Arabidopsis VRN genes mediate vernalization, the process by which a long period of cold induces a mitotically stable state that leads to accelerated flowering during later development. VRN1 encodes a protein that binds DNA in vitro in a non–sequence-specific manner and functions in stable repression of the major target of the vernalization pathway, the floral repressor FLC . Overexpression of VRN1 reveals a vernalization-independent function for VRN1 , mediated predominantly through the floral pathway integrator FT , and demonstrates that VRN1 requires vernalization-specific factors to target FLC .

Allelic variation at the FRI (FRIGIDA) and FLC (FLOWERING LOCUS C) loci are major determinants of flowering time in Arabidopsis accessions. Dominant alleles of FRI confer a vernalization requirement causing plants to overwinter vegetatively. Many early flowering accessions carry loss-of-function fri alleles containing one of two deletions. However, some accessions categorized as early flowering types do not carry these deletion alleles. We have analyzed the molecular basis of earliness in five of these accessions: Cvi, Shakhdara, Wil-2, Kondara, and Kz-9. The Cvi FRI allele carries a number of nucleotide differences, one of which causes an in-frame stop codon in the first exon. The other four accessions contain nucleotide differences that only result in amino acid substitutions. Preliminary genetic analysis was consistent with Cvi carrying a nonfunctional FRI allele; Wil-2 carrying either a defective FRI or a dominant suppressor of FRI function; and Shakhdara, Kondara, and Kz-9 carrying a functional FRI allele with earliness being caused by allelic variation at other loci including FLC. Allelic variation at FLC was also investigated in a range of accessions. A novel nonautonomous Mutator-like transposon was found in the weak FLC allele in Landsberg erecta, positioned in the first intron, a region required for normal FLC regulation. This transposon was not present in FLC alleles of most other accessions including Shakhdara, Kondara, or Kz-9. Thus, variation in Arabidopsis flowering time has arisen through the generation of nonfunctional or weak FRI and FLC alleles.

▪ Abstract At a certain stage in their life cycle, plants switch from vegetative to reproductive development. This transition is regulated by multiple developmental and environmental cues. These ensure that the plant switches to flowering at a time when sufficient internal resources have been accumulated and the environmental conditions are favorable. The use of a molecular genetic approach in Arabidopsis has resulted in the identification and cloning of many of the genes involved in regulating floral transition. The current view on the molecular function of these genes, their division into different genetic pathways, and how the pathways interact in a complex regulatory network are summarized.

The regulation of ion and pH homeostasis of endomembrane organelles is critical for functional protein trafficking, sorting and modification in eukaryotic cells. pH homeostasis is maintained through the activity of vacuolar H⁺-ATPases (V-ATPases) pumping protons (H⁺) into the endomembrane lumen, and counter-action by cation/proton exchangers such as the NHX family of Na⁺(K⁺)/H⁺ exchangers. In plants, V-ATPase activity at the trans-Golgi network/early endosome (TGN/EE) is important for secretory and endocytic trafficking, however the role of the endosomal antiporters NHX5 and NHX6 in endomembrane trafficking is unclear. Here we show through genetic, pharmacological, and live-cell imaging approaches that double knockout of NHX5 and NHX6 results in the impairment of endosome motility, protein recycling at the TGN/EE, but not in the secretion of integral membrane proteins. Furthermore, we report that nhx5 nhx6 mutants are partially insensitive to osmotic swelling of TGN/EE induced by the monovalent cation ionophore monensin, and to late endosomal swelling by the phosphatidylinositol 3/4-kinase inhibitor wortmannin, demonstrating that NHX5 and NHX6 function to regulate the luminal cation composition of endosomes.