Gametophytic epigenetic regulators, MEDEA and DEMETER, synergistically suppress ectopic shoot formation in Arabidopsis.

KEY MESSAGE: The gametophytic epigenetic regulators, MEA and DME, extend their synergistic role to the sporophytic development by regulating the meristematic activity via restricting the gene expression in the shoot apex. The gametophyte-to-sporophyte transition facilitates the alternation of generations in a plant life cycle. The epigenetic regulators DEMETER (DME) and MEDEA (MEA) synergistically control central cell proliferation and differentiation, ensuring proper gametophyte-to-sporophyte transition in Arabidopsis. Mutant alleles of DME and MEA are female gametophyte lethal, eluding the recovery of recessive homozygotes to examine their role in the sporophyte. Here, we exploited the paternal transmission of these mutant alleles coupled with CENH3-haploid inducer to generate mea-1;dme-2 sporophytes. Strikingly, the simultaneous loss of function of MEA and DME leads to the emergence of ectopic shoot meristems at the apical pole of the plant body axis. DME and MEA are expressed in the developing shoot apex and regulate the expression of various shoot-promoting factors. Chromatin immunoprecipitation (ChIP), DNA methylation, and gene expression analysis revealed several shoot regulators as potential targets of MEA and DME. RNA interference-mediated transcriptional downregulation of shoot-promoting factors STM, CUC2, and PLT5 rescued the twin-plant phenotype to WT in 9-23% of mea-1-/-;dme-2-/- plants. Our findings reveal a previously unrecognized synergistic role of MEA and DME in restricting the meristematic activity at the shoot apex during sporophytic development.

Epigenetically mismatched parental centromeres trigger genome elimination in hybrids.

Wide crosses result in postzygotic elimination of one parental chromosome set, but the mechanisms that result in such differential fate are poorly understood. Here, we show that alterations of centromeric histone H3 (CENH3) lead to its selective removal from centromeres of mature Arabidopsis eggs and early zygotes, while wild-type CENH3 persists. In the hybrid zygotes and embryos, CENH3 and essential centromere proteins load preferentially on the CENH3-rich centromeres of the wild-type parent, while CENH3-depleted centromeres fail to reconstitute new CENH3-chromatin and the kinetochore and are frequently lost. Genome elimination is opposed by E3 ubiquitin ligase VIM1. We propose a model based on cooperative binding of CENH3 to chromatin to explain the differential CENH3 loading rates. Thus, parental CENH3 polymorphisms result in epigenetically distinct centromeres that instantiate a strong mating barrier and produce haploids.

Cantil - a new organ or a morphological oddity?

Cantil is reported as a new-found organ specific to the model plant Arabidopsis thaliana that is prominent only in short-day-grown wild-type accessions or long-day-grown genetic mutants with delayed vegetative to reproductive transition. Here, we show that cantils (previously known as nubbins) arise as one of the many phenotypic consequences of aneuploidy resulting from chromosome dosage imbalances in Arabidopsis polyaneuploids despite normal reproductive transition in long-day photoperiods. Without a demonstrated function or adaptive significance, we view cantils as a morphological oddity rather than a separate organ, and as a manifestation of physiological perturbations triggered by genetic and environmental factors. We also note a striking phenotypic resemblance between 'cantil' and 'gynophore', a floral morphological structure that is naturally present in the allopolyploid Arabidopsis suecica.

Understanding and exploiting uniparental genome elimination in plants: insights from Arabidopsis thaliana.

Uniparental genome elimination (UGE) refers to the preferential exclusion of one set of the parental chromosome complement during embryogenesis following successful fertilization, giving rise to uniparental haploid progeny. This artificially induced phenomenon was documented as one of the consequences of distant (wide) hybridization in plants. Ten decades since its discovery, attempts to unravel the molecular mechanism behind this process remained elusive due to a lack of genetic tools and genomic resources in the species exhibiting UGE. Hence, its successful adoption in agronomic crops for in planta (in vivo) haploid production remains implausible. Recently, Arabidopsis thaliana has emerged as a model system to unravel the molecular basis of UGE. It is now possible to simulate the genetic consequences of distant crosses in an A. thaliana intraspecific cross by a simple modification of centromeres, via the manipulation of the centromere-specific histone H3 variant gene, CENH3. Thus, the experimental advantages conferred by A. thaliana have been used to elucidate and exploit the benefits of UGE in crop breeding. In this review, we discuss developments and prospects of CENH3 gene-mediated UGE and other in planta haploid induction strategies to illustrate its potential in expediting plant breeding and genetics in A. thaliana and other model plants.

Natural epialleles of Arabidopsis SUPERMAN display superwoman phenotypes.

Epimutations are heritable changes in gene function due to loss or gain of DNA cytosine methylation or chromatin modifications without changes in the DNA sequence. Only a few natural epimutations displaying discernible phenotypes are documented in plants. Here, we report natural epimutations in the cadastral gene, SUPERMAN(SUP), showing striking phenotypes despite normal transcription, discovered in a natural tetraploid, and subsequently in eleven diploid Arabidopsis genetic accessions. This natural lois lane(lol) epialleles behave as recessive mendelian alleles displaying a spectrum of silent to strong superwoman phenotypes affecting only the carpel whorl, in contrast to semi-dominant superman or supersex features manifested by induced epialleles which affect both stamen and carpel whorls. Despite its unknown origin, natural lol epialleles are subjected to the same epigenetic regulation as induced clk epialleles. The existence of superwoman epialleles in diverse wild populations is interpreted in the light of the evolution of unisexuality in plants.

Genome Elimination by Tailswap CenH3: In Vivo Haploid Production in Arabidopsis thaliana.

Artificial production of haploids is one of the important sought-after goals of plant breeding and crop improvement programs. Conventionally, haploid plants are generated by in vitro (tissue) culture of haploid plant gametophytes, pollen (male), and embryo sac (female). Here, we describe a facile, nontissue culture-based in vivo method of haploid production through seeds in the model plant, Arabidopsis thaliana. This method involves simple crossing of any desired genotype of interest to a haploid-inducing strain (GFP-tailswap) to directly obtain haploid F1 seeds. The described protocol can be practiced by anyone with basic experience in growing A. thaliana plants and will be of interest to Arabidopsis research community.