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Chromosome condensation, a process mediated by the condensin complex, is essential for proper chromosome segregation during cell division. Unlike rapid mitotic chromosome condensation, meiotic chromosome condensation occurs over a relatively long prophase I and is unusually complex due to the coordination with chromosome axis formation and homolog interaction. The molecular mechanisms that regulate meiotic chromosome condensation progression from prophase I to metaphase I are unclear. Here, we show that the Arabidopsis thaliana meiotic PHD-finger protein MMD1/DUET is required for progressive compaction of prophase I chromosomes to metaphase I bivalents. The MMD1 PHD domain is required for its function in chromosome condensation and binds to methylated histone tails. Transcriptome analysis and qRT-PCR showed that several condensin genes exhibit significantly reduced expression in mmd1 meiocytes. Furthermore, MMD1 specifically binds to the promoter region of the condensin subunit gene CAP-D3 to enhance its expression. Moreover, cap-d3 mutants exhibit similar chromosome condensation defects, revealing an MMD1-dependent mechanism for regulating meiotic chromosome condensation, which functions in part by promoting condensin gene expression. Together, these discoveries provide strong evidence that the histone reader MMD1/DUET defines an important step for regulating the progression of meiotic prophase I chromosome condensation. [ABSTRACT FROM AUTHOR]

Drought dramatically affects plant growth and crop yield, but previous studies primarily examined responses to drought during vegetative development. Here, to study responses to drought during reproductive development, we grew Arabidopsis thaliana plants with limited water, under conditions that allowed the plants to initiate and complete reproduction. Drought treatment from just after the onset of flowering to seed maturation caused an early arrest of floral development and sterility. After acclimation, plants showed reduced fertility that persisted throughout reproductive development. Floral defects included abnormal anther development, lower pollen viability, reduced filament elongation, ovule abortion, and failure of flowers to open. Drought also caused differential expression of 4153 genes, including flowering time genes FLOWERING LOCUS T , SUPPRESSOR OF OVEREXPRESSION OF CO1 , and LEAFY , genes regulating anther and pistil development, and stress-related transcription factors. Mutant phenotypes of hypersensitivity to drought and fewer differentially expressed genes suggest that DEHYDRATION RESPONSE ELEMENT B1A may have an important function in drought response in flowers. A more severe filament elongation defect under drought in myb21 plants demonstrated that appropriate stamen development requires MYB DOMAIN PROTEIN 21 under drought conditions. Our study reveals a regulatory cascade in reproductive responses and acclimation under drought. [ABSTRACT FROM AUTHOR]

Aliphatic alcohols naturally exist in many organisms as important cellular components; however, their roles in extracellular polymer biosynthesis are poorly defined. We report here the isolation and characterization of a rice (Oryza sativa) male-sterile mutant, defective pollen wall (dpw), which displays defective anther development and degenerated pollen grains with an irregular exine. Chemical analysis revealed that dpw anthers had a dramatic reduction in cutin monomers and an altered composition of cuticular wax, as well as soluble fatty acids and alcohols. Using map-based cloning, we identified the DPW gene, which is expressed in both tapetal cells and microspores during anther development. Biochemical analysis of the recombinant DPW enzyme shows that it is a novel fatty acid reductase that produces 1-hexadecanol and exhibits >270-fold higher specificity for palmiltoyl-acyl carrier protein than for C16:0 CoA substrates. DPW was predominantly targeted to plastids mediated by its N-terminal transit peptide. Moreover, we demonstrate that the monocot DPW from rice complements the dicot Arabidopsis thaliana male sterile2 (ms2) mutant and is the probable ortholog of MS2. These data suggest that DPWs participate in a conserved step in primary fatty alcohol synthesis for anther cuticle and pollen sporopollenin biosynthesis in monocots and dicots. [ABSTRACT FROM AUTHOR]

In flowering plants, sink tissues rely on transport of carbohydrates from photosynthetic tissues (sources) for nutrition and energy. However, how sugar partitioning in plants is regulated at the molecular level during development remains unknown. We have isolated and characterized a rice (Oryza sativa) mutant, carbon starved anther (csa), that showed increased sugar contents in leaves and stems and reduced levels of sugars and starch in floral organs. In particular, the csa mutant had reduced levels of carbohydrates in later anthers and was male sterile. The csa mutant had reduced accumulation of 14C-labeled sugars in anther sink tissue. CSA was isolated by map-based cloning and was shown to encode an R2R3 MYB transcription factor that was expressed preferentially in the anther tapetal cells and in the sugar-transporting vascular tissues. In addition, the expression of MST8 , encoding a monosaccharide transporter, was greatly reduced in csa anthers. Furthermore, CSA was found to be associated in vivo and in vitro with the promoter of MST8. Our findings suggest that CSA is a key transcriptional regulator for sugar partitioning in rice during male reproductive development. This study also establishes a molecular model system for further elucidation of the genetic control of carbon partitioning in plants. [ABSTRACT FROM AUTHOR]

The requirement of CUL1 for Arabidopsis embryogenesis suggests that Skp1--CUL1--F-box protein (SCF) complexes play important roles during embryo development. Among the 21 Arabidopsis Skp1-like genes (ASKs), it is unknown which ASK gene(s) is essential for embryo development. In this study, we demonstrate a vital role for ASK1 and ASK2 in Arabidopsis embryogenesis and postembryonic development through analysis of the ask1 ask2 double mutant. Our detailed analysis indicates that the double mutations in both ASK1 and ASK2 affect cell division and cell expansion/elongation and cause a developmental delay during embryogenesis and lethality in seedling growth. The expression patterns of ASK1 and ASK2 were examined further and found to be consistent with their roles in embryogenesis and seedling development. We propose that mutations in ASK1 and ASK2 abolish all of the ASK1- and ASK2-based SCF and non-SCF complexes, resulting in alteration of gene expression and leading to defects in growth and development. [ABSTRACT FROM AUTHOR]

The modification of tRNA is important for accurate, efficient protein translation. A number of tRNA-modifying enzymes were found to influence various developmental processes in distinct organisms. However, few genetic or molecular studies have focused on genes encoding tRNA-modifying enzymes in green plant organelles. Here, we discovered that PDD OL , a natural variation allele of PLEIOTROPIC DEVELOPMENTAL DEFECTS ( PDD ), leads to pleiotropic developmental defects in a near-isogenic line (NIL) generated by introgressing the wild rice Oryza longistaminata into the rice ( Oryza sativa ) cv 187R. Map-based cloning revealed that PDD encodes an evolutionarily conserved tRNA-modifying GTPase belonging to the tRNA modification E family. The function of PDD was further confirmed by genetic complementation experiments and mutant analysis. PDD mRNA is primarily expressed in leaves, and PDD is localized to chloroplasts. Biochemical analyses indicated that PDD 187R forms homodimers and has strong GTPase activity, whereas PDD OL fails to form homodimers and has weak GTPase activity. Liquid chromatography-coupled tandem quadrupole mass spectrometry revealed that PDD is associated with the 5-methylaminomethyl-2-thiouridine modification of chloroplast tRNA. Furthermore, compared to 187R, NIL- PDD OL has severely reduced levels of proteins involved in photosynthesis and ribosome biogenesis but increased levels of plastid-encoded RNA polymerase subunits. Finally, we demonstrate that the defect due to PDD OL alters chloroplast gene expression, thereby affecting communication between the chloroplast and the nucleus., (© 2020 American Society of Plant Biologists. All rights reserved.)

The Arabidopsis thaliana MALE STERILITY1 (MS1) gene encodes a nuclear protein with Leu zipper-like and PHD-finger motifs and is important for postmeiotic pollen development. Here, we examined MS1 function using both cell biological and molecular biological approaches. We introduced a fusion construct of MS1 and a transcriptional repression domain (MS1-SRDX) into wild-type Arabidopsis, and the transgenic plants showed a semisterile phenotype similar to that of ms1. Since the repression domain can convert various kinds of transcriptional activators to dominant repressors, this suggested that MS1 functioned as a transcriptional activator. The Leu zipper-like region and the PHD motif were required for the MS1 function. Phenotypic analysis of the ms1 mutant and the MS1-SRDX transgenic Arabidopsis indicated that MS1 was involved in formation of pollen exine and pollen cytosolic components as well as tapetum development. Next, we searched for MS1 downstream genes by analyzing publicly available microarray data and identified 95 genes affected by MS1. Using a transgenic ms1 plant showing dexamethasone-inducible recovery of fertility, we further examined whether these genes were immediately downstream of MS1. From these results, we discuss a role of MS1 in pollen and tapetum development and the conservation of MS1 function in flowering plants.

In flowering plants, tapetum degeneration is proposed to be triggered by a programmed cell death (PCD) process during late stages of pollen development; the PCD is thought to provide cellular contents supporting pollen wall formation and to allow the subsequent pollen release. However, the molecular basis regulating tapetum PCD in plants remains poorly understood. We report the isolation and characterization of a rice (Oryza sativa) male sterile mutant tapetum degeneration retardation (tdr), which exhibits degeneration retardation of the tapetum and middle layer as well as collapse of microspores. The TDR gene is preferentially expressed in the tapetum and encodes a putative basic helix-loop-helix protein, which is likely localized to the nucleus. More importantly, two genes, Os CP1 and Os c6, encoding a Cys protease and a protease inhibitor, respectively, were shown to be the likely direct targets of TDR through chromatin immunoprecipitation analyses and the electrophoretic mobility shift assay. These results indicate that TDR is a key component of the molecular network regulating rice tapetum development and degeneration.

Anther development involves the formation of several adjacent cell types required for normal male fertility. Only a few genes are known to be involved in early anther development, particularly in the establishment of these different cell layers. Arabidopsis thaliana BAM1 (for BARELY ANY MERISTEM) and BAM2 encode CLAVATA1-related Leu-rich repeat receptor-like kinases that appear to have redundant or overlapping functions. We characterized anther development in the bam1 bam2 flowers and found that bam1 bam2 anthers appear to be abnormal at a very early stage and lack the endothecium, middle, and tapetum layers. Analyses using molecular markers and cytological techniques of bam1 bam2 anthers revealed that cells interior to the epidermis acquire some characteristics of pollen mother cells (PMCs), suggesting defects in cell fate specification. The pollen mother-like cells degenerate before the completion of meiosis, suggesting that these cells are defective. In addition, the BAM1 and BAM2 expression pattern supports both an early role in promoting somatic cell fates and a subsequent function in the PMCs. Therefore, analysis of BAM1 and BAM2 revealed a cell-cell communication process important for early anther development, including aspects of cell division and differentiation. This finding may have implications for the evolution of multiple signaling pathways in specifying cell types for microsporogenesis.

In plants, reproductive development requires normal meiosis, which involved several highly coordinated events. Such meiotic events are regulated in a number of ways in yeast and animal systems, including transcriptional and checkpoint control mechanisms. Although a number of mutations that affect different aspects of meiosis have been characterized in plants, very little is known about the regulation of plant meiosis at the molecular level. In particular, no meiosis-specific transcriptional regulators have been identified in plants, and checkpoint control has not been observed during plant meiosis. We report here the isolation and characterization of a new Arabidopsis male-sterile mutant that exhibits meiotic defects. Meiocytes from mutant plants appeared normal up to diakinesis, when they exhibited signs of apoptosis, including defects in chromosome behavior, cytoplasmic shrinkage, and chromatin fragmentation, followed by cell death before cytokinesis. Therefore, the mutant was named male meiocyte death1 (mmd1). The MMD1 gene was cloned using Dissociation transposon tagging and encodes a plant homeo domain domain-containing protein. MMD1 is expressed preferentially during male meiosis. Our results suggest that MMD1 may be involved in the regulation of gene expression during meiosis and that the mmd1 mutation triggers cell death in male meiocytes.

Xie and colleagues previously isolated the Arabidopsis COI1 gene that is required for response to jasmonates (JAs), which regulate root growth, pollen fertility, wound healing, and defense against insects and pathogens. In this study, we demonstrate that COI1 associates physically with AtCUL1, AtRbx1, and either of the Arabidopsis Skp1-like proteins ASK1 or ASK2 to assemble ubiquitin-ligase complexes, which we have designated SCF(COI1). COI1(E22A), a single amino acid substitution in the F-box motif of COI1, abolishes the formation of the SCF(COI1) complexes and results in loss of the JA response. AtRbx1 double-stranded RNA-mediated genetic interference reduces AtRbx1 expression and affects JA-inducible gene expression. Furthermore, we show that the AtCUL1 component of SCF(COI1) complexes is modified in planta, where mutations in AXR1 decrease the abundance of the modified AtCUL1 of SCF(COI1) and lead to a reduction in JA response. Finally, we demonstrate that the axr1 and coi1 mutations display a synergistic genetic interaction in the double mutant. These results suggest that the COI1-mediated JA response is dependent on the SCF(COI1) complexes in Arabidopsis and that the AXR1-dependent modification of the AtCUL1 subunit of SCF(COI1) complexes is important for JA signaling.