Progress on tree architecture and key genes of its regulation in peach

Abstract：Peach (Prunus persica L.) is one of the most important fruit trees in China. The yield and cultivation area of peach in China rank firstly in world. The unique management and horticulture practices of fruit trees, such as pruning, spraying and harvesting strategies, maximized production and orchard efficiency. The control of tree size is critical for the optimization of productivity and for limiting the amount of labor and inputs needed for orchard management. The exploring of the molecular mechanism of tree architecture and breeding of new cultivar which is suitable for labor-saving cultivation in economically important fruit trees attracts attention of many researchers. The study of the characteristics of tree architecture mainly focuses on height, internode length, branch orientation and distribution. A large number of studies have been carried out on the heredity of these characteristics, and many achievements have been made. Previous studies indicated the plant dwarfism was controlled by a recessive gene (dw/dw) and related to GA (Giberellic acid) biosynthesis and signaling genes. A recessive dwarfism trait (dw) in peach trees is caused by a nonsense mutation (from a tryptophan to a premature stop codon, gid1cW162* ) within GA receptor PpeGID1c. The tree size was regulated by modification of GID1c expression, and the fruit development was not affected. Another single nonsynonymous nucleotide mutant in PpeGID1c (gid1cS191F ) was observed in Fenhuashouxingtao, a GA-insensitive dwarf mutant. The mutant site in PpeGID1c (gid1cS191F ) was also co-segregated with dwarf phenotype. Three genotypes, including gid1cW162* / gid1cW162* , gid1cS191F /gid1cS191F and gid1cW162* /gid1cS191F , were observed in other 12 dwarf cultivars. Meanwhile, gid1cS191F was unable to interact with the growth-repressor DELLA1,the degradation of which is normal induced by its interaction with GID1. SD9238 is a semi-dwarf mutant controlled by a single dominant gene. The temperature-sensitive semi-dwarf (Tssd) gene was located 500 kb region in scaffold 3 using SLAF sequencing and SNP analysis. Variation tree architecture in peach trees is associated with a single-dominant gene named broomy (br). Pillar trees (br/br) has vertical branches and upright trees (Br/br) has upright growth habit with less vertical branches. In peach, the vertical branch growth was caused by a SNP or a 3-Kb insertion in TAC1 belongs to IGT gene family. The variation expression of TAC1 was existed in pillar, standard and upright peach. TAC1 promoted horizontal branch growth and the pillar phenotype was due to lacking of expression. And PpeTAC1 is specifically expressed within or near actively growing vegetative and reproductive tissues, including flower buds and branch attachment sites. Recent studies indicated TAC1 regulates branch angle in response to light downstream of photosynthetic signals. LAZY1 was also belong to IGT gene family. LAZY1 has been shown to have the opposite effect of TAC1 on branch angle. The auxin concentration of shoots in pillar trees during the growing season were higher than standard phenotype. Some studies showed LAZY1 protein interacted with some proteins related to auxin signaling and transport pathway in the nucleus and at the plasma membrane, respectively. The LAZY1-mediated upward branch might be by the formation of asymmetric auxin concentration in the gravitropic response pathway. A pendulous weeping peach phenotype with elliptical branch growth was caused by a deletion in WEEP gene which might altered growth direction and contributes to gravitropic perception of response. The MAX genes were first identified in Arabidopsis thaliana, and the mutants of MAX genes led to increased branching. Elevated auxin concentrations and higher gene expression of MAX3 in roots and MAX4 in stems were found in pillar rather than standard trees in peach. In addition, new plant growth regulators, such as strigolactone, may affect the modes of action of root-originating signals. Based on the above results, ideal tree architecture is proposed in this study. Moreover, it provides a reference for molecular mechanism of tree architecture and improvement of germplasm by molecular design and gene editing in peach.