Abiotic stresses often disrupt protein folding and induce endoplasmic reticulum （ER） stress. There is a sophisticated ER quality control （ERQC） system to mitigate the effects of malfunctioning proteins and maintain ER homeostasis. The accumulation of misfolded proteins in the ER activates the unfolded protein response （UPR） to enhance ER protein folding and the degradation of misfolded proteins mediate by ER- associated degradation （ERAD）. That ERQC reduces abiotic stress damage has been well studied in mammals and yeast. However, in plants, both ERAD and UPR have been studied separately and found to be critical for plant abiotic stress tolerance. In this study, we discovered that UPR-associated transcription factors AtbZlP17, AtbZIP28 and AtbZIP60 responded to tunicamycin （TM） and NaCl induced ER stress and subsequently enhanced Arabidopsis thaliana abiotic stress tolerance. They regulated the expression level of ER chaperones and the HRD1-complex components. Moreover, overexpression of AtbZIP17, AtbZIP28 and AtbZIP60 could restore stress tolerance via ERAD in the HRD1-complex mutant hrd3a-2, which suggested that UPR and ERAD have an interactive mechanism in Arabidopsts.
Dear Editor, Successful sexual reproduction in plants requires exten- sive communication between male and female gametophytes, their gametes, and with the surrounding sporophytic tissues （Dresselhaus and Franklin-Tong 2013）. Over the past two decades, many peptides including small cysteine-rich peptides （C.RPs） and non-CRPs were found to play key roles in plant reproduction, mainly acting as signaling cues in this male- female communication （Marshall et al.2011）.
The 76 amino acid protein ubiquitin （Ub） is highly conserved in all eukaryotic species. It plays important roles in many cellular processes by covalently attaching to the target proteins. The best known function of Ub is marking substrate proteins for degra- dation by the 26S proteasome. In fact, other consequences of ubiquitination have been discovered in yeast and mammals, such as membrane trafficking, DNA repair, chromatin modification, and protein kinase activation. The common mechanism underlying these processes is that Ub serves as a signal to sort proteins to the vacuoles or lysosomes for degradation as opposed to 26S proteasome-dependent degradation. To date, several reports have indicated that a similar function of Ub also exists in plants. This review focuses on a summary and analysis of the recent research progress on Ub acting as a signal to mediate endocytosis and endosomal trafficking in plants.
Anaphase-promoting complex/cyclosome （APC/C）, a multisubunit E3 ligase, plays a critical role in cell cycle control, but the functional characterization of each subunit has not yet been completed. To investigate the function of APC1 in Arabidopsis, we analyzed four mutant alleles of APC1, and found that mutation in APC1 resulted in significantly reduced plant fertility, accumulation of cyclin B, and disrupted auxin distribution in embryos. The three mutant alleles apcl-1, apcl-2 and apcl-3 shared variable defects in female gametogenesis including degradation, abnormal nuclear number, and disrupted polarity of nuclei in the embryo sac as well as in embryogenesis, in which embryos were arrested at multiple stages. All of these defects are similar to those previously identified in apc4. The mutant apcl-4, in which the T-DNA was inserted after the transmembrane domain at the C-terminus, showed much more severe phenotypes; that is, most of the ovules were arrested at the one-nucleate female gametophyte stage （stage FG1）. In the apcl apc4 double mutants, the fertility was further reduced by one-third in apcl-ll＋ apc4-1/＋, and in some cases no ovules even survived in siliques of apcl-4/＋ apc4-1/＋. Our data thus suggest that APC1, an essential component of APC/C, plays a synergistic role with APC4 both in female gametogenesis and in embryogenesis.
Extreme environments such as salt stress often increase the reactive oxygen species （ROS） levels （Abogadallah, 2010; Miller et al., 2010）, which cause protein denaturation and damage （Kelsen et al., 2008）. In eukaryotic cells, the endo- plasmic reticulum （ER） lumen is highly oxidative environment, which help newly synthesized proteins form intermolecular or intramolecular disulfide bonds between their cysteine residues. However, the ROS must be regulated according to the folding load to protect the cell from the consequences of oxidative folding.
Ubiquitination of proteins is one of the critical regulatory mechanisms in eukaryotes. In higher plants, protein ubiquitination plays an essential role in many biological processes, including hormone signaling, photomorphogenesis, and pathogen defense. However, the roles of protein ubiquitination in the repro- ductive process are not clear. In this study, we identified four plant-specific RING-finger genes designated Aberrant_Pollen Development 1_ （APD1） to APD4, as regulators of pollen mitosis II （PMII） in Arabidopsis thaliana （L.）. The apdl apd2 double mutant showed a significantly increased percentage of bicellular-like pollen at the mature pollen stage. Further downregulation of the APD3 and APD4 transcripts in apdl apd2 by RNA interference （RNAi） resulted in more severe abnormal bicellular-like pollen phenotypes than in apdl apd2, suggesting that cell division was defective in male gametogenesis. All of the four genes were expressed in multiple stages at different levels during male gametophyte development. Confocal analysis using green florescence fusion proteins （GFP） GFP-APD1 and GFP-APD2 showed that APDs are associated with intracellular membranes. Furthermore, APD2 had E2-dependent E3 ligase activity in vitro, and five APD2-interacting proteins were identified. Our results suggest that these four genes may be involved, redundantly, in regulating the PMII process during male gametogenesis.
