Ethereal Site

In eukaryotes, sRNAs have been found to control cellular metabolism, growth and differentiation, to maintain genome integrity, and to combat viruses and mobile genetic elements . These regulatory sRNAs have been classified into at least six groups, including microRNAs (miRNAs), heterochromatic small interfering RNAs (hc-siRNAs), trans-acting small interfering RNAs (tasiRNAs), natural antisense small interfering RNAs (nat-siRNAs), repeat-associated small interfering RNAs (ra-siRNAs), and in metazoans, the piwi-interacting RNAs (piRNAs) .miRNAs are derived from single-stranded RNA precursors that are transcribed by RNA polymerase II to generate self-complementary fold-back structures (stem-loop or hairpin) processed subsequently by DICER-like 1 (DCL1) in association with other protein factors [6,8]. Distinct from miRNA biogenesis, small interfering RNAs (siRNAs) are generated from long double-stranded RNAs that are converted from single-stranded RNAs by plant RNA-dependent RNA polymerases (RDRs), which usually give rise to transcript-wide, distinct siRNA species from both strands dependent on the choice of DCL proteins involved [8]. Biogenesis of predominant small 21-nucleotide siRNAs requires RDR6 and DCL4 while that of predominant 24-nucleotide siRNAs requires RDR2 and DCL3. miRNAs and siRNAs are incorporated into different RNA-induced silencing complexes (RISCs) , where one of the Argonaute (AGO) factors catalyzes sequence-specific endonucleotic cleavage of targeted gene transcripts [12,13] in the cases of miRNAs and 21-nucleotide siRNAs , or translational repression occasionally for some miRNAs [14], or induction of DNA methylation in the case of 24-nucleotide siRNAs [. The biogenesis of tasiRNAs exploits both miRNA and 21-nucleotide siRNA biogenesis pathways, and requires all the factors necessary for miRNA and 21-nucleotide siRNA production, including DCL1, RDR6 and DCL4 as well as other protein factors [5,17]. In Arabidopsis, the transcript from a trans-acting siRNA (TAS) gene is first cleaved by one of three specialized miRNAs (miR173, miR390 and miR828), and then either the 3'-cleaved (in the case of miRNA828, miR173) or the 5'-cleaved transcript fragments (in the case of miR390) are converted into double-stranded RNAs by RDR6 and subsequently diced into phased 21-nucleotide siRNAs by DCL4 to generate multiple but distinct tasiRNA species, some of which are found to further guide sequence-specific cleavage of their targeted gene transcripts through the RISC [5,17-20]. To date, only four TAS gene families have been identified inArabidopsis and their biogenesis has been extensively characterized [21].In plants, miRNAs are the second most abundant sRNAs [22], acting as powerful endogenous regulators. For example, many distinct miRNAs target transcripts encoding an array of transcription factors that control plant development and phase transition in Arabidopsis, maize and woody species [23-25], while others are involved in stress response and disease resistance[26-28]. In humans, it is estimated that at least 30% of genes are regulated by miRNAs [29], further underscoring their fundamental importance. Whether a similar proportion of plant genes are subjected to miRNA-mediated regulation is unknown; however, a large number of miRNAs have been identified, characterized and reported in diverse plant species, including Arabidopsis], rice , maize , poplar , grape , soybean , orange and peanut. The latest release of published miRNAs (miRBase 17) contains over 15,000 miRNA gene loci in over 140 species, and over 17,000 distinct mature miRNA sequences . Like many gene regulatory systems, miRNAs show both conservation and diversity among plant lineages. Many miRNAs are conserved in angiosperms or even embryophyta [, while a significant number of miRNAs or miRNA families show species-specificity, reflecting their fast evolving, functionally diverging natures .Apple (Malus × domestica) is a major temperate fruit crop worldwide. Its fruit is a widely consumed and rich source of phytochemicals, which may play a key role in reducing chronic disease risk in humans. As a perennial species, apple undergoes many distinct developmental programs and inducible responses during its life cycle that cannot be easily replicated or investigated in annual model species such as Arabidopsis. For example, apple requires a long period of juvenility (5 to 7 years) before flowering  and its reproductive cycle lasts for nearly a year, as fruit forms from flower buds initiated during the previous summer. Its fruit development, which spans the spring, summer and fall seasons, comprises fruit enlargement, color changes, texture improvement and ripening, all of which are directly relevant to crop productivity and quality. That apple trees produce fruit over a period spanning several decades is another important consideration for investigation of plant longevity. Thus, apple trees represent an important model for investigating the fundamental biology of a wide range of specialized strategies and programs to adapt or respond to seasonal and perhaps climatic changes as well as biotic and abiotic stress challenges while implementing multiple coordinated developmental events necessary for perennial fruit production. In addition to its importance as a new genomic model for tree fruit and Rosaceae studies, the discovery of genetic mechanisms that regulate fruit development and quality or stress responses and disease resistance could enhance the molecular breeding of apple for horticulturally important traits.