Essential roles of SKIP for plant growth and stress tolerance
SKIP protein, as a spliceosome component, played a vital role in maintaining cell viability in yeast (PRP45), C. elegans (CeSKIP) and O. sativa (OsSKIPa) [16, 20, 22]. The homozygous mutant of AtSKIP in Arabidopsis resulted in death of plants, which was similar to the phenotype of severe growth arrest and even death of O. sativa with suppression of OsSKIPa and embryonic arrest of the CeSKIP mutant in C. elegans, which indicated that AtSKIP was also required for maintaining cell viability and normal growth in Arabidopsis. The indispensable role of SKIP homolog in keeping normal cell viability and growth might be conserved in plants, which was supported by that the plant SKIP homologs possessed the conserved SKIP domain for cell viability identified in PRP45. However, it needs further study to confirm whether the SKIP homologs in soybean could have the conserved function for cell viability.
Drought and high-salinity repressed plant growth and limited seed yield. SKIP homologs in Arabidopsis and rice could improve the tolerance to drought and high-salinity [22, 23]. In this study, the ectopic expression of GmGBP1 in Arabidopsis could enhance the tolerance to drought. However, GmGBP1-ox plants showed a salt-sensitive appearance, although atskip-i plants reduced the tolerance to salt as previous report . Thus, the tolerance of SKIP homologs to salt might be not conserved in plant. Heat, one of serious environmental stresses, affected the growth of plants and the productivity of crops. There was no report about heat tolerance of SKIP homologs previously. Our result showed that GmGBP1-ox plants increased the tolerance to heat whereas atskip-i plants reduced the tolerance to heat, indicating that AtSKIP has the ability of heat tolerance.
Reactive oxygen species (ROS) could be induced by abiotic stresses, and over-accumulation of ROS could lead to cell damage and even death. SOD was used to eliminate ROS, and MDA was the intermediate product during the elimination of ROS [29, 30]. In this study, the raised content of ROS in GmGBP1-ox plants treated with salinity and the decreased content of ROS in GmGBP1-ox plants treated with heat (or drought) indicated that the altered stress tolerance of GmGBP1-ox plants may be partially due to the regulation of the activity of ROS-eliminating.
GmGBP1 regulates flowering time
Interestingly, in this study ectopic expression of GmGBP1 in Arabidopsis induced earlier flowering in LDs and late flowering in SDs. The finding of a flowering-regulation function from a SKIP homolog has not yet been reported in any plants so far. The specific function of GmGBP1 in regulating flowering time might be especially useful for developing soybean cultivars with the adaptability to broad grow regions.
Flowering time was controlled by several signaling pathways, such as the day-length, vernalization, autonomous pathways and gibberellin signal pathway [1, 5–7], and it could be measured by scoring the number of rosette leaves at flowering time and the number of days from germination to bolting in Arabidopsis . The number of rosette leaves at flowering time and the number of days from germination to bolting measured in GmGBP1-ox plants were both reduced in LDs, suggesting that ectopic expression of GmGBP1 induced early flowering. atskip-i plants delayed flowering in LDs supported the early flowering of GmGBP1-ox plants in another hand. However, when GmGBP1 ectopicly expressed in myb33 plants, no early flowering could be observed in LDs, indicating that the early flowering function of GmGBP1 might depend on the existence of MYB33. When the transgenic lines were transferred into SDs, the phenotypes of flowering were all changed. The flowering times of GmGBP1-ox and GmGBP1-ox/myb33 plants were both delayed significantly, and atskip-i displayed the early flowering phenotype in SDs.
The expression levels of numerous flowering-related genes could be induced by day-length or gibberellin. The expression level of GmGBP1 was regulated by both day-length and gibberellin, suggesting that GmGBP1 might participate in both day-length and gibberellin signal pathways. In atskip
i plants of Arabidopsis, the expression levels of more than 10 flowering-related genes were affected. In particular, the mRNA levels of several flowering integrators (FT
LFY and SOC1) were significantly lower. Our results showed that FT and LFY were both up-regulated in GmGBP1-ox plants in LDs, but LFY showed no change when GmGBP1 was ectopic expression in myb33 mutants. Considering the interaction between GmGBP1 and GmGAMYB1, and the conserved function on photoperiodic flowering of FT homologs in soybean [32–34], GmGBP1 might regulate flowering time in two ways by its function as a transcription factor and interaction in LDs. On one hand, GmGBP1 could regulate the expressions of flowering-related genes in day-length signal pathway, and on the other hand, GmGBP1 could bind with GmGAMYB1 in gibberellin signal pathway to control flowering time.
Many reports had revealed that GA pathway played a key role in flowering under SD condition when other regulatory pathways that promoted flowering were not active [1, 7, 35–37]. However, the late flowering of GmGBP1-ox plants indicated that there might be another way to control flowering in SDs. FLC was a negative regulator of floral initiation and an integrator of the autonomous and vernalization pathways. FLC could directly down-regulated FT and SOC1 to repress the flowering in these pathways [8, 9, 38]. SVP, a MADS box transcription factor, could interact with FLC and acted as partially redundant repressors of flowering time with FLC. The negative action on the phenotypes of the down-regulated FLC and SVP in atskip-i plants in LDs suggested that flowering time was regulated crossly by several signal pathways and SKIP might take part in more than two ways. In SDs, the expression of FLC was up-regulated in GmGBP1-ox plants, and was down-regulated in atskip-i plants as that in LDs. The late flowering of GmGBP1-ox plants in SDs elucidated that GmGBP1 might delay flowering time in SDs through autonomous pathway by improving the expression level of FLC, a key flowering inhibitor factor.
In general, GmGBP1 might regulate flowering time by three signal pathways. GmGBP1 positively controlled the flowering time by regulating CO, FT, LFY and GAMYB directly or indirectly in photoperiodic and gibberellin pathways in LDs, GmGBP1 repressed flowering by regulating FLC and SVP in autonomous pathway in SDs.
Diverse functions of SKIP homologs in plant
SKIP had a conserved SKIP domain with an S-N-W-K-N peptide signature, and was considered as a cofactor for transcription regulation in all eukaryotes so far. However, the derived or additional functions of the SKIP homologs varied among species. Transgenic rice that overexpressed OsSKIPa exhibited stress tolerances (abscisic acid, salt, mannitol) at both seedling and reproductive stages . Overexpression of the AtSKIP gene in Arabidopsis modulated the induction of salt tolerance, dehydration resistance and insensitivity towards abscisic acid under stress conditions ; However, ectopic expression of GmGBP1 in Arabidopsis reduced tolerance to NaCl, but increased tolerance to drought and heat in this study.
Suppression of OsSKIP resulted in growth arrest of rice due to the reduced cell viability in the active growth regions . A decrease in AtSKIP expression led to altered plant development with the phenotype of reduced inflorescence stems and smaller rosette leaves . In this study, the homozygous atskip mutant was lethal to the growth, and the knockdown of AtSKIP showed late flowering and increased number of rosette leaves. The ectopic expression of GmGBP1 induced premature flowering of Arabidopsis in LDs. All the study on SKIP homologs in plant revealed that SKIPs might play a vital role in the growth and development of plant, but have different function on the stress tolerance and flowering in plant.
Plant SKIPs were divided into four groups based on the SKIP sequences by phylogenetic tree (Figure 1D). The different groups among OsSKIP, AtSKIP and GmGBP1 indicated the diversity of their functions. SKIP could be induced by various abiotic stresses, phytohormones treatments and day-length, but the expression patterns of SKIP varied among rice, Arabidopsis and soybean [22, 23]. The transcriptional levels of OsSKIP, AtSKIP and GmGBP1 were all up-regulated by ABA, NaCl and drought (PEG6000 or mannitol), but had different scales. GmGBP1 could be induced by day-length and heat, but no related report for OsSKIP and AtSKIP in rice and Arabidopsis. The various functions of SKIP homologs might be also owing to the diversification of SKIP-interacting proteins SIPs. Previous studies showed that the SKIP homologs, PRP45 (yeast), BX42 (Drosophila), CeSKIP (C. elegans), HvSKIP (barely) and OsSKIP (rice) had 34, 13, 5, 1 and 35 interacted proteins, respectively [16, 20–22, 40, 41]. Nevertheless, few SIPs could match each other among species. For example, both HvSKIP and GmGBP1 interacted with GAMYB but OsGAMYB was not included in the 35 OsSIPs [21, 22]. All the data indicated that SKIP might participate in distinct functions through the interaction with diverse proteins.