CENH3 variation mediates parental centromere imbalance.

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.

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.

Vacuolar storage of iron (Fe) is important for Fe homeostasis in plants. When sufficient, the excess Fe could be stored in vacuoles for remobilization in case of Fe deficiency. Although the mechanism of Fe remobilization from vacuoles is critical for crop development under low Fe stress, the transporters that mediate vacuolar Fe translocation into the cytosol in rice remains unknown. Here, we showed that under higher Fe 2+ concentrations, the Δccc1 yeast mutant transformed with rice natural resistance-associated macrophage protein 2 (OsNRAMP2) became more sensitive to Fe toxicity. In rice protoplasts and transgenic plants expressing Pro35S: OsNRAMP2-GFP, OsNRAMP2 was localized to tonoplast. Vacuolar Fe contents in osnramp2 knockdown lines were higher than in the wild-type, while the growth of osnramp2 knockdown plants was significantly influenced by Fe deficiency. Furthermore, the germination of osnramp2 knockdown plants was arrested. Inversely, the vacuolar Fe contents of Pro35S: OsNRAMP2-GFP lines were significantly lower than in the wild-type, and overexpression of OsNRAMP2 increased shoot biomass under Fe deficiency. Taken together, we propose that OsNRAMP2 transports Fe from the vacuole to the cytosol and plays a pivotal role in seed germination.

Uniparental genome elimination is a dramatic case of centromeric failure, resulting in the postzygotic loss of a parental chromosome set. Genome partitioning during cell division relies on centromere attachment to spindle fibers through kinetochores. Centromeres are epigenetically specified by CENP-A (CENH3), a conserved centromeric specific histone H3 variant. In Arabidopsis, CENH3 modification results in haploid inducers, whose genome is eliminated frequently when crossed to the wild type. To investigate the underlying mechanism, we dissected the timing and molecular features of genome elimination. In zygotes and early embryos from genome elimination crosses, CENH3 occupied only the centromeres contributed by the wild-type parent. Haploid inducer chromosomes had defective kinetochores and missegregated, often forming micronuclei. This uniparental loss of centromere identity is initiated by the removal of altered CENH3 at fertilization, while wild-type CENH3 persists and maintains strong centromeric identity. Weak centromeres were capable of rebuilding functional kinetochores, but often failed when in competition with normal ones. We induced a similar weak state by mitotic dilution of wild-type CENH3. Furthermore, weakness was suppressed by crosses of haploid inducers to other variants of haploid inducers, and enhanced by mutations in ​VIM1,​ a ubiquitin ligase known to modify CENH3 and centromeric DNA methylation.. The differential stability of altered CENH3 during reproduction has important genetic and evolutionary implications.