Small RNAs, emerging regulators critical for the development of horticultural traits

Small RNAs (sRNAs) have been recently recognized as key genetic and epigenetic regulators in various organisms, ranging from the modification of DNA and histone methylations to the modulation of the abundance of coding or non-coding RNAs. In plants, major regulatory sRNAs are classified as respective microRNA (miRNA) and small interfering RNA (siRNA) species, with the former primarily engaging in posttranscriptional regulation while the latter in transcriptional one. Many of these characterized sRNAs are involved in regulation of diverse biological programs, processes, and pathways in response to developmental cues, environmental signals/stresses, pathogen infection, and pest attacks. Recently, sRNAs-mediated regulations have also been extensively investigated in horticultural plants, with many novel mechanisms unveiled, which display far more mechanistic complexity and unique regulatory features compared to those studied in model species. Here, we review the recent progress of sRNA research in horticultural plants, with emphasis on mechanistic aspects as well as their relevance to trait regulation. Given that major and pioneered sRNA research has been carried out in the model and other plants, we also discuss ongoing sRNA research on these plants. Because miRNAs and phased siRNAs (phasiRNAs) are the most studied sRNA regulators, this review focuses on their biogenesis, conservation, function, and targeted genes and traits as well as the mechanistic relation between them, aiming at providing readers comprehensive information instrumental for future sRNA research in horticulture crops. Genetics: Tiny keys to important traits Studying small RNAs in horticultural plants may reveal the genetic mechanisms underlying many unique and valuable traits, and help improve plant breeding programs. Small RNAs, short RNA molecules only 20–24 bases long, switch genes on or off to control many aspects of plant growth, development, and reproduction. They are well-studied in model plants, but are only just being investigated in horticultural species. In a review of small RNA research in horticultural plants, Rui Xia at South China Agricultural University and co-workers report that horticultural plants contain many novel sRNAs that hold the keys to important traits such as juvenile-to-adult transition in fruit trees, fruit size, fruit quality, and disease resistance. Better understanding of these miRNAs may help in engineering varieties with earlier production of tastier frui