Progress in research on the dwarfing mechanism of pear dwarfing rootstocks

Abstract: Pear, belonging to the genus Pyrus, is the third-largest fruit crop in China, and its cultivated area, yield, and export volume rank first in the world. Most pear varieties in China have tall trees, exuberant growth and long life. Vigorous rootstock cultivation techniques are widely used in production, which has resulted in some problems, such as too large tree canopy, bearing fruit late, difficult manage-ment, or excessive tree vigor, poor orchard ventilation and light transmission, poor fruit quality and so on. As a new cultivation mode, the dwarfing and dense planting has the advantages of early- bearing fruit and high yield, good quality, convenient management and rapid variety renewal, which can make up for the shortcomings of the traditional vigorous growing mode. There are two main measures to achieve dwarfing cultivation of fruit trees, one is to use the dwarfing rootstock, and the other is to use the dwarfing varieties. At present, there are not many pear dwarfing varieties available, so the use of dwarfing rootstocks is the main way to realize the dwarfing and intensive cultivation of pear. However, the dwarfing mechanism of pear dwarfing rootstocks has not been revealed, and it is of great theoretical and practical significance to investigate the dwarfing mechanism of pear dwarfing rootstock to improve the cultivation technology and accelerate the breeding process of pear dwarfing rootstock. In recent years, while the selection and breeding of pear dwarf rootstocks are carried out in order, the research on dwarfing mechanisms is also gradually deepened and has been made a great progress. About the dwarfing mechanism of dwarfing rootstock, the early research was mainly focused on the tissue structure and other aspects. Through the observation of the anatomical structure of different kinds of rootstocks, scholars at home and abroad found that the root/skin ratio, branch/skin ratio, leaf structure, xylem vessel structure, and phloem sieve tube structure were closely related to the height of the plant. Besides, with the increase in the length of the interstock segment, the dwarfing effect will be more obvious. The material transport in plants includes the transport of water, mineral elements, and assimilation products. The rootstock affects the growth and development of the tree and achieves the dwarfing efficacy by influencing the absorption and utilization of water and nutrients by the scions. The response of leaves to soil mineral elements in the whole tree is the most sensitive, and the content level can indicate the absorption and utilization of soil mineral elements. Therefore, the material transport characteristics of pear dwarfing rootstocks can be reflected by the content of mineral elements in scion leaves. At the same time, phenolic acids/derivatives and flavonoids have been proved to be two important categories of metabolites involved in the regulation of dwarfing of pear rootstock. Silencing of the gene for hydroxycinnamoyl- CoA shikimate/quinate hydroxycinnamoyl transferase (HCT), a lignin biosynthesis enzyme, was previously shown to redirect the metabolic flux from the lignin biosynthesis pathway to the flavonoid pathways. The accumulation of flavonoids will seriously slow down plant growth and achieve the dwarfing effect. In addition, the concentrations of two monosaccharides, D-sorbitol and D-mannitol are drastically reduced in the roots with“OHF51”interstock present, which is also one of the reasons for tree dwarfing. The relationship between plant hormones and dwarfing is one of the current research hotspots. Most scholars believe that plant dwarfing is the result of cell division or reduced elongation, which is usually regulated by plant hormones. Studies have shown that gibberellins (GAs), abscisic acid (ABA), auxin (IAA), cytokinin (CKs), brassinolide (BR), and other hormones are related to the dwarfing ability of rootstocks. With the rapid development of molecular biotechnology, great progress has been made in the study on dwarfing genes in pear rootstocks. According to the current research results, the genes leading to the dwarfing of pear rootstock can be divided into two types: one is the dwarfing gene, that is, the dwarfing gene directly controls the plant characteristic to make it dwarf, and plays a direct role; the other includes plant hormone synthesis and signal transduction related enzyme genes and microtubulin genes, which play an indirect role. At present, most of the current studies focus on single gene regulation of dwarfism in pear trees, and there are few studies on the regulatory relationship of multiple genes on dwarfism, which need to be combined with genomics, transcriptomics, proteomics and metabolomics, and other high-throughput data for in-depth and systematic studies. Therefore, as for the research on the dwarfing mechanism of pear rootstock, future work should focus on the common regulatory relationship among various hormone metabolic pathways on dwarfing. Meanwhile, combined with the technology of system biology, the dwarfing traits are further helpful to fine QTL mapping by Bulked Segregant Analysis (BSA) or Genome Wide Association Study (GWAS), and key QTL loci are comprehensively analyzed to clarify the upstream and downstream regulatory relationship between each gene.