bZIP17 regulates the expression of genes related to seed storage and germination, reducing seed susceptibility to osmotic stress.

Low temperatures, salinity, and drought cause significant crop losses. These conditions involve osmotic stress, triggering transcriptional remodeling, and consequently, the restitution of cellular homeostasis and growth recovery. Protein transcription factors regulate target genes, thereby mediating plant responses to stress. bZIP17 is a transcription factor involved in cellular responses to salinity and the unfolded protein response. Because salinity can also produce osmotic stress, the role of bZIP17 in response to osmotic stress was assessed. Mannitol treatments induced the transcript accumulation and protein processing of bZIP17. Transcriptomic analyses showed that several genes associated with seed storage and germination showed lower expression in bzip17 mutants than in wild-type plants. Interestingly, bZIP17 transcript was more abundant in seeds, and germination analyses revealed that wild-type plants germinated later than bzip17 mutants in the presence of mannitol, but no effects were observed when the seeds were exposed to ABA. Finally, the transcript levels of bZIP17 target genes that control seed storage and germination were assessed in seeds exposed to mannitol treatments, which showed lower expression levels in bzip17 mutants compared to the wild-type seeds. These results suggest that bZIP17 plays a role in osmotic stress, acting as a negative regulator of germination through the regulation of genes involved in seed storage and germination.

Regulation of Carotenoid Biosynthesis by Shade Relies on Specific Subsets of Antagonistic Transcription Factors and Cofactors.

Carotenoids are photosynthetic pigments essential for the protection against excess light. During deetiolation, their production is regulated by a dynamic repression-activation module formed by PHYTOCHROME-INTERACTING FACTOR1 (PIF1) and LONG HYPOCOTYL5 (HY5). These transcription factors directly and oppositely control the expression of the gene encoding PHYTOENE SYNTHASE (PSY), the first and main rate-determining enzyme of the carotenoid pathway. Antagonistic modules also regulate the responses of deetiolated plants to vegetation proximity and shade (i.e. to the perception of far-red light-enriched light filtered through or reflected from neighboring plants). These responses, aimed to adapt to eventual shading from plant competitors, include a reduced accumulation of carotenoids. Here, we show that PIF1 and related photolabile PIFs (but not photostable PIF7) promote the shade-triggered decrease in carotenoid accumulation. While HY5 does not appear to be required for this process, other known PIF antagonists were found to modulate the expression of the Arabidopsis (Arabidopsis thaliana) PSY gene and the biosynthesis of carotenoids early after exposure to shade. In particular, PHYTOCHROME-RAPIDLY REGULATED1, a transcriptional cofactor that prevents the binding of true transcription factors to their target promoters, was found to interact with PIF1 and hence directly induce PSY expression. By contrast, a change in the levels of the transcriptional cofactor LONG HYPOCOTYL IN FAR RED1, which also binds to PIF1 and other PIFs to regulate shade-related elongation responses, did not impact PSY expression or carotenoid accumulation. Our data suggest that the fine-regulation of carotenoid biosynthesis in response to shade relies on specific modules of antagonistic transcriptional factors and cofactors.

bZIP17 and bZIP60 Regulate the Expression of BiP3 and Other Salt Stress Responsive Genes in an UPR-Independent Manner in Arabidopsis thaliana.

Plants can be severely affected by salt stress. Since these are sessile organisms, they have developed different cellular responses to cope with this problem. Recently, it has been described that bZIP17 and bZIP60, two ER-located transcription factors, are involved in the cellular response to salt stress. On the other hand, bZIP60 is also involved in the unfolded protein response (UPR), a signaling pathway that up-regulates the expression of ER-chaperones. Coincidentally, salt stress produces the up-regulation of BiP, one of the main chaperones located in this organelle. Then, it has been proposed that UPR is associated to salt stress. Here, by using insertional mutant plants on bZIP17 and bZIP60, we show that bZIP17 regulate the accumulation of the transcript for the chaperone BiP3 under salt stress conditions, but does not lead to the accumulation of UPR-responding genes such as the chaperones Calnexin, Calreticulin, and PDIL under salt treatments. In contrast, DTT, a known inducer of UPR, leads to the up-regulation of all these chaperones. On the other hand, we found that bZIP60 regulates the expression of some bZIP17 target genes under conditions were splicing of bZIP60 does not occur, suggesting that the spliced and unspliced forms of bZIP60 play different roles in the physiological response of the plant. Our results indicate that the ER-located transcription factors bZIP17 and bZIP60 play a role in salt stress but this response goes through a signaling pathway that is different to that triggered by the unfolded protein response.

Plant proximity perception dynamically modulates hormone levels and sensitivity in Arabidopsis.

The shade avoidance syndrome (SAS) refers to a set of plant responses initiated after perception by the phytochromes of light enriched in far-red colour reflected from or filtered by neighbouring plants. These varied responses are aimed at anticipating eventual shading from potential competitor vegetation. In Arabidopsis thaliana, the most obvious SAS response at the seedling stage is the increase in hypocotyl elongation. Here, we describe how plant proximity perception rapidly and temporally alters the levels of not only auxins but also active brassinosteroids and gibberellins. At the same time, shade alters the seedling sensitivity to hormones. Plant proximity perception also involves dramatic changes in gene expression that rapidly result in a new balance between positive and negative factors in a network of interacting basic helix-loop-helix proteins, such as HFR1, PAR1, and BIM and BEE factors. Here, it was shown that several of these factors act as auxin- and BR-responsiveness modulators, which ultimately control the intensity or degree of hypocotyl elongation. It was deduced that, as a consequence of the plant proximity-dependent new, dynamic, and local balance between hormone synthesis and sensitivity (mechanistically resulting from a restructured network of SAS regulators), SAS responses are unleashed and hypocotyls elongate.

The bHLH proteins BEE and BIM positively modulate the shade avoidance syndrome in Arabidopsis seedlings.

The shade avoidance syndrome (SAS) refers to a set of plant responses initiated after perception by the phytochromes of light with a reduced red to far-red ratio, indicative of vegetation proximity or shade. These responses, including elongation growth, anticipate eventual shading from potential competitor vegetation by overgrowing neighboring plants or flowering to ensure production of viable seeds for the next generation. In Arabidopsis thaliana seedlings, the SAS includes dramatic changes in gene expression, such as induction of PHYTOCHROME RAPIDLY REGULATED 1 (PAR1), encoding an atypical basic helix-loop-helix (bHLH) protein that acts as a transcriptional co-factor to repress hypocotyl elongation. Indeed, PAR1 has been proposed to act fundamentally as a dominant negative antagonist of conventional bHLH transcription factors by forming heterodimers with them to prevent their binding to DNA or other transcription factors. Here we report the identification of PAR1-interacting factors, including the brassinosteroid signaling components BR-ENHANCED EXPRESSION (BEE) and BES1-INTERACTING MYC-LIKE (BIM), and characterize their role as networked positive regulators of SAS hypocotyl responses. We provide genetic evidence that these bHLH transcriptional regulators not only control plant growth and development under shade and non-shade conditions, but are also redundant in the control of plant viability. Our results suggest that SAS responses are initiated as a consequence of a new balance of transcriptional regulators within the pre-existing bHLH network triggered by plant proximity, eventually causing hypocotyls to elongate.

Identification and characterization of genes differentially expressed in cherimoya (Annona cherimola Mill) after exposure to chilling injury conditions.

Cherimoyas (Annona cherimola), like other subtropical/tropical fruits, are susceptible to damage from exposure to temperatures between 0 and 5 °C (chilling injury, CI), which may affect fruit quality. To increase our understanding of the molecular mechanisms involved in the CI response, a forward suppression subtractive hybridization (SSH) cDNA library was constructed. In this work, we obtained 75 genes that could potentially be involved in the CI response. The CI induced activation of genes that are involved in a range of metabolic pathways, such as primary metabolism, transport, and endomembrane traffic, among others. We also characterized the expression of 12 selected genes in different A. cherimola tissues by polymerase chain reaction (PCR), and we confirmed the differential expression of a subset in CI fruits by real-time quantitative PCR (qPCR). The expression of six A. cherimola genes: annexin (AcAnex), UDP-glucose pyrophosphorylase (AcUGP), syntaxin of plants 71 (AcSyp71), 1-aminocyclopropane-1-carboxylic-acid synthase (AcACS), ubiquitin carrier-like protein (AcUCP), and enolase (AcEnol), was up-regulated after cold storage for 12 days at 0 °C. These results imply that selected genes could be related to the development of internal browning observed in cherimoyas after exposure to CI conditions. The information generated in this study provides new clues that may aid in understanding the cherimoya ripening process.

The shade avoidance syndrome in Arabidopsis: a fundamental role for atypical basic helix-loop-helix proteins as transcriptional cofactors.

The shade avoidance syndrome (SAS) refers to a set of plant responses aimed at anticipating eventual shading by potential competitors. The SAS is initiated after perception of nearby vegetation as a reduction in the red to far-red ratio (R:FR) of the incoming light. Low R:FR light is perceived by the phytochromes, triggering dramatic changes in gene expression that, in seedlings, eventually result in an increased hypocotyl elongation to overgrow competitors. This response is inhibited by genes such as PHYTOCHROME RAPIDLY REGULATED 1 (PAR1), PAR2 and LONG HYPOCOTYL IN FR 1 (HFR1), which are transcriptionally induced by low R:FR. Although PAR1/PAR2 and HFR1 proteins belong to different groups of basic helix-loop-helix (bHLH) transcriptional regulators, they all lack a typical basic domain required for binding to E-box and G-box motifs in the promoter of target genes. By overexpressing derivatives of PAR1 and HFR1 we show that these proteins are actually transcriptional cofactors that do not need to bind DNA to directly regulate transcription. We conclude that protein-protein interactions involving the HLH domain of PAR1 and HFR1 are a fundamental aspect of the mechanism by which these proteins regulate gene expression, most likely through interaction with true transcription factors that do bind to the target genes and eventually unleash the observed SAS responses.