OsbZIP62/OsFD7, a functional ortholog of Flowering Locus D (FD), regulates floral transition and panicle development in rice

We have characterized a rice bZIP protein coding gene OsbZIP62/OsFD7 that expresses preferentially in SAM and during early panicle developmental stages in comparison to other OsFDs characterised till date. Surprisingly, unlike OsFD1, OsFD7 interacts directly and more efficiently with OsFTLs; the interaction is strongest with OsFTL1 followed by Hd3a and RFT1, as confirmed by FLIM-FRET analysis. Also, OsFD7 is phosphorylated at its C-terminal end by OsCDPK41 and OsCDPK49 in vitro and this phosphorylated moiety is recognized by OsGF14 proteins. OsFD7 RNAi transgenics were late flowering; the transcript levels of some floral meristem identity genes (e.g. OsMADS14, OsMADS15 and OsMADS18) were also down-regulated. It was quite interesting to note that these RNAi lines exhibited dense panicle morphology with increase in the number of primary and secondary branches resulting in longer panicles and more seeds probably due to downregulation of Sepallata (SEP) family genes. In comparison to other FD-like proteins characterized thus far from rice, it appears that OsFD7 may have undergone diversification during evolution resulting in the acquisition of newer functions and thus playing dual role in floral transition and panicle development in rice.HighlightOsbZIP62/OsFD7 interacts with major flowering regulators participating in the processes of floral transition as well as panicle and floral organ development.

Roles of C/EBP class bZip proteins in the growth and cell competition of Rp (‘Minute’) mutants in Drosophila

Reduced copy number of ribosomal protein (Rp) genes adversely affects both flies and mammals. Xrp1 encodes a reportedly Drosophila-specific AT-hook, bZIP protein responsible for many of the effects including the elimination of Rp mutant cells by competition with wild type cells. Irbp18, an evolutionarily conserved bZIP gene, heterodimerizes with Xrp1 and with another bZip protein, dATF4. We show that Irbp18 is required for the effects of Xrp1, whereas dATF4 does not share the same phenotype, indicating that Xrp1/Irbp18 is the complex active in Rp mutant cells, independently of other complexes that share Irbp18. Xrp1 and Irbp18 transcripts and proteins are upregulated in Rp mutant cells by auto-regulatory expression that depends on the Xrp1 DNA binding domains and is necessary for cell competition. We show that Xrp1 is conserved beyond Drosophila, although under positive selection for rapid evolution, and that at least one human bZip protein can similarly affect Drosophila development.

Protein phosphatase 2A regulates the nuclear accumulation of the Arabidopsis bZIP protein VIP1 under hypo-osmotic stress

Hypo-osmotic stress induces nuclear accumulation of the transcription factor VIP1. We show that protein phosphatase 2A (PP2A) regulates this process, and that VIP1 interacts with PP2A B''-family subunits.

Functional divergence between soybean FLOWERING LOCUS T orthologues FT2a and FT5a in post-flowering stem growth

Genes in the FLOWERING LOCUS T (FT) family integrate external and internal signals to control various aspects of plant development. In soybean (Glycine max), FT2a and FT5a play a major role in floral induction, but their roles in post-flowering reproductive development remain undetermined. Ectopic overexpression analyses revealed that FT2a and FT5a similarly induced flowering, but FT5a was markedly more effective than FT2a for the post-flowering termination of stem growth. The down-regulation of Dt1, a soybean orthologue of Arabidopsis TERMINAL FLOWER1, in shoot apices in early growing stages of FT5a-overexpressing plants was concomitant with highly up-regulated expression of APETALA1 orthologues. The Dt2 gene, a repressor of Dt1, was up-regulated similarly by the overexpression of FT2a and FT5a, suggesting that it was not involved in the control of stem termination by FT5a. In addition to the previously reported interaction with FDL19, a homologue of the Arabidopsis bZIP protein FD, both FT2a and FT5a interacted with FDL12, but only FT5a interacted with FDL06. Our results suggest that FT2a and FT5a have different functions in the control of post-flowering stem growth. A specific interaction of FT5a with FDL06 may play a key role in determining post-flowering stem growth in soybean.

Mitochondrial UPR repression duringPseudomonas aeruginosainfection requires the bZIP protein ZIP-3

Mitochondria generate most cellular energy and are targeted by multiple pathogens during infection. In turn, metazoans employ surveillance mechanisms such as the mitochondrial unfolded protein response (UPRmt) to detect and respond to mitochondrial dysfunction as an indicator of infection. The UPRmtis an adaptive transcriptional program regulated by the transcription factor ATFS-1, which induces genes that promote mitochondrial recovery and innate immunity. The bacterial pathogenPseudomonas aeruginosaproduces toxins that disrupt oxidative phosphorylation (OXPHOS), resulting in UPRmtactivation. Here, we demonstrate thatPseudomonas aeruginosaexploits an intrinsic negative regulatory mechanism mediated by theCaenorhabditis elegansbZIP protein ZIP-3 to repress UPRmtactivation. Strikingly, worms lackingzip-3were impervious toPseudomonas aeruginosa-mediated UPRmtrepression and resistant to infection. Pathogen-secreted phenazines perturbed mitochondrial function and were the primary cause of UPRmtactivation, consistent with these molecules being electron shuttles and virulence determinants. Surprisingly,Pseudomonas aeruginosaunable to produce phenazines and thus elicit UPRmtactivation were hypertoxic inzip-3–deletion worms. These data emphasize the significance of virulence-mediated UPRmtrepression and the potency of the UPRmtas an antibacterial response.