Research progress on pistil abortion in fruit trees

Abstract: Pistil development is directly related to the flowering and fruiting of fruit trees. The pistil abortive phenomenon is prevalent in fruit trees, which has significant impacts on the yield and quality of the fruit. There are one or more separate pistils in a single gynoecium, which are considered to be composed of carpels. A pistil of the hermaphrodite flower is typically composed of three parts: ovary, style and stigma. The ovary is the expanded basal portion of the pistil and it provides the structural support and develops into seeds after fertilization. The style is the elongated section of the pistil; it is a hollow tube through which pollen tubes grow to reach the ovary. The stigma is the apical structure of the pistil, which is used to accept and recognize pollen. The pistil plays a significant role in fruit trees sexual reproduction, the development of the pistil consists of the development of the ovary, the stylar development, and the pollination and fertilization of the fruit trees. Ovules of most fruit trees consist of four parts: an outer layer called the integuments, the nucellus, the funiculus and the female gametophyte. The parenchyma cells in the interfaces of the carpel edge develop into placeta, one or more cells under the placenta epidermis differentiate into the ovule primordia. The ovule primordia undergoes elongated growth and gives rise to a finger-like nucellus. The archespore which is the initiation of the female gametophyte is formed at the tip of the ovule primordia. The archesporial directly differentiates into the megasporocyte, which twice undergoes meiotic divisions and forms four megaspores. Three megaspores near the chalazal ultimately degenerate, only the chalazal-most megaspores continue to undergo thrice nuclear divisions and develop into mature embryosacs, which consists of a central cell, two synergids, and three antipodal cells. If one of the tache is impeded during the development of the fruit floral pistil, the pistil would be abortive. Similarly, the impeded stylar development can also lead to the pistil's abortion during the floral pistil development. In addition, upon fertilization, the ovules develop into seeds. The pollination and fertilization impeded by internal and external factors during pistil development can also result in the pistil's abortion. The manifestation of the pistil abortion shows diversity, so are the reasons that caused the pistil abortion. Variations in pistil are connected with different factors such as gene regulation, light, humidity, temperature, nutritional conditions and so on and so forth. There is great significance for improving the yield and quality of the fruit by studying the molecular regulation mechanism, seeking the factors influencing the development of the pistil, and discovering the way for overcoming the pistil abortion. Thus, the studies on pistil abortion have been paid given a lot of attention since the 1950 s. Though the gene regulatory networks controlling the development of the pistil are as yet unclear, great efforts have been made and a lot of progress has been achieved. This review will focus mainly on pistil abortion in fruit trees with emphasis on the possible causes of pistil abortion from genetic, physiological and biochemical characteristics, and external circumstances. From the previous experimental results, it can be concluded that pistil abortion is subject to internal and external factors. The internal factors include gene regulation, programmed cell death, and changes of the internal storage substance. In order to understand the mechanism of plant pistil abortion, the latest program in pistil abortion at home and abroad are mentioned, for example, some latest studies on the floral development“ABCDE”model and other related genes. Certain correlations exist among the internal factors, and the environment factors inducing signals which affect the accumulation and transfer of plant nutrients and internal growth substances, which regulate the expression of related genes that impede plant pistil development. External factors include nutrient supply imbalance and meteorological factors. Moreover,four expression forms of pistil abortion are illustrated briefly in this review, which include integument dysplasia, embryo sac hampering, style structural abnormalities, and the pistil differentiation process which can be stagnate or slow. The effective ways to overcome pistil abortion are also summarized, such as spraying the plant growth regulator, fertilization, and measures for facilities protection. What is more, the researches on pistil abortion are faced with a significant challenge. On one hand, the juvenescent phase of fruit trees is so long that it is hard to observe the flower phenotype. On the other hand, most of the fruit tree genome database data is not comprehensive enough, although lots of the fruit species' genome is available.And last but not the least, because of transgenic and other molecular biological technique constraints, the fruit pistil development research is still at the basic level. Therefore, the effective ways to overcome pistil abortion and the research prospects of pistil abortion are all topics that have been covered in this review. In the end, the future prospect of additional research on fruit pistil abortion was proposed.