Research progress in miRNAs in fruit trees

Abstract: The functional studies, mostly from model species, have revealed that miRNAs are majorpost-transcriptional regulators of gene expression in plants and are implicated in fundamental biologicalprocesses, such as plant development and abiotic/biotic stress responses. miRNAs have been detected indifferent parts of fruit trees, including leaves, inflorescence, roots and fruits, and there are significantdifferences in expression levels among different parts or in different developmental processes. With therapid development of sequencing technology, a substantial number of miRNAs have been identified in aseries of fruit crops sofar, while the target mRNAs and functions remain largely uncharacterized. Thepresent review summarizes the progress in miRNA research in fruit crops, especially the role of miR-NAs on the economically important species, such as grapes, peaches, pears, apples, oranges, and so on.Some studies show grapes have the ability of anti-abiotic/biotic stress attributed to the significant ex-pression of miRNAs and the miRNAs also respond to exogenous hormones. The above-mentioned abi-otic/biotic stress responses to miRNAs are also observed in apples, oranges, pears and peaches. In ap-ples, miRNAs regulates the metabolic pathways of IAA, ABA, GA, and thus regulates the process offlower bud differentiation and juvenile period. Moreover, the activation of miRNAs influences the fruitsize of apples as well. Boron and magnesium are implicated in citrus tree development and fruit quality, and some studies have found that miRNAs are widely involved in the response of citrus to boron andmagnesium by regulating auxin synthesis, leaf morphogenesis, antioxidant system and boron transport.In citrus, miRNAs are also active in apomixis, somatic embryogenesis and cytoplasmic male sterile hy-brids. Color is a part of fruit quality. The miRNAs are involved in carotenoid metabolism and then influ-ence the citrus fruit color. Some research have showed orange juice sacs are related to the expression ofmiRNAs, while the quantity of juice sacs affects the taste of citrus. This function of fruit quality regula-tion is also found in pears. The target gene of miRNAs in pears encodes some enzymes, which is relatedto the formation of lignin and stone cells. The role of miRNAs in bud dormancy and dormancy releaseof pears was also detected. Except for the above-mentioned fruit trees, twith some additional fruit trees, the miRNAs and its targe genes have been researched, such as papayas, bananas, strawberries, litchi, etc. We also discuss the future miRNA research prospects in fruit trees: ( Ⅰ) To use RLM-RACE, 3'PPMRACE, degradome sequencing and genetic transformation technology to enhance the ability of pre-dicting and validating miRNAs target genes, ( Ⅱ) to enhance the research on miRANs and its targetgenes in fruit trees by the creation of efficient transformation germplasm resources of fruit trees, tran-sient transformation, virus vector-mediated transformation or natural variation materials of fruit tree va-rieties for some fruit trees with low efficiency of genetic transformation,  ( Ⅲ) to construct molecularregulation of important biological processes mediated by target miRNAs-target genes by means of vari-ous genomics and other bioinformatics methods, to combine the results of functional genomics, and tosystematically analyse the regulation mechanism of plant development or stress response, in which thattarget miRNAs and its Target genes are involved, (Ⅳ) to discover many conservative, non-conservativeand fruit-specific miRNAs and (Ⅴ) to declare how miRNAs regulates some specific traits of fruit trees, such as flower bud formation, bud dormancy, juvenile transformation, fruit coloring and cell engineer-ing breeding, etc.