Japanese apricot (Prunus mume Sieb. et Zucc) is an important economic fruit crop in China and Japan, with more than 200 cultivars in China . Japanese apricot fruit has consistently been one of the most valuable processing materials used in the food and wine-making industries and is believed to contain many physiochemicals beneficial to human health. However, the phenomenon of imperfect flowers is common and seriously affects production yields. The percentage of imperfect flowers depends on the cultivar; the highest is 75.15% and the average is 35% . Imperfect flowers are characterised by pistils below the stamens, withered pistils or the absence of pistils, and hence, they fail to bear fruit . Several environmental factors and physiological processes have been shown to affect pistil development [1, 4]. Previous research indicated that several miRNAs and multiple target genes are involved in flower development in model plants [5–12]. More recently, real-time quantitative reverse transcription polymerase chain reaction and in situ hybridisation showed that the PmAG mRNA was highly-expressed in the sepals, carpels and stamens, and a weak signal was detected in the seeds and nutlets. No expressions were detected in the leaves or petals, but no significant differential was expressed between perfect and imperfect flowers . Meanwhile, comparative proteomic analyses were performed on perfect and imperfect flowers, and several differently-expressed proteins were identified . However, the type of molecular mechanism involved in pistil abortion remains unknown in Japanese apricot.
Small RNAs (sRNAs) are low molecular weight RNAs with regulatory functions. Based on differences in biogenesis and action, sRNAs are grouped into two categories: short interfering RNAs (siRNAs) and microRNAs (miRNAs) [14, 15]. MicroRNAs are non-coding RNAs, 21–24 nucleotides (nt) long, which regulate gene expression at the post-transcriptional level [16–18]. In plants, miRNAs are processed from the stem-loop regions of long primary transcripts by a Dicer-like enzyme and are loaded into silencing complexes, where they generally direct the cleavage of complementary mRNAs . Identified in plants less than 10 years ago [19, 20], miRNAs are already known to play numerous crucial roles at each major stage of development, typically at the cores of gene regulatory networks, targeting genes that are themselves regulators, such as those that encode transcription factors, suggesting that plant miRNAs are master regulators [18, 21]. Among non-transcription factor targets, many miRNAs encode F-box proteins or ubiquitin-conjugating enzymes implicated in targeting selected proteins for proteasomal degradation, indicating miRNAs play a role in regulating protein stability and plant development [22, 23]. miR156, miR163, miR169, miR172, miR398 and miR399 play important roles in flowering-time regulation and belong to ambient temperature-responsive miRNAs in plants [9, 24]. miR172 has also acquired specialised species-specific functions in other aspects of plant development, such as cleistogamy and tuberisation .
The fact that a large number of the known miRNAs in the plant kingdom, from mosses and ferns to higher flowering plants, are evolutionarily conserved has been used as a practical indicator for the identification or prediction of miRNAs using homology searches in other species [25, 26]. Recently developed, next-generation, high-throughput sequencing technologies provide a powerful tool for identifying, as well as quantifying, miRNAs. These technologies open up the possibility of exploring sRNA populations in economically important species such as Arabidopsis thaliana[27, 28], Oryza sativa[29, 30], Populus trichocarpa[31, 32], Vitis vinifera, Arachis hypogaea, Citrus sinensis, Citrus trifoliate, Medicago truncatula[37, 38], Glycine max, Carthamus tinctorius
Cucumis sativus, Rehmannia glutinosa and others. By means of high-throughput sequencing, miR164 and miR169 were shown to be drought-responsive miRNAs in Medicago truncatula. miR167, miR1857 and miR172a are involved in the mutant trait formation of lycopene accumulation in sweet orange .
Although miRNAs have been extensively studied in the past, there has be no systematic examination of miRNAs performed on the Japanese apricot. To investigate the role of miRNAs on the pistil development of Japanese apricot, high-throughput sequencing technology (Solexa) was employed to survey sRNA populations from perfect and imperfect flower buds.