Histone H3K4 methyltransferase DcATX1 promotes ethylene induced petal senescence in carnation

Petal senescence is controlled by a complex regulatory network. Epigenetic regulation like histone modification influences chromatin state and gene expression. However, involvement of histone methylation in regulating petal senescence is still largely unknown. Here, we found that the trimethylation of histone H3 at Lysine 4 (H3K4me3) is increased during the ethylene induced petal senescence in carnation (Dianthus caryophyllus L.). The H3K4me3 levels are positively associated with the expression of transcription factor DcWRKY75, ethylene biosynthetic genes DcACS1 and DcACO1, and senescence associated genes (SAGs) DcSAG12 and DcSAG29. Further, we identified that carnation DcATX1 (ARABIDOPSIS HOMOLOG OF TRITHORAX1) encodes a histone lysine methyltransferase which can methylate H3K4. Knockdown of DcATX1 delays ethylene induced petal senescence in carnation, which is associated with the downregulated expression of DcWRKY75, DcACO1 and DcSAG12. While overexpression of DcATX1 exhibits the opposite effects. DcATX1 promotes the transcription of DcWRKY75, DcACO1 and DcSAG12 by targeting to their promoters to elevate the H3K4me3 levels. Overall, our results demonstrate that DcATX1 is a H3K4 methyltransferase that promotes the expression of DcWRKY75, DcACO1 and DcSAG12 by regulating H3K4me3 levels, thereby accelerating ethylene induced petal senescence in carnation. This study further indicates that epigenetic regulation is important for plant senescence process.

Genome-Wide Analysis of Purple Acid Phosphatase Genes in Brassica rapa and Their Association with Pollen Development and Phosphorus Deprivation Stress

PAPs (purple acid phosphatases) belong to the metallo-phosphoesterase superfamily and play important roles in developmental processes, phosphorus foraging, and recycling. However, the specific functions of BrPAPs in Brassica rapa are poorly understood. In this study, 39 BrPAPs were identified and divided into three major clades and nine subgroups. In 8 of the 39 BrPAPs, some invariant amino acid residues were lost or shifted. Based on an expression profiling analysis, BrPAP11, 14, 20, 24, 29, and 34 were specifically expressed in fertile floral buds, indicating their critical roles during pollen development. A total of 21 BrPAPs responded to Pi deprivation in either shoots or roots. Of these, BrPAP4, 5, 19, and 21 were upregulated in roots under Pi depravation conditions, while BrPAP12 was upregulated in the roots in normal conditions. BrPAP28 was upregulated in shoots under Pi depravation conditions, indicating its function shifted compared with its Arabidopsis homolog, AtPAP26. The present work contributes to further investigation of BrPAPs as candidate genes for genetic improvement studies of low phosphorus tolerance as well as for creating male sterile lines based on gene editing methods in Brassica rapa.

The DREAM complex represses growth in response to DNA damage in Arabidopsis

The DNA of all organisms is constantly damaged by physiological processes and environmental conditions. Upon persistent damage, plant growth and cell proliferation are reduced. Based on previous findings that RBR1, the only Arabidopsis homolog of the mammalian tumor suppressor gene retinoblastoma, plays a key role in the DNA damage response in plants, we unravel here the network of RBR1 interactors under DNA stress conditions. This led to the identification of homologs of every DREAM component in Arabidopsis, including previously not recognized homologs of LIN52. Interestingly, we also discovered NAC044, a mediator of DNA damage response in plants and close homolog of the major DNA damage regulator SOG1, to directly interact with RBR1 and the DREAM component LIN37B. Consistently, not only mutants in NAC044 but also the double mutant of the two LIN37 homologs and mutants for the DREAM component E2FB showed reduced sensitivities to DNA-damaging conditions. Our work indicates the existence of multiple DREAM complexes that work in conjunction with NAC044 to mediate growth arrest after DNA damage.

The canonical α-SNAP is essential for gametophytic development in Arabidopsis

The development of male and female gametophytes is a pre-requisite for successful reproduction of angiosperms. Factors mediating vesicular trafficking are among the key regulators controlling gametophytic development. Fusion between vesicles and target membranes requires the assembly of a fusogenic soluble N-ethylmaleimide sensitive factor attachment protein receptors (SNAREs) complex, whose disassembly in turn ensures the recycle of individual SNARE components. The disassembly of post-fusion SNARE complexes is controlled by the AAA+ ATPase N-ethylmaleimide-sensitive factor (Sec18/NSF) and soluble NSF attachment protein (Sec17/α-SNAP) in yeast and metazoans. Although non-canonical α-SNAPs have been functionally characterized in soybeans, the biological function of canonical α-SNAPs has yet to be demonstrated in plants. We report here that the canonical α-SNAP in Arabidopsis is essential for male and female gametophytic development. Functional loss of the canonical α-SNAP in Arabidopsis results in gametophytic lethality by arresting the first mitosis during gametogenesis. We further show that Arabidopsis α-SNAP encodes two isoforms due to alternative splicing. Both isoforms interact with the Arabidopsis homolog of NSF whereas have distinct subcellular localizations. The presence of similar alternative splicing of human α-SNAP indicates that functional distinction of two α-SNAP isoforms is evolutionarily conserved.

Defective Repression of OLE3::LUC 1 (DROL1) is specifically required for the splicing of AT–AC-type introns in Arabidopsis

An Arabidopsis mutant named defective repression of OLE3::LUC 1 (drol1) was originally isolated as a mutant with defects in the repression of OLEOSIN3 (OLE3) after seed germination. In this study, we show that DROL1 is an Arabidopsis homolog of yeast DIB1, a subunit of U5 snRNP in the spliceosome, but comprises a subfamily specific to a certain class of eukaryotes. Comprehensive analysis of intron splicing by RNA-Seq analysis of drol1 mutants revealed reduced splicing of most of the minor introns with AT–AC dinucleotide termini. Thirty-nine genes, including those playing important roles in the response to abiotic stress, exhibited reduced splicing of AT–AC-type introns in drol1 mutants. In addition, drol1 mutant seedlings showed growth arrest, similar to that caused by the activation of abscisic acid signaling, as a result of reduced splicing of AT–AC-type introns in some genes. These results indicate that DROL1 is specifically involved in the splicing of introns with AT–AC termini, and splicing of these minor introns plays an important role in plant growth and development.

Citrus reticulata CrRAP2.2 Transcriptional Factor Shares Similar Functions to the Arabidopsis Homolog and Increases Resistance to Xylella fastidiosa

Xylella fastidiosa is a worldwide multihost pathogen that causes diseases in different crops. It is considered a new global threat and substantial efforts have been made in order to identify sources of resistance. Indeed, many genes have been associated with resistance to X. fastidiosa, but without functional validation. Here, we describe a C. reticulata gene homologous to the transcriptional factor RAP2.2 from Arabidopsis thaliana that increases resistance to citrus variegated chlorosis (CVC). This gene was previously detected in C. reticulata challenged with X. fastidiosa. Bioinformatics analysis together with subcellular localization and auto-activation assays indicated that RAP2.2 from C. reticulata (CrRAP2.2) is a transcriptional factor orthologous to AtRAP2.2. Thus, we used A. thaliana as a model host to evaluate the functional role of CrRAP2.2 in X. fastidiosa resistance. The inoculation of X. fastidiosa in the A. thaliana rap2.2 mutant resulted in a larger bacterial population, which was complemented by CrRAP2.2. In addition, symptoms of anthocyanin accumulation were higher in the mutant, whose phenotype was restored by CrRAP2.2, indicating that they have conserved functions in plant defense response. We therefore transformed C. sinensis with CrRAP2.2 and verified a positive correlation between CVC resistance and gene expression in transgenic lines. This is the first study using A. thaliana as model host that characterizes the function of a gene related to X. fastidiosa defense response and its application in genetic engineering to obtain citrus resistance to CVC.

Epigenetic silencing of a multifunctional plant stress regulator

The central regulator of the ethylene (ET) signaling pathway, which controls a plethora of developmental programs and responses to environmental cues in plants, is ETHYLENE-INSENSITIVE2 (EIN2). Here we identify a chromatin-dependent regulatory mechanism at EIN2 requiring two genes: ETHYLENE-INSENSITIVE6 (EIN6), which is a H3K27me3 demethylase also known as RELATIVE OF EARLY FLOWERING6 (REF6), and EIN6 ENHANCER (EEN), the Arabidopsis homolog of the yeast INO80 chromatin remodeling complex subunit IES6 (INO EIGHTY SUBUNIT). Strikingly, EIN6 (REF6) and the INO80 complex redundantly control the level and the localization of the repressive histone modification H3K27me3 and the histone variant H2A.Z at the 5’ untranslated region (5’UTR) intron of EIN2. Concomitant loss of EIN6 (REF6) and the INO80 complex shifts the chromatin landscape at EIN2 to a repressive state causing a dramatic reduction of EIN2 expression. These results uncover a unique type of chromatin regulation which safeguards the expression of an essential multifunctional plant stress regulator.

The PLUTO plastidial nucleobase transporter also transports the thiamin precursor hydroxymethylpyrimidine

In plants, the hydroxymethylpyrimidine (HMP) and thiazole precursors of thiamin are synthesized and coupled together to form thiamin in plastids. Mutants unable to form HMP can be rescued by exogenous HMP, implying the presence of HMP transporters in the plasma membrane and plastids. Analysis of bacterial genomes revealed a transporter gene that is chromosomally clustered with thiamin biosynthesis and salvage genes. Its closest Arabidopsis homolog, the plastidic nucleobase transporter (PLUTO), is co-expressed with several thiamin biosynthetic enzymes. Heterologous expression of PLUTO in Escherichia coli or Saccharomyces cerevisiae increased sensitivity to a toxic HMP analog, and disrupting PLUTO in an HMP-requiring Arabidopsis line reduced root growth at low HMP concentrations. These data implicate PLUTO in plastidial transport and salvage of HMP.