Table 2. Key Characteristics of Reproductive phasiRNAs.
| Features | 21-nt PhasiRNA | References | 24-nt PhasiRNA | Reference(s) |
|---|---|---|---|---|
| Species distribution | Originated with angiosperms or possibly gymnosperms (such as Norway spruce) | ( Xia et al., 2015a; Chen et al., 2019) | Originated in and widespread in angiosperms | (Kakrana et al., 2018; Xia et al., 2019) |
| Tissue specificity and enrichment | Premeiotic anthers | (Zhai et al., 2015; Araki et al., 2020) | Meiotic anthers; also premeiotic anthers in wheat and barley | (Zhai et al., 2015; Xia et al., 2019; Bélanger et al., 2020) |
| miRNA trigger | miR2118 in monocots | ( Xia et al., 2015a; Araki et al., 2020) | miR2275 in most monocots; other angiosperms have diverse modes of biogenesis, with or without miR2275 as a trigger, and with or without RDR6 (i.e., from inverted repeats) | (Kakrana et al., 2018; Xia et al., 2019) |
| miRNA trigger localization | Epidermis | (Zhai et al., 2015; Araki et al., 2020; Huang et al., 2020b) | Tapetum and other layers | (Zhai et al., 2015; Kakrana et al., 2018; Huang et al., 2020b) |
| Dicer for biogenesis and its localization | DCL4, likely in all cell types | (Song et al., 2012) | In grasses, DCL5 in the tapetum; in eudicots, the absence of DCL5 but presence of both 24-nt phasiRNAs and 24-nt Pol IV siRNAs suggests a dual role for DCL3 | (Huang et al., 2020a) |
| PHAS precursor loci | 21-PHAS loci are mostly lncRNAs | (Xia et al., 2015a; Zhai et al., 2015; Araki et al., 2020) | 24-PHAS loci are mostly lncRNAs | (Zhai et al., 2015; Kakrana et al., 2018; Ono et al., 2018; Xia et al., 2019; Bélanger et al., 2020) |
| phasiRNA localization | Tapetum and germ cells | (Zhai et al., 2015; Huang et al., 2020b) | Tapetum and pollen mother cells, meiocytes | (Zhai et al., 2015; Huang et al., 2020b) |
| phasiRNA function | Largely unknown; at least two 21-PHAS loci (PMS1 and PMS3) in rice are required for full male fertility under long-day conditions in N58S rice, rescued with short days; molecular mechanism still unknown, possibly PTGS or transcriptional gene silencing | (Ding et al., 2012; Fan et al., 2016) | Largely unknown; loss of 24-nt phasiRNAs in rice eat1 and maize dcl5 mutants impacts tapetal development and male fertility; dcl5 phenotype rescued under slow-growth environmental conditions; molecular mechanism still unknown | (Ono et al., 2018; Teng et al., 2020) |
nt, nucleotide.