Identification and physicochemical properties of GRF protein in pineapple
Based on multiple sequence alignments and hidden Markov models of conserved domains QLQ and WRC, 21 candidate AcGRF genes were initially identified. Then, after using NCBI Conserved Domain Search and SMART to predict the QLQ and WRC conserved domains of AcGRFs, the sequences containing only one conserved domain were removed, and finally 8 genes were annotated as pineapple GRF genes. The validated AcGRF gene sequences were available in Additional file 1. After chromosomal location analysis, 8 GRF genes could be mapped on the linkage groups and were renamed from AcGRF1 to AcGRF8 based on their order on the linkage groups (Fig.1).
Gene characteristics, including coding sequence length, protein sequence length, exon number, protein molecular weight (MW), isoelectric point (PI), and subcellular localization results are listed in Additional file 2. Among the 8 AcGRF proteins, AcGRF4 containing 249 amino acids (aa) was the smallest protein, whereas the largest one was AcGRF6 containing 612 amino acids. All AcGRF proteins have molecular weights between 25.1KDa(AcGRF4)-99.3KDa(AcGRF7) and isoelectric points between 6.57(AcGRF5)-9.63(AcGRF4). The number of AcGRFs exons is between 2(AcGRF1)-5(AcGRF8). The predicted subcellular localization results showed that 7 AcGRF proteins were localized in the nucleus and one in the cell membrane (AcGRF4). Both the hydrophilicity and hydrophobicity results of the proteins were less than 0, indicating that these were all hydrophilic proteins.
Sequence and phylogenetic analysis of AcGRFs
The phylogenetic relationship of the AcGRF proteins was examined by multiple sequence alignment of their QLQ and WRC domains. In order to deeply study the evolutionary relationship among GRFs in pineapple, Oryza sativa, Arabidopsis, and Solanum Lycopersicum, a neighbor-joining phylogenetic tree was constructed using MEGA11 software. As shown in Figure 2a, QLQ and WRC domain sequences are highly conserved. All AcGRF proteins have the highly conserved sequence RTDGKKWRC, and 7 AcGRFs have a conserved QLQ (Gln-Leu-Gln) domain, the only exception being AcGRF5, whose QLQ domain composition is (Gln-Met-Gln).
The phylogenetic analysis(Fig.2b) indicated that the pineapple GRF proteins could be divided into three large groups(the group I, II, and III). Among the 8 AcGRFs proteins,3 belong to group I, 3 to group II, and 2 to group III. The GRF members in the group I can be further clustered into two subgroups (Ia and Ib), and group III can be further clustered into three subgroups(IIa, IIb, IIc). Among them, subgroup IIa is the largest, with 10 GRFs members, 6 monocots members, and 4 dicots members. Subgroup IIc is the smallest, containing only 4 dicot members; subgroup IIb contains 6 GRF members, 5 dicot members, accounting for 83%, and 1 monocot member, accounting for 17%, indicating that This subgroup of genes may have more important functions in dicots. In group III, there are 5 monocots members and 3 dicots members, with 62.5% of monocots members and 37.5% of dicots members, indicating that this family of genes may play a more important role in monocots. Monocots and dicots GRFs were almost evenly distributed in other subgroups, suggesting that these subgroups may have a similar status to monocots and dicots. The phylogenetic tree showed that AcGRFs were more closely related to OsGRFs, while AtGRFs were closely related to SlGRFs, which may be related to the fact that pineapple and Oryza sativa are monocots while Arabidopsis and Solanum Lycopersicum are dicots.
Unrooted phylogenetic trees of pineapple, Oryza sativa, Arabidopsis, and Solanum Lycopersicum GRF proteins. Phylogenetic tree constructed using the neighbor-joining method with a bootstrap value of 1000. Different colored arcs represent different families or subgroups. Different colors and fonts represent pineapple, Oryza sativa, Arabidopsis, and Solanum Lycopersicum GRF domains, respectively. GRF proteins from pineapple, Oryza sativa, Arabidopsis, and Solanum Lycopersicum are denoted by Ac, Os, At, and Sl prefixes, respectively.
Gene structure and motif composition analysis of AcGRFs
The exon-intron organizations of all the identified AcGRF genes were examined to gain more insight into the evolution of the GRF family in pineapple. As shown in Figure 3, all 8 AcGRF genes have intron-exon structures, and the number of exons varies between 2-5. AcGRF8 has the largest number of exons, AcGRF1 contains only two exons, and most AcGRFs genes contain 3-4 exons (Fig. 3d).
Protein motifs are the basic units of protein structure, which directly determine the function of a particular protein. To elucidate the diversity of AcGRF proteins, we used the online program MEME to analyze the conserved motifs of 8 AcGRF proteins (Fig. 3b and Additional file 3). There are between 4-10 conserved motifs of GRF proteins in pineapple. Motif1 and motif2 are present in all AcGRF proteins, and they represent the N-terminal WRC and QLQ-specific domains. For subgroup IIa members (AcGRF6 and AcGRF7), all Motifs were contained, while other group members contained varying amounts of Motifs. In conclusion, AcGRFs from within the same group has similar protein structures, although the number of motif arrangements is slightly different in some subgroup. Similar motif arrangements among AcGRF proteins suggest that protein structure is conserved within the specific subgroups. Gene structure and motif characteristics strongly support the phylogenetic relationship of AcGRFs.
synteny analysis of AcGRFs
Gene duplication plays an important role in promoting gene expansion, the three whole-genome duplications in Arabidopsis have been directly responsible for >90% of the increase in transcription factors, signal transducers, and developmental genes [23]. To gain a possible mechanism for the expansion of AcGRFs, we investigated gene duplication events in pineapple (Fig.4a). A total of two GRF pairs with duplication events (AcGRF8/Aco017250.1, AcGRF1/AcGRF2) were identified(Additional file 4), according to club’s definition of tandem duplication: a chromosomal region within 200kb containing two or more genes were defined as a tandem duplication event (Holub, 2001). Both duplication events identified were segmental duplications. This result suggests that fragment duplication plays a more critical role in the amplification of GRFs in the pineapple genome.
Given that comparative collinearity, maps are helpful for the study of evolutionary traits, we also established comparative collinearity maps of four species, Arabidopsis, Solanum Lycopersicum, Oryza sativa, and Musa, with pineapple (Fig. 4b). According to the results of collinearity analysis, we found 8 and 18 homologous gene pairs in monocots plants Oryza sativa and Musa, respectively, while 4 and 7 homologous genes were found in dicots plants Arabidopsis and Solanum Lycopersicum, indicating that the GRF gene has evolved over a longer period in pineapple, Arabidopsis and Solanum Lycopersicum, while it has undergone a shorter selective evolution with the GRF gene in Musa. Some AcGRFs were found to be collinear with multiple genes (especially between pineapple and Musa), such as AcGRF4 and AcGRF7, suggesting that these genes may play a major role in the expansion of the GRF gene family.
Interestingly, some genes, such as AcGRF4, were only colinear in the monocots plant Oryza sativa and Musa but not in the dicots plant Arabidopsis and Solanum Lycopersicum, suggesting that this gene was formed after the differentiation of monocots and dicots. Similarly, we also found that some genes (AcGRF2 and AcGRF7) were collinear in other species, suggesting that these genes may have existed before the differentiation of monocots and dicots plants. To understand the evolution of the GRF gene family more clearly, we successively calculated the Ka, Ks, and Ka/Ks of the gene pairs (Additional file 5). It may have experienced strong purifying selection pressure during evolution.
Identification and analysis of homeopathic elements in AcGRFs promoters
Cis-acting regulatory elements are important molecular switches in gene transcription regulation, involved in the regulation of gene transcription in plant growth and development and response to various biotic and abiotic stresses[24]. To further investigate the potential biological functions of AcGRFs genes in pineapple, we extracted the upstream 2000bp region sequences of all AcGRF genes and identified their homeopathic elements using the PlantCare online website. We analyzed the promoter sequences of eight AcGRFs genes. The results show that AcGRFs promoter elements are involved in auxin response, gibberellin response, abscisic acid response, jasmonic acid response, drought induction, meristem expression, endosperm expression, low-temperature response anaerobic induction, light response, etc. (Fig.5 Additional file 6), indicating that the AcGRF genes may be involved in a variety of stress and hormone responses. Among these elements, light-responsive elements accounted for the largest proportion (43.8% of all active elements), followed by anaerobic induction (14.2%), and abscisic acid and jasmonic acid responses also accounted for a larger proportion, 10.1%, and 9.5%, respectively. It suggests that AcGRFs may play an important role in light response and abscisic acid and jasmonic acid response, and may be involved in photomorphogenesis, plant senescence, and other processes, thereby regulating hypocotyl elongation, tissue and organ formation, stem elongation, and fruit ripening. Among the 8 AcGRFs, AcGRF8 has the most active elements (28), suggesting that it may be involved in more biological functions.
Expression patterns of AcGRFs in different tissues
To gain insight into the potential functions of GRF genes in pineapple, RNA-Seq data from 14 different pineapple tissue expression profiles corresponding to major stages of whole pineapple fruit development were used to create a heatmap of AcGRFs expression (Fig.6a, Additional File 7). We found that almost all AcGRFs had relatively high expression in various tissues early in development (eg, ovary, petals, receptacles, and sepals), revealing that AcGRFs may play a role in these tissues early in development. However, of the 8 pineapple GRFs, 5 GRFs were relatively highly expressed in the ovary and 1 was relatively highly expressed in the petals, suggesting that these genes play regulatory roles in the growth and development of these specific tissues.
To further analyze the expression patterns of GRF genes in different pineapple floral organs, we also created a heatmap of AcGRFs for 27 different pineapple floral organ samples (Fig. 6b, Additional file 7). The results showed that all genes except AcGRF3 had relatively high expression in the ovary and stamens, while AcGRF3 was relatively highly expressed in the pistil. Consistent with the previous transcriptome data, the more detailed transcriptome data also showed that AcGRFs had higher transcriptional levels in the early stages of development of various tissues and organs, further indicating that AcGRFs may play their functions in the early stages of growth and development.
Effects of Different Treatments on GRF Gene Expression in Pineapple
To further understand the effect of the pineapple GRF gene on biotic stress and hormone treatment, we used QRT-PCR to analyze the expression patterns of some pineapple GRF members under different treatments (Fig.7). Overall, some GRF genes were significantly induced or repressed by multiple abiotic factors. For example, under low-temperature treatment, the expression levels of AcGRF2 and AcGRF7 increased significantly at 8h and 12h and then gradually decreased, while the expression levels of AcGRF4 and AcGRF5 showed a decreasing trend after treatment, especially AcGRF5 was significantly increased at 2h after low-temperature treatment. reduce. During heat treatment, AcGRF2, AcGRF4, AcGRF5, and AcGRF7 were significantly reduced in different periods, especially AcGRF2 and AcGRF7, and the difference in expression reached a significant level 2h after treatment. For gibberellin treatment, the expression levels of AcGRF2, AcGRF4, and AcGRF7 were significantly different after 2h, while AcGRF5 was only significantly different at 8h. These results indicated that AcGRFs were involved in phytohormone and abiotic stress responses to varying degrees.