MYB75 transcription factor participates in modulation of plant cadmium tolerance
Previous research has proved that plants exposed to Cd stress show transcriptional change of genes involved in phenylpropanoid metabolisms and anthocyanin accumulation (Dai et al., 2012, Herbette et al., 2006), thus we further investigated the mechanism underlying Cd stress-regulated anthocyanin accumulation. To test the impact of Cd stress on anthocyanin accumulation, seeds of the wild type were germinated on one-half-strength Murashige and Skoog (1/2 MS) agar plates containing either 0, 25, 50, 75, 100 µM CdCl2 for 3 d. As shown in Fig. 1A, anthocyanin accumulation was elevated in wild type under Cd stress. Quantification of anthocyanin also verified that the anthocyanin level increased with the Cd concentration (Fig. 1B). Previous evidence demonstrated that anthocyanin biosynthesis derives from flavonoid biosynthetic pathway and three anthocyanin-specific genes encoding dihydroflavonol 4-reductasae (DFR), leucoanthocyanidin dioxygenase (LDOX), UDP-glucose: flavonoid 3-oglucosyl transferase (UF3GT) have been identified, and expression of these genes is regulated by MYB-bHLH-WD40 (MWB) protein complex (Xu et al., 2015). We hence determined the expression of regulatory gene in wild type without or with 25, 50, 75, 100 µM CdCl2. MYB75 transcription was notably induced in response to Cd stress (Fig. 1B, Fig. S1A). However, Cd treatment would not alter the expression of other regulatory gene (MYB90, TT8 and EGL3) (Fig. S1B-D). These results suggest that MYB75 participates in Cd stress-induced anthocyanin accumulation.
Above findings lead us conclude that MYB75 transcription factor is involved in modulation of plant Cd tolerance. Consequently, pap1-D seedlings, the activation tag mutant constitutively overexpresses MYB75/PAP1 and myb75-c that MYB75 knockout mutant using the CRISPR-Cas9 system were used for determining the MYB75 function in plant Cd tolerance. Arabidopsis seedlings grown vertically in 1/2 MS agar plates without CdCl2 for 3 d, then were moved to 1/2 MS agar plates without or with 50 or 75 µM CdCl2 for 7 d. As can be seen from Fig. 1C, when grown on 1/2 MS media without CdCl2, pap1-D and myb75-c exhibited no difference compared with wild type. Nevertheless, pap1-D showed higher tolerance in response to Cd stress compared with wild type (Fig. 1C). By comparison, we observed that myb75-c was more sensitive to Cd stress than wild type (Fig. 1C). These results were further verified by quantification of both the root length and fresh weight (Fig. 1D). Overall, these results suggest that MYB75 is involved in regulation of plant Cd tolerance.
It has previously been observed that Cd exposure weakens the photosystem, we next confirmed that whether MYB75 regulated Cd stress-induce photosystem damage. We maintained Arabidopsis seedlings grown in 1/2 MS media without or with 75 µM CdCl2 for 21 d. From the Fig. 2A we can see that Cd exposure accelerated chlorophyll degradation. Quantification of chlorophyll verified that pap1-D showed more chlorophyll content compared with wild type (Fig. 2B). Meanwhile, the chlorophyll content of myb75-c mutant was significantly lower than that of wild type (Fig. 2B). Ion leakage can indicate the degree of damage in plants caused by environmental stresses, thus we analyzed the ion leakage under Cd exposure. As shown in Fig. 2C, ion leakage was much higher in myb75-c mutant than in wild type. Nonetheless, ion leakage of pap1-D was significantly lower than that of wild type (Fig. 2C).
We further determined the photosystem II (PSII) photochemistry by detecting Chlorophyll fluorescence including Fv/Fm and ΦPSII. Compared with wild type, Fv/Fm of pap1-D showed no visible difference without CdCl2 (Fig. 2D). When exposed to Cd stress, pap1-D exhibited higher levels of Fv/Fm than that of wild type (Fig. 2D). In contrast, levels of Fv/Fm were lower in myb75-c than in wild type (Fig. 2D). Similarly, levels of ΦPSII were also higher in pap1-D and lower in myb75-c compared with wild type (Fig. 2E). These results reveal that MYB75 transcription factor alleviates Cd stress-induced photosystem damage. Together with above results, we conclude that MYB75 transcription factor participates in modulation of plant Cd tolerance.
MYB75 positively regulates plant Cd tolerance
To further examine the impact of MYB75 on plant Cd tolerance, we generated transgenic plants expressing MYB75 driven by the constitutive 35S promoter in wild type background. The 35S: MYB75 #7 and 35S: MYB75 #10 transgenic plants exhibited increased tolerance to Cd stress compared with wild type when grown on 1/2 MS media with 50 or 75 µM CdCl2 (Fig. 3A). Quantitative analysis of root length and fresh weight further confirmed these results (Fig. 3B, C), indicating that MYB75 positively regulates plant Cd stress.
Next, the 35S: MYB75 #7 and 35S: MYB75 #10 transgenic plants grown on 1/2 MS media without or with 75 µM CdCl2 were examined. The chlorophyll content of 35S: MYB75 #7 and 35S: MYB75 #10 was significantly higher than that of wild type (Fig. 4A, B). As can be seen from Fig. 4C, ion leakage of 35S: MYB75 #7 and 35S: MYB75 #10 transgenic plants was significantly lower than that of wild type. When exposed to Cd stress, 35S: MYB75 #7 and 35S: MYB75 #10 transgenic plants exhibited higher levels of Fv/Fm and ΦPSII than that of wild type (Fig. 4D, E). Taken together, MYB75 transcription factor enhances plant Cd tolerance under Cd exposure.
MYB75 transcription factor-elevated Cd tolerance is involved in antioxidant system
Much work so far has focused on the key function of MYB75 transcription factor in regulation of anthocyanin accumulation. Given that anthocyanin is critical for scavenge ROS via their antioxidant capability (Li et al., 2017, Nakabayashi et al., 2014), we further examined the ROS levels in MYB75-overexpressing lines and myb75-c mutant under normal or Cd stress conditions. Nitroblue tetrazolium (NBT) staining indicated that content of superoxide (O2·−) was much lower in pap1-D, 35S: MYB75 #7 and 35S: MYB75 #10 plants compared with wild type under Cd stress (Fig. 5A). On the contrary, O2·− content of myb75-c was much higher than that of wild type under Cd stress (Fig. 5A). These results were further ascertained by quantification of O2·− content (Fig. 5B). Moreover, we observed that pap1-D, 35S: MYB75 #7 and 35S: MYB75 #10 plants accumulated less hydrogen peroxide (H2O2) content than wild type under Cd stress (Fig. 5C). In contrast, H2O2 content of myb75-c was much higher than that of wild type under Cd stress (Fig. 5C). Quantification of H2O2 content also confirmed these results (Fig. 5D). Collectively, these results demonstrated that MYB75 transcription factor plays positive role in protection of plants from Cd exposure by alleviating oxidative damage.
Extensive research has shown that plants have highly effective antioxidant mechanisms involving superoxide dismutase (SOD) and catalase (CAT) to scavenge ROS. Cd exposure led to decreases in antioxidant enzyme activities (Fig. 5E, F). It is mostly likely that Cd2+ inhibits the function of antioxidant enzyme. Intriguingly, diminution of SOD and CAT activities in myb75-c mutant were significantly aggravated under Cd exposure (Fig. 5E, F). Oppositely, SOD and CAT activities of pap1-D, 35S: MYB75 #7 and 35S: MYB75 #10 plants were significantly higher than that of wild type (Fig. 5E, F). In summary, MYB75 transcription factor positively regulates Cd tolerance through activating antioxidant system and alleviating oxidative damage.
MYB75 transcription factor stimulates GSH‑dependent PC synthesis pathway and Cd accumulation
Reduced glutathione (GSH)-oxidized glutathione (GSSG) conversion by ROS homeostasis in plant cell has been intensively investigated (Noctor et al., 2012). To clarify whether ROS homeostasis affects GSH content, we examined catalase-overexpressing plants (35S: CAT2 and 35S: CAT3) under Cd exposure. When compared with wild type, 35S: CAT2 and 35S: CAT3 plants exhibited less H2O2 content, but more GSH and PC content under Cd stress (Fig. S2). Given that MYB75 transcription factor declined the ROS levels via anthocyanin and antioxidant enzyme, we next determined the GSH levels without or with Cd treatment. As shown in Fig. 6A, no significant difference was detected in total glutathione (GSH plus 2GSSG) between the wild type, the pap1-D, 35S: MYB75 #7, 35S: MYB75 #10 plants, and the myb75-c mutants without Cd treatment. Cd exposure significantly depressed GSH concentrations in these plants (Fig. 6A). Nevertheless, compared with wild type, GSH levels was higher in pap1-D, 35S: MYB75 #7, 35S: MYB75 #10 plants and lower in myb75-c mutants (Fig. 6A). Previous research has established that phytochelatin (PC), the important component involved in chelating Cd2+, derived directly from GSH via the PC synthase (PCS). The PC content was elevated significantly in pap1-D, 35S: MYB75 #7, 35S: MYB75 #10 plants and diminished in myb75-c mutants compared with wild type under Cd exposure, suggesting MYB75 transcription factor positively regulates PC levels. Taken together, these results verified that MYB75 positively regulates Cd tolerance through stimulating GSH‑dependent PC synthesis pathway.
Given the observed change of PC, we further test whether MYB75 affects Cd content through measuring Cd content under Cd stress. As can be seen from Fig. 6C, pap1-D, 35S: MYB75 #7, 35S: MYB75 #10 plants showed higher Cd content in roots and shoots than wild type. On the contrary, Cd content was reduced in myb75-c under Cd stress compared with that in the wild type. These results suggest that MYB75 mediates Cd accumulation in roots and shoots under Cd stress.
MYB75 transcription factor directly regulates the Cd tolerance-related gene expression
Transcription factor found to be influencing Cd tolerance have been explored in several studies (Agarwal et al., 2020, Zhang et al., 2019). Therefore, we further investigated whether MYB75 regulates Cd tolerance at transcriptional level. We determined Cd tolerance-related gene expression such as ACBP2, ABCC2, GSH1, PDR8, ATM3, and PDF2.5. Intriguingly, the transcription levels of GSH1, PDR8, ATM3, and PDF2.5 were induced by Cd stress, but these gene expression in pap1-D, 35S: MYB75 #7, 35S: MYB75 #10 and myb75-c exhibited no difference compared with wild type (Fig. S3). However, under Cd stress, the transcription levels of ACBP2, which binds Cd2+, were significantly higher in pap1-D, 35S: MYB75 #7, 35S: MYB75 #10 plants than in wild type, while its expression levels in myb75-c mutant were significantly lower than that in the wild type (Fig. 7A). We also noticed that expression levels of ABCC2, an ABCC-type phytochelatin transporter, were elevated in pap1-D, 35S: MYB75 #7, 35S: MYB75 #10 plants, but reduced in myb75-c mutant without or with Cd stress (Fig. 7B).
On the basis of positive impact of MYB75 on ACBP2 and ABCC2 expression, we further surveyed the GUS activity by transient expression analysis in Nicotiana benthamiana. From the Fig. 7C-E and Fig. S4, we can see that MYB75 prompted the expression levels of ProACBP2: GUS and ProABCC2: GUS, indicating that MYB75 has the capacity of prompting reporter activity driven by the promoters of ACBP2 and ABCC2. Moreover, we performed luciferase (LUC) reporter transactivation assays in Arabidopsis protoplasts. Promoters of ACBP2 and ABCC2 were fused with LUC gene to generate promoter-LUC reporter constructs (Fig. 7F). These reporter constructs were co-expressed with empty vector or MYB75-HA in wild type protoplasts treated with MG132, and the reporter gene expression was used to evaluate MYB75 transcriptional activity. We noticed that MYB75 induced ProACBP2-LUC expression compared with empty vector (Fig. 7G). Consistently, MYB75 also induced ProABCC2-LUC expression (Fig. 7H). Based on these findings, we conclude that MYB75 positively regulates ACBP2 and ABCC2 expression.
MYB75 transcription factor directly binds to the promoter of ACBP2 and ABCC2
Taking the above observations into account, we further examined the binding of MYB75 to the promoter regions of ACBP2 and ABCC2 in vitro and in vivo. MYB-recognizing element (MRE) has been identified as the core binding motif of MYB75. We found two MREs within the promoters of ACBP2 and ABCC2 (Fig. 8A, B). To test whether MYB can directly bind to the promoter regions of these target genes, we firstly performed electrophoretic mobility shift assay (EMSA). The results revealed that MYB75 protein tagged with maltose binding protein (MBP) (MBP-MYB75) bound to the P1 probe of ACBP2 and C1 probe ABCC2, while the binding was abolished by mutation of MYB75 binding sites in the probes (Fig. 8A, B). Interestingly, MBP-MYB75 weakly bound to the P2 probe in the promoter of ACBP2, but did not bind to the C2 in the promoter of ABCC2 (Fig. 8A, B). We next employed chromatin immunoprecipitation (ChIP) to further test the affinity of MYB75 for promoters of ACBP2 and ABCC2. We immunoprecipited HA-MYB75 protein from 35S: MYB75 #7 transgenic plants treated with MG132 by using anti-GFP antibody. TA3, a retrotransposable element, was used as the internal control. The ChIP-qPCR results revealed that MYB75 significantly enriched the fragments containing P1 and P2 of ProACBP2, C1 of ProABCC2 (Fig. 8C, D). These results indicate that MYB75 directly regulates the transcription of ACBP2 and ABCC2 by binding to their promoters.