and enhance the regulatory network map of the blooming pathway.