Relatively little is known about how metals such as iron are effluxed from cells, a necessary step for transport from the root to the shoot. Ferroportin (FPN) is the sole iron efflux transporter identified to date in animals, and there are two closely related orthologs in Arabidopsis thaliana, IRON REGULATED1 (IREG1/FPN1) and IREG2/FPN2. FPN1 localizes to the plasma membrane and is expressed in the stele, suggesting a role in vascular loading; FPN2 localizes to the vacuole and is expressed in the two outermost layers of the root in response to iron deficiency, suggesting a role in buffering metal influx. Consistent with these roles, fpn2 has a diminished iron deficiency response, whereas fpn1 fpn2 has an elevated iron deficiency response. Ferroportins also play a role in cobalt homeostasis; a survey of Arabidopsis accessions for ionomic phenotypes showed that truncation of FPN2 results in elevated shoot cobalt levels and leads to increased sensitivity to the metal. Conversely, loss of FPN1 abolishes shoot cobalt accumulation, even in the cobalt accumulating mutant frd3. Consequently, in the fpn1 fpn2 double mutant, cobalt cannot move to the shoot via FPN1 and is not sequestered in the root vacuoles via FPN2; instead, cobalt likely accumulates in the root cytoplasm causing fpn1 fpn2 to be even more sensitive to cobalt than fpn2 mutants.

Molybdenum (Mo) is an essential micronutrient for plants, serving as a cofactor for enzymes involved in nitrate assimilation, sulfite detoxification, abscisic acid biosynthesis, and purine degradation. Here we show that natural variation in shoot Mo content across 92 Arabidopsis thaliana accessions is controlled by variation in a mitochondrially localized transporter (Molybdenum Transporter 1 - MOT1) that belongs to the sulfate transporter superfamily. A deletion in the MOT1 promoter is strongly associated with low shoot Mo, occurring in seven of the accessions with the lowest shoot content of Mo. Consistent with the low Mo phenotype, MOT1 expression in low Mo accessions is reduced. Reciprocal grafting experiments demonstrate that the roots of Ler-0 are responsible for the low Mo accumulation in shoot, and GUS localization demonstrates that MOT1 is expressed strongly in the roots. MOT1 contains an N-terminal mitochondrial targeting sequence and expression of MOT1 tagged with GFP in protoplasts and transgenic plants, establishing the mitochondrial localization of this protein. Furthermore, expression of MOT1 specifically enhances Mo accumulation in yeast by 5-fold, consistent with MOT1 functioning as a molybdate transporter. This work provides the first molecular insight into the processes that regulate Mo accumulation in plants and shows that novel loci can be detected by association mapping.

A requirement for vernalization, the process by which prolonged cold exposure provides competence to flower, is an important adaptation to temperate climates that ensures flowering does not occur before the onset of winter. In temperate grasses, vernalization results in the up-regulation of VERNALIZATION1 (VRN1) to establish competence to flower; however, little is known about the mechanism underlying repression of VRN1 in the fall season, which is necessary to establish a vernalization requirement. Here, we report that a plant-specific gene containing a bromo-adjacent homology and transcriptional elongation factor S-II domain, which we named REPRESSOR OF VERNALIZATION1 (RVR1), represses VRN1 before vernalization in Brachypodium distachyon. That RVR1 is upstream of VRN1 is supported by the observations that VRN1 is precociously elevated in an rvr1 mutant, resulting in rapid flowering without cold exposure, and the rapid-flowering rvr1 phenotype is dependent on VRN1. The precocious VRN1 expression in rvr1 is associated with reduced levels of the repressive chromatin modification H3K27me3 at VRN1, which is similar to the reduced VRN1 H3K27me3 in vernalized plants. Furthermore, the transcriptome of vernalized wild-type plants overlaps with that of nonvernalized rvr1 plants, indicating loss of rvr1 is similar to the vernalized state at a molecular level. However, loss of rvr1 results in more differentially expressed genes than does vernalization, indicating that RVR1 may be involved in processes other than vernalization despite a lack of any obvious pleiotropy in the rvr1 mutant. This study provides an example of a role for this class of plant-specific genes.A common adaptation for optimal timing of flowering in temperate climates is the evolution of a vernalization requirement. Vernalization is the process by which plants become competent to flower after prolonged exposure to the cold temperatures of winter (1). Cold exposure alone, however, is typically not sufficient to induce flowering; rather, it often must be followed by an inductive photoperiod, such as the increasing day lengths of spring. The combination of a requirement for a prolonged period of cold followed by a requirement for increasing day lengths has the adaptive value of preventing flowering in the fall season before the onset of winter, which would likely compromise reproductive success (2).To date, information about the vernalization response in grasses has largely been derived from studies of existing allelic variation in wheat and barley (for reviews, see refs. 3-5). Wheat and barley varieties can be classified as either spring or winter types. Spring varieties do not require vernalization to flower rapidly in inductive long days (LD), whereas vernalization enables rapid flowering of winter varieties (5, 6). A current molecular model of vernalization in temperate grasses consists of a regulatory loop including the genes VERNALIZATION1 (VRN1), VERNALIZATION2 (VRN2), and VERNALIZATION3

Prolonged exposure to winter cold enables flowering in many plant species through a process called vernalization. In Arabidopsis, vernalization results from the epigenetic silencing of the floral repressor FLOWERING LOCUS C (FLC) via a Polycomb Repressive Complex 2 (PRC2)-mediated increase in the density of the epigenetic silencing mark H3K27me3 at FLC chromatin. During cold exposure, a gene encoding a unique, cold-specific PRC2 component, VERNALIZATION INSENSITIVE 3 (VIN3), which is necessary for PRC2-mediated silencing of FLC, is induced. Here we show that SET DOMAIN GROUP 7 (SDG7) is required for proper timing of VIN3 induction and of the vernalization process. Loss of SDG7 results in a vernalization-hypersensitive phenotype, as well as more rapid cold-mediated up-regulation of VIN3. In the absence of cold, loss of SDG7 results in elevated levels of long noncoding RNAs, which are thought to participate in epigenetic repression of FLC. Furthermore, loss of SDG7 results in increased H3K27me3 deposition on FLC chromatin in the absence of cold exposure and enhanced H3K27me3 spreading during cold treatment. Thus, SDG7 is a negative regulator of vernalization, and loss of SDG7 creates a partially vernalized state without cold exposure.vernalization | flowering time | SET DOMAIN GROUP 7 S easonal timing of flowering is critical for reproductive success in many plant species. The timing of flowering is often strongly influenced by seasonal variables, such as day length and temperature. Vernalization, the process by which exposure to the prolonged cold of winter enables flowering in the spring, is an example of a temperature effect on flowering (1).Arabidopsis thaliana contains both vernalization-requiring accessions (winter annual) and accessions that do not require vernalization (summer annual). The vernalization requirement is conferred by FRIGIDA (FRI)-mediated up-regulation of the potent flowering repressor FLOWERING LOCUS C (FLC) (2, 3). The presence of active alleles of FRI and FLC ensures that flowering is repressed in the fall season. Summer annual accessions do not require vernalization to flower, because these accessions typically contain a mutant allele of FRI that is not able to up-regulate FLC (4).On perception of a sufficiently long period of cold, vernalization results in the chromatin-level suppression of FLC, which in turn provides competence to flower (3, 5). Thus, in Arabidopsis, vernalization is an environmentally induced epigenetic switch (6).Vernalization-mediated FLC silencing is associated with an increase of two chromatin modifications involving trimethylation of histone 3 at lysine 9 (H3K9me3) and at lysine 27 (H3K27me3) (7,8). The increase in H3K27me3 results from the action of the Polycomb Repressive Complex 2 (PRC2) on FLC chromatin (9). Many of the PRC2 components are conserved among plants, animals, and fungi (10, 11); however, a plant-specific PRC2-associated protein, VERNALIZATION INSENSITIVE 3 (VIN3), is required for vernalization and is uniquely expressed during cold (6). Thus, the...

Studies of natural genetic variation for the vernalization requirement in Arabidopsis have revealed two genes, FRIGIDA (FRI) and FLOWERING LOCUS C (FLC), that are determinants of the vernalization-requiring, winter-annual habit. In this study, we show that FLC EXPRESSOR LIKE 4 (FLL4) is essential for up-regulation of FLC in winter-annual Arabidopsis accessions and establishment of a vernalization requirement. FLL4 is part of the FLC EXPRESSOR (FLX) gene family and both are non-redundantly involved in flowering-time control. Epistasis analysis among FRI, FLL4, FLX and autonomous-pathway genes reveals that FRI fve exhibits an extreme delay of flowering compared to fri fve, but mutants in other autonomous-pathway genes do not, indicating that FVE acts most antagonistically to FRI. FLL4 may represent a new member of a FRI-containing complex that activates FLC.