# Molecular and Hormonal Mechanisms Regulating Fleshy Fruit Ripening
## 5 Non-Coding RNAs  
Diverse types of noncoding RNAs have been reported, including microRNAs (miRNAs), small interfering RNAs (siRNA), trans-acting siRNAs (tasiRNA), long noncoding RNAs (lncRNAs), natural antisense transcripts (NATs) and circular RNAs (circRNAs), which play a variety of roles in an RNA network regulating gene expression during development and ripening.  
MicroRNAs (miRNAs) are small non-coding RNAs, of typically 20-24 nt, which regulate gene expression either transcriptionally or post-transcriptionally through sequence complementarity with target genes and/or their mRNAs. Ripening-induced DNA hypomethylulation is associated with decreased siRNA levels, consistent with reduced RdDM activity [200]. These 24 nt siRNAs are further methylated by HEN1 [235; 236; 237] and interact with scaffold RNAs and participate in the RNA-directed DNA methylation (RdDM) gene silencing pathway.  
Recent progress in understanding the role of miRNAs in regulating fruit development and quality characteristics during ripening and senescence has been summarised by [PERSON] et al. [238; 239]. miR172a is inhibited by MADS-RIN transcription factor, and several miRNAs have been shown to modulate the ethylene signal transduction pathway (miR1917) [238] and ethylene biosynthetic genes _ACS8_ (miR159) and different _ACO_ genes (miR414, miR396b, miR397, miR1917) [239]. Recently, miR164 was found to cleave _AdNAC6/7_ mRNAs and inhibit their transcriptional regulatory effects on ripening-related genes in kiwifruit [20].  
CircRNAs can arise from exons (exonic circRNA), introns (intronic circRNA) and intergenic regions [240; 241], and expression of some exonic circRNAs and their parent genes are significantly positively correlated [242]. Most plant circRNAs show developmental-/stress-specific expression patterns, as reported in animals [243], and are involved in regulating fruit colouration [244; 245], fruit development [246], fruit chilling response [247] and the fruit ethylene biosynthetic pathway [248].  
LncRNAs are broadly defined as non-coding RNAs longer than 200 nucleotides in length [249]. Recent work suggests that lncRNAs play critical roles in transcriptional and post-transcriptional regulation [250], as well as in epigenetic modification, cell differ entiation and development [251], and they can recruit polycomb-repressive complexes, alter trans-interactions and chromatin function, activate or suppress gene transcription and expression, alter RNA splicing or mRNA stability or activate mRNA translation and protein post-translational modifications [252; 253]. Several lncRNAs are reported to regulate tomato fruit development and ripening [254; 255], although knowledge of the transcriptional regulation mechanism of lncRNAs is limited [256]. Multiple lncRNAs regulate the ethylene biosynthesis pathway by controlling the expression of different _ACS_ and _ACO_ targets, lncRNA2018 targets _ACS2_, while lncRNA2113 and lncRNA2118 target _ACO2_[239]. The lncRNA2155 is involved in the regulation of tomato fruit colour, and repression of _lncRNA1459_ inhibited lycopene accumulation [239]. Tomato lncRNA192 and lncRNA291 target _flavovol synthase_ (_FLS_), while the targets of lncRNA135, lncRNA027 and lncRNA143 are _L-ascorbate peroxidase 3_ (_APX3_), _lipoxygenase_ (_LOX_) and _vitamin K_ _cyoxide reductase_ (_VKOR_), which are related to the fruit flavour in tomato [239]. Additionally, the target gene of tomato lncRNA2180 and lncRNA TCONS_00028129 is _pectinesterase_ (_PE_), and the target gene of lncRNA TCONS_00007262 is _pectate lyase_ (_PL_), which indicate that lncRNAs may regulate fruit texture change [239]. Further clarification of the diverse mechanisms of action of lncRNAs is required in order to fully understand their role in modulating ripening.