In the SUMOylation pathway SCE
In the SUMOylation pathway, SCE1 as an E2 enzyme has an important role in mediating the conjugation of SUMO to target proteins. The accumulated SUMO conjugates in plants are associated with abiotic stress responses (Castro et al, 2012). We performed a drought stress analysis on WT plants, induced by PEG6000, to observe the LMK 235 of the OsSCE1 gene in relation to drought stress. The OsSCE1 gene in leaves of WT plants was upregulated after PEG6000 treatment for more than 3 h (Fig. 2-B), revealing that OsSCE1 expression is strongly induced by dehydration.
To further validate OsSCE1 gene function in relation to drought stress, we established OsSCE1-OX (overexpressed) and OsSCE1-KD (knockdown) transgenic rice plants. Drought stress analysis using PEG6000 showed the selected OsSCE1-OX line (S15.1) had a lower tolerance to drought stress than WT plants (Fig. 6). In S15.1 line, expression of OsSCE1 was upregulated (Fig. 4). The upregulation of OsSCE1 suggests a role for SUMOylation in the response of plants to drought stress. Increased SUMOylation levels attenuate ABA-mediated growth inhibition and induce stress responsive genes (Lois et al, 2003). Conversely, the OsSCE1-KD rice line (AS250.9) was less sensitive to drought stress than the WT (Fig. 6). Analysis of expression levels showed that for both OsSCE1-KD lines (AS242.4 and AS250.9), the OsSCE1 transcript was downregulated (Fig. 4). This may decrease SUMOylation levels in OsSCE1-KD lines. Whereas SUMOylation results in repression or activation of transcription factor activity, de-SUMOylation mediates the opposite response (Miura et al, 2007a). These findings may reveal SUMOylation by OsSCE1 plays a negative role in rice response to drought stress, as OsSCE1-KD rice showed improved drought tolerance and vice versa. Similar findings were previously reported by Miura et al (2013) and Zhang et al (2013) for the characterization of Arabidopsis E3 ligase genes MMS21 and SIZ1, respectively. Plants deficient in MMS21 and SIZ1 display an improved tolerance to drought. Moreover, constitutive expression of MMS21 was reported to reduce drought tolerance. Moreover, Karan and Subudhi (2012) reported overexpression of the S. alterniflora E2 gene SaSce9 improved drought tolerance in Arabidopsis, which may suggest more complex regulation mechanisms are involved in SUMOylation.
The negative regulatory role of OsSCE1 in the drought response is further supported by the accumulation of more proline in the OsSCE1-KD line (Fig. 7). Proline is a compatible solute accumulated by plants under water deficit conditions, and acts as an osmolyte for osmotic adjustment by stabilizing sub-cellular structures, scavenging free radicals and buffering cellular redox potential under stress conditions (Hayat et al, 2012). The phenotype of the OsSCE1-KD line was less sensitive to drought stress than that of WT, and the OsSCE1-KD line had higher proline levels than WT. The proline levels were lower in the overexpressed OsSCE1-OX lines, which also demonstrated a more susceptible phenotype. This correlates well with a previous report wherein Arabidopsis E3-suppressed mutants (mms21) exhibit improved drought tolerance by accumulating higher level of free proline compared to WT and MMS21- overexpressed plants (Zhang et al, 2013).
In summary, OsSCE1 may play a role as a negative regulator in the drought stress response in rice, and the suppression of proline biosynthesis is a possible tolerance mechanism related to this role.
Introduction Autophagy is generally believed to be a process in which nonspecific degradation of cytoplasmic content, including organelles, occurs . Recent studies have revealed that there is specificity in autophagy that allows the degradation of specific cargo , . Such a selective type of autophagy includes mitophagy, the process whereby damaged mitochondria are removed from the cell , . Mitophagy is crucial for many physiological processes such as development and differentiation, and its deregulation has been implicated in Parkinson's disease, cardiovascular disease, as well as various infections and cancers , . Mitophagy has been shown to be mediated predominantly by the E3 ubiquitin ligase parkin, which is recruited to the mitochondria by PINK1 (PTEN-induced putative kinase protein 1) , , , . Recent studies have identified that another mitochondrial E3 ubiquitin ligase, Mulan/Hades/GIDE (mitochondrial ubiquitin ligase activator of NF-κB; hereafter referred to as Mulan), is involved in mitophagy , , . Our studies have identified Mulan as a specific substrate of the mitochondrial Omi/HtrA2 and showed that inactivation of Omi/HtrA2 protease leads to the accumulation of the Mulan protein and increased mitophagy .