9901 scion enhanced the rooting of YJ rootstock in 9901/YJ plants
During hydroponic culturing, the grafted willows started rooting at the bases of the rootstocks. Obviously, the 9901/9901 grafts rooted faster than the YJ/YJ plants (Fig. 1). The total root length of the 9901/9901 grafts was 2.66 times that of the YJ/YJ by the 22nd d after grafting, indicating that 9901 has a strong rooting potential than YJ (Fig. 2). The 9901/YJ plants rooted faster than YJ/YJ plants, and the total root length of the 9901/YJ grafts by the 22nd d after grafting was 1.93 times that of the YJ/YJ grafts. This revealed that the 9901 scions, with a fast rooting potential, had a certain promotive effect on the rooting of the YJ rootstocks.
Degs In Homologous And Heterologous Willow Grafts
A total of 49,425, 48,176 and 48,713 genes were identified in the rootstock tissues of 9901/9901, YJ/YJ, and 9901/YJ grafted plants, respectively (Fig. 3), using a transcriptome analysis. In total, 2,948 DEGs were identified between YJ/YJ and 9901/YJ, which probably resulted in their different rooting performances. Among them, 1,687 and 1,261 genes were significantly up- and down-regulated, respectively, in 9901/YJ compared with in YJ/YJ.
A total of 2,309 DEGs (S1 Fig) were annotated in the GO database. The DEGs related to biological processes were grouped in 21 terms, which were mainly involved in metabolic (1,100 genes), cellular (983 genes), and single-organism (916 genes) processes. The DEGS related to cellular component were grouped in 15 terms, which were mainly involved in cell (999 genes), cell part (975 genes), membrane (931 genes), and organelle (571 genes). The DEGS related to molecular function were grouped in 15 terms, which were mainly involved in catalytic activity (1,226 genes), binding (1,170 genes), and transporter activity (216 genes).
SNP analysis revealed 692 shoot-to-root mobile mRNAs in the 9901/YJ graft
SNP analysis was conducted between the homologous genes of two grafts (YJ/YJ, 9901/YJ), which showed 692 of 48,713 mRNAs in rootstock of 9901/YJ came from the scion. Among them, 536 mRNAs were annotated in the GO database (Fig. 4). In total, 16 annotated terms were related to biological processes, including metabolic processes (253 mRNAs), cellular processes (234 mRNAs), and single-organism processes (163 mRNAs). There were 14 annotated terms related to cellular component, including cell (240 mRNAs), cell part (239 mRNAs), and membrane (201mRNAs), and there were 11 annotated terms related to molecular function, including binding (301 mRNAs) and catalytic activity (259 mRNAs).
An example was given in detail. The ABF (ABA responsive element binding factor) (K14432) gene was 966 bp in both the parents but displayed three SNPs at site of 135, 200, and 259 bp in the grafted plants (Fig. 5). The SNPs resulted in coding sequence changes in two amino acids, which further affected the secondary structure of the 9901-sourced homologous protein having less alpha helix and extension chain in comparison with the YJ-sourced protein, even far from the SNP site (Fig. 6). The polymorphisms in the proteins might really contribute to graft performance.
Characterization Of The Mobile Mrnas In Grafted Willows
The transfer ratio of 455 mobile mRNAs were estimated (the others, having less than 30 reads, were not considered), and they ranged from 7.14–68.75% with an average of 27.30%. Among them, 29.89%, 23.74%, and 20.88% of the mRNAs had transfer ratio of 10–20%, 20–30%, and 30–40%, respectively.
The maximum size of a mobile mRNAs was 10,710 bp and the minimum was 186 bp, with an average of 1,657 bp, which was 19.64% greater than the average 1,385 bp length of all willow transcripts. On the basis of mRNA length, the 455 mobile mRNAs were classified into several groups. The 1,000–1,500bp group contained the largest proportion, at 26.59%, of all the mobile mRNAs, followed by the 501–1,000 bp and 1,501–2,000 bp groups. However, a transfer ratio analysis revealed that mRNAs > 4,001 bp were preferred for transfer, accounting for 2.40% of the mobile mRNAs in the group, followed by the 3,501–4,000 and 2,501–3,000-bp mRNAs (Table 1).
Table 1
mRNA length and transfer ratio in the rootstock of 9901/YJ
mRNA length (bp) | Mobile mRNA amount | Total transcribed gene amount | Mobile mRNA ratio in total transcribed gene of the group (%)a | Mobile mRNA propotion in all mobile mRNA (%)b |
0-500 | 44 | 4883 | 0.90 | 9.67 |
501–1000 | 96 | 12127 | 0.79 | 21.10 |
1001–1500 | 121 | 11219 | 1.08 | 26.59 |
1501–2000 | 65 | 6441 | 1.01 | 14.29 |
2001–2500 | 46 | 3444 | 1.34 | 10.11 |
2501–3000 | 35 | 1901 | 1.84 | 7.69 |
3001–3500 | 17 | 1151 | 1.48 | 3.74 |
3501–4000 | 11 | 558 | 1.97 | 2.42 |
4001- | 20 | 835 | 2.40 | 4.40 |
a A ratio between the mobile mRNAs amount in the group and the total transcribed gene amount in the group; |
b A ratio between the mobile mRNAs amount in the group and the total mobile mRNAs (455) in the rootstock of 9901/YJ. |
The GC contents of all the transcripts in willow were calculated, and ranged from 28.70–67.84%, with an average of 44.37%. Comparably, the GC contents of the 455 mobile mRNAs ranged from 36.40–57.89%, with an average of 44.18%. The transcribed mRNAs were grouped on the basis of their GC content (Table 2). Among them, the mRNAs with GC contents of less than 35% were not transferred. The highest proportion, at 64.18%, of all the mobile mRNAs had GC contents of 40–45%, followed by mRNAs with GC contents of 45–50% and 50–55%. The transfer ratio calculation showed that the mRNAs with GC contents of 50–55% had the largest value at 1.20%, followed by the mRNAs with GC contents of 40–45% and 35–40%.
Table 2
GC content and transfer ratio in the rootstock of 9901/YJ
GC content (%) | Mobile mRNA amount | Total transcribed gene amount | Mobile mRNA ratio in total transcribed gene of the group (%) | Mobile mRNA proportion in all mobile mRNA (%) |
25–30 | 0 | 2 | 0 | 0 |
30–35 | 0 | 72 | 0 | 0 |
35–40 | 13 | 1353 | 0.96 | 2.86 |
40–45 | 292 | 25776 | 1.13 | 64.18 |
45–50 | 128 | 13471 | 0.95 | 28.13 |
50–55 | 21 | 1752 | 1.20 | 4.62 |
55- | 1 | 133 | 0.75 | 0.22 |
The expression abundances (presented as FPKM values) of the mobile mRNAs (Table 3) revealed minimum and maximum expression levels of 0.42 and 817.09, respectively, with an average of 19.81. Comparably, the average expression level of all the transcribed genes was 17.74% lower, at 16.83, than that of the mobile mRNAs. The mobile mRNAs were classified into several groups on the basis of the expression abundance. Among them, the mRNAs with expression value less than 10 formed the largest group, representing 63.30% of the mobile mRNAs, followed by the mRNAs with expression value of 10–20, representing 17.58% of the mobile mRNAs. The transfer efficiency evaluation showed that mRNAs with expression value of 10–20 accounted for 1.12% of the transcribed genes, followed by the mRNAs with expression values of 40–50.
Table 3
Gene expression abundance and transfer ratio in the rootstock of 9901/YJ
mRNA expression abundance (FPKM) | Mobile mRNAs amount | Total transcribed gene amount | Mobile mRNAs ratio in total transcribed gene of the group (%) | Mobile mRNAs proportion in all mobile mRNAs (%) |
0–10 | 288 | 33241 | 0.87 | 63.3 |
10–20 | 80 | 7113 | 1.12 | 17.58 |
20–30 | 27 | 3168 | 0.85 | 5.93 |
30–40 | 12 | 1734 | 0.69 | 2.64 |
40–50 | 12 | 1077 | 1.11 | 2.64 |
50–60 | 4 | 631 | 0.63 | 0.88 |
In comparison with the previous reports, several mobile mRNAs were recognized in association with rooting such as ABCB29 [14], ARR5 [15], ERF9 [16], and ABF [17]. PCR tests showed them well expressed in 9901 rootstock of 9901/9901 plants, but less in YJ rootstock of YJ/YJ plants (Fig. 7). Obviously, these genes had more transcripts in YJ rootstock of 9901/YJ plants than in YJ rootstock of YJ/YJ plants, potentially gained from its 9901scion and resulted in their improved rooting ability.
KEGG pathways of the mobile mRNAs participate in root formation
A total of 142 (20.52%) mobile mRNAs were annotated to 85 pathways in the KEGG database. The main pathways included spliceosome, biosynthesis of amino acids, plant–pathogen interaction, and protein processing in endoplasmic reticulum (Table 4), which enriched with 11, 11, 10, and 10 mobile mRNAs, respectively.
Table 4
The main pathways of the mobile mRNAs involved
Pathway | ko_id | P_value | Mobile mRNAs amount |
Spliceosome | ko03040 | 0.0606 | 11 |
Biosynthesis of amino acids | ko01230 | 0.1549 | 11 |
Plant-pathogen interaction | ko04626 | 0.0588 | 10 |
Protein processing in endoplasmic reticulum | ko04141 | 0.1816 | 10 |
Plant hormone signal transduction | ko04075 | 0.5378 | 8 |
RNA transport | ko03013 | 0.3316 | 7 |
Carbon metabolism | ko01200 | 0.7074 | 7 |
Ribosome | ko03010 | 0.9146 | 7 |
Fructose and mannose metabolism | ko00051 | 0.1093 | 4 |
Carbon fixation in photosynthetic organisms | ko00710 | 0.2043 | 4 |
Notably, the plant hormone signal transduction was enriched with eight mobile mRNAs (S2 Fig), which participated in auxin (AUX), cytokinin (CK), gibberellin (GA), abscisic acid (ABA), jasmonates(JAs), and salicylic acid(SA) metabolism, with transfer ratio of 14.72–68.29% (Table 5). They may directly or indirectly participate in root formation as previously reported.
Table 5
The mobile mRNAs involved in the plant hormone signal transduction pathway
Mobile mRNAs | Gene function | Expression abundance* | Transfer ratio |
9901/9901 | YJ/YJ | 9901/YJ |
AUX/IAA (K14484) | activation of the downstream transcription factor ARF | 0.50(0) | 0(0.14) | 0.37(0.26) | 58.73% |
CRE1 (K14489) | transmembrane histidine kinase cytokinin receptor activity | 0.29(0) | 0(0.03) | 0.12(0.12) | 50.00% |
B-ARR (K14491) | sequence-specific DNA binding | 0.45(0) | 0(0.11) | 0.04(0.04) | 50.00% |
TF (K16189) | protein dimerization activity | 0.10(0) | 0(0.09) | 0.04(0.06) | 40.00% |
ABF (K14432) | sequence-specific DNA binding | 4.20(0) | 0(0.84) | 0.08(0.40) | 16.67% |
ABF (K14432) | sequence-specific DNA binding | 0.20(0) | 0(0.31) | 0.03(0.18) | 14.29% |
JAZ (K13464) | transcription corepressor activity | 0.80(0) | 0(0.31) | 0.28(0.13) | 68.29% |
NRP1 (K14508) | Regulatory protein | 0.23(0) | 0(0.38) | 0.10(0.37) | 21.28% |
* The data outside the parentheses means the expression abundance of 9901 homologous gene in rootstock of grafts, while the data inside the parentheses means the expression abundance of YJ homologous gene in rootstock of grafts. |