Small RNA Sequencing Data
In order to explore the role of miRNAs in the regulation of tea plant flower development, we used high-throughput sequencing to construct miRNA libraries of FBH, MBH and ZDH. The Q20 percentage of raw data exceeded 98%, and the Q30 percentage was above 96%, with the ratios of GC content above 47%. As a result, a total of 13 976 247, 16 644 199, and 13 976 692 clean reads were obtained for FBH, MBH and ZDH, respectively. The length of plant small RNA ranged from 18 ~ 30 nt, and a total of 11 704 253, 14 693 758 and 13 045 116 small RNA sequence read were identified for FBH, MBH and ZDH, respectively (Table 2). The results show that the small RNA in the three miRNA libraries were mainly distributed in the range of 21 ~ 24 nt, of which 24 nt was the most abundant (Fig. 1). In regard to the position of the small RNA after length screening to the reference sequence, the ratio of mapped small RNA for for FBH, MBH and ZDH was 60.49%, 53.61% and 56.4%, respectively. The number of known miRNAs were 108703, 134730 and 105547, novel miRNA were 64681, 74929 and 53515, rRNA were 411500, 420492 and 295026, ta-siRNA were 33333, 40746 and 21517, snRNA were 8441, 8086 and 2975, and snoRNA were 4114, 3020 and 3227, resoectively (Table 3).
Table 2
Summary Dataset of small RNA and transcriptome libraries
Category
|
male parent(FBH)
|
female parent(MBH)
|
sterile flowers(ZDH)
|
Raw reads
|
14270452
|
16985205
|
14315444
|
Clean reads
|
13976247
|
16644,199
|
13976692
|
GC content
|
48.74%
|
47.94%
|
47.24%
|
Q20
|
98.35%
|
98.17%
|
96.73%
|
Q30
|
97.14%
|
96.84%
|
96.73%
|
Total sRNA
|
11704253
|
13045116
|
14693758
|
Table 3
small RNA classification statistics
types
|
male parent(FBH)
|
female parent(MBH)
|
sterile flowers(ZDH)
|
Mapped sRNA
|
7079588 (100.00%)
|
8286830 (100.00%)
|
6993877 (100.00%)
|
Known_miRNA
|
108703 (1.54%)
|
134730 (1.63%)
|
105547 (1.51%)
|
rRNA
|
411500 (5.81%)
|
420492 (5.07%)
|
295026 (4.22%)
|
tRNA
|
2 (0.00%)
|
4 (0.00%)
|
0 (0.00%)
|
snRNA
|
8441 (0.12%)
|
8086 (0.10%)
|
2975 (0.04%)
|
snoRNA
|
4114 (0.06%)
|
3020 (0.04%)
|
3227 (0.05%)
|
novel miRNA
|
64681 (0.91%)
|
74929 (0.90%)
|
53515 (0.77%)
|
ta-siRNA
|
33333 (0.47%)
|
40746 (0.49%)
|
21517 (0.31%)
|
Others
|
6448814 (91.09%)
|
7604823 (91.77%)
|
6512070 (93.11%)
|
Table 4
Differentially expressed miRNA
miRNA
|
RPM (FBH)
|
RPM (MBH)
|
RPM (ZDH)
|
ZDH vs FBH
|
ZDH vs MBH
|
ath-miR156a-5p
|
605.19
|
492.24
|
16.74
|
down
|
down
|
ath-miR156j
|
314.01
|
305.67
|
27.90
|
down
|
down
|
ath-miR157a-5p
|
1941.16
|
1845.92
|
72.55
|
down
|
down
|
ath-miR157d
|
45.67
|
35.73
|
0.00
|
down
|
down
|
ath-miR160a-3p
|
0.00
|
0.00
|
11.16
|
up
|
up
|
ath-miR160a-5p
|
274.05
|
194.52
|
1679.73
|
up
|
up
|
ath-miR164a
|
856.40
|
1004.34
|
396.22
|
down
|
down
|
ath-miR164c-5p
|
770.76
|
829.67
|
284.61
|
down
|
down
|
ath-miR167a-5p
|
6257.40
|
7034.34
|
1618.35
|
down
|
down
|
ath-miR167d
|
23596.55
|
31491.75
|
1819.24
|
down
|
down
|
ath-miR169b-5p
|
11.42
|
11.91
|
0.00
|
down
|
down
|
ath-miR172a
|
17.13
|
47.64
|
94.87
|
up
|
up
|
ath-miR172e-3p
|
62.80
|
27.79
|
217.64
|
up
|
up
|
ath-miR2111a-5p
|
5.71
|
7.94
|
0.00
|
down
|
down
|
ath-miR319a
|
99045.01
|
99008.67
|
250503.15
|
up
|
up
|
ath-miR319c
|
6645.63
|
4354.78
|
15190.13
|
up
|
up
|
ath-miR396a-5p
|
14181.91
|
12988.91
|
3833.80
|
down
|
down
|
ath-miR396b-5p
|
149720.80
|
107615.02
|
44387.31
|
down
|
down
|
Screening and Analysis of the miRNAs
To identify known miRNAs, we compared our data with known miRNA data of Arabidopsis in the miRBase 21.0 database (http://www.mirbase.org/ftp.shtml). A total of 55 known miRNAs were identified and divided into 27 miRNA families. Among these miRNA families, miR156 and miR396 contained four members, miR159, miR166, miR169, miR172 and miR399 contained three members, miR157, miR160, miR164, miR167, miR169, miR170, miR171, miR319, miR390, miR395, miR398 and miR858 contained 2 members, and miR162, miR165, miRK2111, miR393, miR394, miR403, miR408 and miR8175 contained 1 member (Table S1). In addition, we identified 90 putative novel miRNAs (Table S2). All novel miRNA sequences were 19 to 25 nt in length, of which the largest proportion, of miRNAs, accounting for 40.00%, were 24 nt. Interestingly, the sequence with A as the first base accounted for 44% of the novel miRNAs, and the sequence with U as the first base comprised the largest proportion of known miRNAs (Fig. 2).
Differentially expressed miRNAs
The expression level of the known and new miRNA in each sample was measured, and the expression level was normalized with TPM (Zhou et al, 2010). A total of 104 differentially expressed miRNAs were detected in FBH, MBH and ZDH (Fig. 3A). Venn diagram analysis revealed the unique and shared miRNAs in the samples (Fig. 3B). There were 86 differentially expressed miRNAs in common between FBH and ZDH, and 66 differentially expressed miRNAs in common between MBH and ZDH. Notably, 37 differentially expressed miRNAs identified in ZDH, and their expression was significantly different from FBH and MBH, but there was no difference between FBH and MBH. Among these 37 miRNAs, 18 miRNAs were up-regulated and 19 miRNAs were down-regulated. Twelve of the miRNAs that were down-regulated included miR156, miR157, miR164, miR167, miR169, miR2111 and miR396 family members, and 6 of the up-regulated miRNAs included miR160, miR172 and miR319 family members (Fig. 3C).
Analysis of miRNA Target Genes
In order to investigate the regulatory effect of the miRNAs on gene expression, the psRobot software was used to predict the target genes by analyzing the known and novel miRNAs. A total of 145 miRNAs were analyzed and 4 007 genes were predicted. Among the predicted genes, 1 200 genes were annotated, including WDR, MYB, bHLH, SPL, WRKY, NAC, APETALA2, AGL, ARF, AP2, as well other transcription factors. The results indicate that miRNAs target multiple genes, and one gene is targeted by multiple miRNAs, suggesting that miRNAs can regulate multiple functions, and that miRNAs have the same or different regulatory effects in tea plants.
In addition, 37 differentially expressed miRNAs have 363 annotated target genes. In order to study the function of miRNAs further, the characteristics of the 363 target genes were analyzed through the GO database,including three biological processes, cellular components, and molecular function. The results showed that a total of 31 subcategories were enriched, and “transcription” and “regulation of transcription” were the most enriched biological process. Cellular component analysis showed that “nucleus”, “cytoplasm” and “plasma membrane” were the most representative subcategories. “DNA binding”, “metal ion binding”, protein binding” and “transcription factor activity” were the most representative groups of molecular function. Furthermore, “photoperiodism/flowering”, “negative regulation of flower development”, “calcium ion transmembrane transport”, and “calmodulin binding” were enriched in 3, 3, 2, and 4 genes, respectively (Fig. 4).
Among the target genes, three auxin response factor genes (ARF18) targeted by miR160a-5p were predicted; nine floral homeotic protein (AP2) and two ethylene-responsive transcription factor (RAP2-7) co-targeted by miR172a, miR172c and miR172e-3p were predicted; two transcription factors (GAMYB) targeted by miR319c were predicted; two calcium-transporting ATPases (ACA12) co-targeted by miR396a-5p and miR396b-5p were predicted; and one ABC transporter I family member (ABCI11) co-targeted by miR172a and miR172c were predicted. In addition, we predicted that miR156 has 142 target genes, including three ATPase family AAA domain-containing proteins (ATD1A), four Squamosa promoter-binding proteins (SPL), two calcium-transporting ATPases (ACA1 and ACA2), two Cyclin-dependent kinases (CKB22), and Floral homeotic protein (MADS2). These results indicate that the differentially expressed miRNAs regulate complex biological processes.
qPCR analysis of miRNA
We randomly selected 12 miRNAs for RT-qPCR analysis to verify the accuracy of the small RNA sequencing. Among these miRNAs, miR156a-5p, miR156j, miR157a-5p, miR157d, miR164a and miR164c-5p were down-regulated, and miR319, amiR319, cmiR172a, miR172e-3p, miR160a-5p and novel_126 were up-regulated in ZDH (Fig. 5). The expression level of these miRNAs in FBH, MBH and ZDH was consistent with the change trend of small RNA sequencing analysis.