Advances in research on male sterility in fruit trees

In the reproductive biology of fruit trees, male sterility, as an important agronomic trait, is directly related to pollination efficiency and breeding strategies. In the industry, this trait has been utilized for the selection of new varieties and the production of seedless fruits. According to different genetic modes, it is usually manifested as cytoplasmic male sterility (CMS) and nuclear male sterility (GMS). GMS is controlled by a single or a few genes in the nuclear genome and follows Mendelian inheritance laws, while CMS is usually caused by the interaction between genes in the mitochondrial genome and nuclear genes, and exhibits non-Mendelian inheritance characteristics. Male sterility in peach and apricot is of the GMS type, controlled by the Ps and Ms genes, respectively, while citrus belongs to the CMS type. Male sterility affects the efficient and labor-saving cultivation of fruit trees and the progress of seedless breeding. The physiological and molecular mechanisms of male sterility in fruit trees remain unknown. Male sterility in fruit trees exhibits diverse morphological manifestations, primarily including pollen abortion, abnormal tapetum structure, anther degeneration, stamen degeneration or morphological abnormality, microsporangium degeneration, and microspore degeneration, etc. Male sterility in fruit trees can be identified through visual inspection, the staining method, the pollen in vitro culture method, and in vivo identification. As a specific tissue that directly contacts microspores, the tapetum undergoes programmed cell death (PCD) during its development. Abnormalities in the structure or function of the tapetum are one of the key factors leading to male sterility in fruit trees. Tapetal programmed cell death (PCD) provides necessary enzymes, sporopollenin precursors, and other nutrients for the normal maturation of microspores and the formation of the pollen exine. During pollen development, the ability of carbohydrates, other nutrients, plant hormones, and antioxidant systems to remove free radicals significantly affects fertility. In addition, abnormal mitosis and meiosis of microspores during pollen development can also cause male sterility in apricots and citrus. Environmental factors, such as extreme temperatures (either too high or too low), insufficient light, drought or flooding stress, and pollutants in the air or soil, can also affect pollen fertility in fruit trees. During the process of microspore formation in peach, the genes regulating male sterility include the PpABCG26, PpCYP703A2, Pp4CL, and Prupe.6G025000 (CLSE6). In grape, the VvMs1, Vvms2, VviINP1, and VviPPR are involved in the regulation of microspore development. The Pbr035883.1 regulates the microspore development in pear. The genes Barnase, CgAP3.2, and CrMER3 affect pollen development in citrus fruits. The CmTAR regulates pollen development in chestnut. The ATP1 regulates pollen development in jujube. PmGRF7 regulates the formation of male sterility in plum. Additionally, methylation affects the pollen fertility in citrus. This study not only summarizes the research progress on the manifestation forms, identification methods, genetic studies, physiological mechanisms, and gene mining of male sterility in deciduous fruit trees such as peach, pear, grape, cherry, jujube, and chestnut, but also reports the research progress on the physiological mechanisms and gene mining of cytoplasmic male sterility in citrus. However, there are few reports on the molecular mechanisms of male sterility in fruit trees. While extensive research on pollen development in many species has focused on the synthesis and transport of sporopollenin, a comprehensive mechanistic understanding of pollen abortion at the molecular level is perhaps best exemplified by studies in citrus. The miR159a-DUO1 module regulates citrus pollen development by modulating auxin biosynthesis and starch metabolism. In the process of peach pollen development, a possible transcriptional regulatory mechanism involving DYT1-TDF1-AMS-MS188-MS1 exists. The polymerization and establishment of sporopollenin involve two crucial steps: synthesis and transport. The synthesis of sporopollenin refers to the synthesis and secretion of its precursors, while its transport refers to the final delivery to the microspores and the formation of the fine exine structure on the microspores. Sporopollenin is produced and accumulated in the tapetum by Acyl-CoA synthetase 5 (ACOS5), while type Ⅲ lipid transfer proteins (LTPs) function as components to transport it from the tapetum to the pollen exine. In the future, research on male sterility in fruit trees will focus on the exploration of male sterility genes and the analysis of their molecular mechanisms. Subsequently, it will involve the study of the mechanisms through which environmental factors influence the expression of these sterile genes. This study can offer significant theoretical support for the simplified cultivation of fruit trees and the breeding of new germplasm.