Research progress on GA signaling pathway and its function in regulating fruit trees growth and development

Abstract：GAs are geranylgeranyl diphosphate (GGDP) -derived phytohormones, control diverse aspects of plant growth and development, such as seed germination, flowering, root development and fruit development. At least 136 GAs have been identified, but only a few GAs such as GA1, GA3, GA4 and GA7 exhibit biological activity. In the past decades, great progress has been made in the understanding of GA signaling transduction and a functional GA-GID1-DELLA-SCFGID2/SLY1/SNE-plant development derepression regulatory model has been established. GID1 was identified from rice in 2005, the loss-offunction gid1 mutant exhibit a severely dwarfed phenotype which was insensitive to exogenous GA. Recombinant glutathione S-transferase (GST) -GID1 showed high affinity only for biologically active GAs, whereas mutated GID1 had no GA-binding affinity. The Arabidopsis genome encodes three GID1 homologous, At GID1 a, At GID1 b and At GID1 c. GA-binding activities of At GID1 a/At GID1 b/At GID1 c were confirmed by biochemical analyses in vitro. With the uncovering of Os GID1 crystal structure, scientists found Os GID1 showed an alpha/beta-hydrolase fold characteristic. GID1 s interact with N-terminal domain of DELLA proteins directly in a bioactive GA-dependent manner and promote the degradation of DELLA proteins via the 26 S proteasome pathway. The DELLA proteins are members of theplant-specific GRAS gene family, acting as negative regulators of GA signaling. They include the conserved N-terminal DELLA and TVHYNP motifs which are responsible for the interaction with GID1 and the conserved C-terminal GRAS domain which mediates the secondary interactions with the GID1 and F-box proteins. DELLA proteins function as transcriptional repressors by blocking the activity of transcription factors such as PIFs, NACs, JAZ, SCL3, IDDs, SPLs etc. GID2 (GIBBERELLIN INSENSITIVE DWARF2) , SLY1 (SLEEY1) and SNE (SNEEZY) all belong to E3 ubiquitin ligase F-box protein. SLY1/GID2/SNE positive regulate GA signaling through mediating the degradation of DELLA proteins via the 26 S proteasome system. The binding of bioactive GAs to GID1 induce the formation of GA-GID1-DELLA protein complex, and then enhances the interaction between DELLA proteins and SCFSLY1/GID2/SNEcomplex. GAs have long been associated with dwarfism in plant and beneficial mutations in GA pathway had been used in fruit trees breeding. A semi-dominant Vvgai allele that contains a gain of function mutation in the DELLA domain had been used in grape dwarfing breeding in the 1500 s, long before the wheat“green revolution”in 1950 s. The peach GID1 c act as the GA receptor and regulate vegetative growth, a nonsense mutation within Ppegid1 c result in dwarf phenotype while fruit development was not impaired. Besides GA perception and signaling genes, GA biosynthesis and inactivation genes also play important role in fruit trees dwarfing breeding. For example, suppression of GA20-oxidase in apple reduced the levels of bioactive GAs, resulting in significant reduction in plant height.The dwarfing phenotype of scion was not affected when grafting on to normally rootstocks. In plum, high expression level of GA inactivation gene Psl GA2 ox associated with low accumulation of bioactive GA1 and GA4, and resulting in smaller leaves, shorter stems, late flowering. Therefore, both GA perceptions, signal transduction, synthesis and inactivation genes are all applicable in fruit trees dwarfing breeding. GAs also play an important role in coordinating fruit development processes, application of GAs could significantly improve fruit size, weight and many other characteristics. Pollination significantly increases GAs content by inducing gibberellin biosynthesis in fruit trees such as apple and grape.In addition, GAs involve in the induction of parthenocarpy in fruit trees. Wittwer firstly demonstrated that exogenous GAs were able to induce tomato parthenocarpy in 1957, thereafter GAs were used to induce parthenocarpy in many fruit trees, such as apple, grape and pear. Plant dormancy refers to the slow growth or temporary cessation of whole plant or some organ growth. GAs are widely used in bud dormancy breaking to improve fruit yield and quality in fruit trees, such as citrus, peach and cherry. GAs are also important signaling substances that relieve seed dormancy. GAs are synthesized in embryo and then transport to aleurone cells to induce a series of hydrolases, such as alpha-amylase, protease and beta-glucanase. Up to now, only a few fruit trees have obtained dwarfing genetic resources, thus GA synthesis inhibitors are widely used in fruit trees to inhibit vegetative growth, including dwarfing fruit trees, shortening internodes, promoting rhizogenesis etc. In 2017, a series of breakthroughs in GA signal transduction have been discovered, such as the degradation mechanism of GA receptor GID1 s, the identification of new DELLA interaction proteins and the function of O-fucosyltransferase in GA signaling pathway.These new discoveries enhance the understanding of GA signaling pathway. The application of GA mutants in fruit trees breeding and the application of GA-related growth regulators in fruit trees cultivation have proved that artificial control of GA signal could significantly enhance the yield of fruit trees. In view of the important roles of GAs in plant growth, this review will introduce the latest research progress in GA signaling pathway and its function in regulating fruit trees growth and development.