Preparation of plant samples
The Tartary buckwheat used in this study was XIQIAO 1. In the appropriate development stage of Tartary buckwheat, we obtained roots, stems, and leaves from 7-day-old seeding, flowers and fruits were got from green fruit stage seedings.
All organ samples were taken from three plants with the same growth condition. To facilitate use in further experiments, the collected plant samples were stored in -80°C refrigerator after rapid treatment with liquid nitrogen. For non-sterile cultures, all plants were grown in sterilized soil, a mixture of vermiculite and soil (1:3, v/v) on the balcony of School of Life Science, Sichuan University [23].
About inhibitive effect of ABA on seed and root growth of Tartary buckwheat
To remove the peel easily, seeds were soaked in water for 20 minutes in advance. Disinfect the seeds using 75% ethanol for 30 s. Replace the ethanol with 5% sodium hypochlorite and disinfect for 15 min, washed with sterilized and distilled water. After that, seeds were planted on Murashige and Skoog (MS) medium supplemented with concentration gradient as 0, 0.3, 0.9 and 1.5µM ABA [5].
The MS medium PH is 6.0, containing 3% (w/v) sucrose and 0.65% (w/v) agar. We recorded data on seed germination every day.
To measure the root length, seeds were placed on MS culture dish for 48 h, then transplanted germinating seeds on the MS supplemented with 0, 0.3, 0.9, and 1.5µM ABA, 3% sugar, and 0.55% agar. After 20 days, the root length was measured and the data were processed (30 Tatary buckwheat seeds were taken as the sample size), to calculate the average length to collect images.
About effect of ABA on leaf stomatal aperture of Tatary buckwheat
The plants were left in the dark for 12 hours to ensure that all stomata are closed. The top leaves of the 20-day-old plants were removed. Then they were immersed in the stomatal opening solution in a climate incubator for 3 h. The stomatal opening solution was made of 10mM MES (pH value 6.15) 10ml, 50mM KCl 5mL, 10µM CaCl2 10µM, and 85 ml deionized water. ABA were added with gradient concentration as 0, 10, 15, 20, 30, 50, and 70µM, respectively. After 3 hours, the subepidermal stmata of the leaves were photographed with a microscope (DMI6000B, Leica, Germany). Three fields were randomly selected from each sample, and 30 stomata were randomly selected from each field to measure its length and width with Image J, and the number of cells in long and short sides were counted.
Homology analysis by constructing phylogenetic trees and predicting protein structures
A phylogenetic tree of Tartary buckwheat has been constructed and Arabidopsis thaliana to identify the phylogenetic relationship between the two species.
The genes of the FtbZIP family were classified into 11 groups after constructing a phylogenetic tree with A. thaliana [24]. In group A, most genes of Athaliana were ABA-responsive element (ABRE) binding factors. According to the phylogenetic relationship of two species, we assumed that the genes in group A of F. tataricum were also related to ABA response. The genes sequences of group A FtbZIP family were obtained from the TBGP database (http://www.mbkbase.org/Pinku1/) [22].
To study the probable function of these genes during the growth of F. tataricum, we constructed the phylogenetic tree of these genes and other genes described as ABRE binding factors from eight other species closely related to F. tataricum. Eight species are monocotyledons, which had a relatively close evolutionary relationship with F. tataricum (B. vulgaris, S. lycopersicum, S. tuberosum, V. vinifera, P. trichocarpa, A. thaliana, T.cacao, G. max.) [22]. Then we got the gene NCBI-IDs of those genes of the eight species by searching the Kegg database (https://www.kegg.jp/) [25], of which KO number was K14432 [26]. Next, we retrieved and consulted the information of those genes in the NCBI database (https://www.ncbi.nlm.nih.gov/) [27], and finally recorded them in supplementary materials (supplementary data 1). We also downloaded the protein sequences of those genes from the NCBI database.
As for the paralogous analysis, at first, the genes in group A of F. tataricum were aligned by using MUSCLE [28] with default parameters. Then the phylogenetic tree was constructed by using RAxML [29] with custom parameters (-x 12345 -p 12345 -# 1000 -m PROTGAMMAJTTF -f a -k -s input -n run_id -T 4). We selected JTTF model by running a script provided by the RAxML author and set the bootstrap value as 1000. As for the ortholog analysis of the genes in group A of F. tataricum with eight other plant species, we used the same pipeline as described above. Finally, the protein structures were predicted by using phyre2 [30] with ab initio modeling methods.
Analysis of FtbZIP in group A of different tissues expression by qRT-PCR
Expression analysis of FtbZIP in group A in different tissues by qRT-PCR. The expression of FtbZIP genes in five organs (fruit, flower, leaf, stem, root) was analyzed by qRT-PCR. we performed three biological replicates. We used the Primer3 program ( http://bioinfo.ut.ee/primer3-0.4.0/) to obtain qRT-PCR primers of FtbZIP genes (Table S1) [31]. In qRT–PCR experiments, the internal reference gene was FtH3 gene. The cDNA was synthesized by The Prime Script RT Reagent Kit (Takara) using 1 g RNA. The cDNA was synthesized with SYBR Premix Ex Taq II (TAKARA). Finally, we analyzed the results and obtained acquired mRNA expression information by the 2−(ΔΔCt) [32]. qRT-PCR was performed using the SYBR Premix Ex TaqⅡ kit (Takara)and an Applied Biosystems 7500 real-time PCR system. FtH3 was used as the internal reference primer for QRT-PCR, and the primer sequences were shown in Table 1.
Test of quatiy variance of FtbZIP with different ABA treatments
The expressions of FtbZIP in A group in Tartary buckwheat treated by concentrations ABA were analyzed. The top leaves of 20-day-old Tartary buckwheat were treated by 0, 15, 30, 50 and 70 µM ABA for 3h. After drying on the surface with absorbent paper, RNA was extracted. The gene expression was analyzed by qrt-PCR method. When analyzing the expression of FtbZIP in A group in Tartary buckwheat treated by ABA at different times, 7-day-old Tartary buckwheat seedlings were treated by 50µM ABA for 0, 0.5, 2, 4, 8, 16, 24 h respectively, and then RNA was extracted for expression analysis using qrt-pcr method.