A Novel Insight into Functional Divergence of the MST Gene Family in Rice Based on Comprehensive Expression Patterns

Abstract

Sugars are critical for plant growth and development as suppliers of carbon and energy, as signal molecules, or as solute molecules for osmotic homeostasis. Monosaccharide transporter (MST) genes are involved in various processes of plant growth and development as well as in response to abiotic stresses. However, the evolution and their roles of MST genes in growth and development and in coping with abiotic stresses in rice are poorly known. Here, we identified 64 MST genes in rice genome, which are classified into seven subfamilies: STP, PLT, AZT, ERD, pGlcT, INT, and XTPH. MST genes are not evenly distributed between chromosomes (Chrs) with a bias to Chr 3, 4, 7, and 11, which could be a result of duplication of fragments harboring MST genes. In total, 12 duplication events were found in the rice MST family, among which, two pairs were derived from fragmental duplications and ten pairs were from tandem duplications. The synonymous and nonsynonymous substitution rates of duplicate gene pairs demonstrated that the MST family was under a strong negative selection during the evolution process. Furthermore, a comprehensive expression analysis conducted in 11 different tissues, three abiotic stresses, five hormone treatments, and three sugar treatments revealed different expression patterns of MST genes and indicated diversified functions of them. Our results suggest that MST genes play important roles not only in various abiotic stresses but also in hormone and sugar responses. The present results will provide a vital insight into the functional divergence of the MST family in the future study.

Keywords: MST family; Oryza sativa; expression analysis; functional divergence; qRT-PCR; rice.

Figure 1

Phylogenetic tree of the monosaccharide transporter (MST) protein sequences in rice. Multiple sequence alignment is conducted by ClustalW. Phylogenetic tree is established by Neighbor-Joining (NJ) clustering method, with 1000 Bootstrap replicates, using MEGA 6.0 software (The Pennsylvania State University, PA, USA). Different colors of arcs indicate different subfamilies.

Figure 2

Chromosomal locations of the monosaccharide transporter (MST) genes. (A) Different colors of genes represent the members from different subfamilies. (B) Pie charts of different sizes indicate the ratio of each subfamily. (C) Pie charts of different sizes indicate the ratio of each chromosome.

Figure 3

Duplication events of the monosaccharide transporter (MST) genes in rice. Different colors of genes represent the members from different subfamilies. The duplicate gene pairs of WGD or segmental/tandem events are shown with blue/red lines.

Figure 4

Phylogenetic tree (A), exon/intron structure (B), and motif compositions (C) of the monosaccharide transporter (MST) genes in rice. A: The Neighbor-Joining (NJ)-Phylogenetic trees are made in the same method with Figure 1B,C: The widths of grey bars at the bottom display relative lengths of genes and proteins. Green boxes and grey lines in B represent exons and introns, respectively. Different boxes in C represent different motifs.

Figure 5

Expression profiles of the MST genes in 11 different tissues by qRT-PCR. Cal: calli from 30 days before subculture; GS: grouting seed at 12–15 days after flowering; Rt: root of 12 days old seedlings; Sh: shoot of 12 days old seedlings; FL: flag leaf at 1 week after heading; SL: unexpanded flag leaves harvested approximately 3 weeks before heading; SinkFLS: flag leaf sheaths harvested from plants 1 week before heading; SourceFLS: flag leaf sheaths harvested from plants 1 week after heading; Nd: the first node on the top at panicle stage; InterN: internode, part between the first node and the second node on the top at panicle stage; Pan5: the length of 5 cm of panicle. The color scale represents relative expression levels based on the 2^−ΔΔCT method: red indicates a high level and green represents a low level of transcript abundance.

Figure 6

Expression changes of the MST genes under three abiotic stresses by qRT-PCR. N0, N1, N3, N6, and N12 represent 0 h, 1 h, 3 h, 6 h, and 12 h after 200 mM NaCl solution treatment. D0, D1, D3, D6, and D12 represent 0 h, 1 h, 3 h, 6 h, and 12 h after roots exposed to the air. P0, P1, P3, P6, and P12 represent 0 h, 1 h, 3 h, 6 h, and 12 h after 20% PEG6000 solution treatment. The fold values were calculated by the 2^−ΔΔCT method. The heat map was made based on the fold changes. The color scale represents fold changes: red indicates high up-regulation and green represents low up-regulation.

Figure 7

Expression changes of the MST genes under ABA, IAA, 6-BA, SA, and GA treatments by qRT-PCR. 0, 1, 3, 6, 12 represents 0 h, 1 h, 3 h, 6 h, and 12 h after hormone treatment. The fold values were calculated by the 2^−ΔΔCT method. The heat map was made based on the fold changes. The color scale represents fold changes with red indicating high and green indicating low up-regulation.

Figure 8

Expression changes of the MST genes in leaf (A) and root (B) under sucrose, glucose, and fructose treatments by qRT-PCR. 0, 1, 3, 6, 12 represents 0 h, 1 h, 3 h, 6 h, and 12 h after hormone treatment. The fold values were calculated by the 2^−ΔΔCT method. The heat map was made based on the fold values. The color scale at the right of each small image represents fold values: red indicates high up-regulation and green represents low up-regulation.

Figure 9

Expression patterns comparison of MST duplicate genes under hormone treatments and abiotic stress treatments. X-axis represents the various points from different treatments. Y-axis on the left indicates the relative values by qRT-PCR. Red numbers represent significant values.