Regulation of Arabidopsis thaliana Ku genes at different developmental stages under heat stress

Multiomics analyses reveal the roles of the ASR1 transcription factor in tomato fruits

The transcription factor ASR1 (ABA, STRESS, RIPENING 1) plays multiple roles in plant responses to abiotic stresses as well as being involved in the regulation of central metabolism in several plant species. However, despite the high expression of ASR1 in tomato fruits, large scale analyses to uncover its function in fruits are still lacking. In order to study its function in the context of fruit ripening, we performed a multiomics analysis of ASR1-antisense transgenic tomato fruits at the transcriptome and metabolome levels. Our results indicate that ASR1 is involved in several pathways implicated in the fruit ripening process, including cell wall, amino acid, and carotenoid metabolism, as well as abiotic stress pathways. Moreover, we found that ASR1-antisense fruits are more susceptible to the infection by the necrotrophic fungus Botrytis cinerea. Given that ASR1 could be regulated by fruit ripening regulators such as FRUITFULL1/FRUITFULL2 (FUL1/FUL2), NON-RIPENING (NOR), and COLORLESS NON-RIPENING (CNR), we positioned it in the regulatory cascade of red ripe tomato fruits. These data extend the known range of functions of ASR1 as an important auxiliary regulator of tomato fruit ripening.

Leaf-Encapsulated Vaccines: Agroinfiltration and Transient Expression of the Antigen Staphylococcal Endotoxin B in Radish Leaves

Transgene introgression is a major concern associated with transgenic plant-based vaccines. Agroinfiltration can be used to selectively transform nonreproductive organs and avoid introgression. Here, we introduce a new vaccine modality in which Staphylococcal enterotoxin B (SEB) genes are agroinfiltrated into radishes (Raphanw sativus L.), resulting in transient expression and accumulation of SEB in planta. This approach can simultaneously express multiple antigens in a single leaf. Furthermore, the potential of high-throughput vaccine production was demonstrated by simultaneously agroinfiltrating multiple radish leaves using a multichannel pipette. The expression of SEB was detectable in two leaf cell types (epidermal and guard cells) in agroinfiltrated leaves. ICR mice intranasally immunized with homogenized leaves agroinfiltrated with SEB elicited detectable antibody to SEB and displayed protection against SEB-induced interferon-gamma (IFN-γ) production. The concept of encapsulating antigens in leaves rather than purifying them for immunization may facilitate rapid vaccine production during an epidemic disease.

Homologous Recombination Defective Arabidopsis Mutants Exhibit Enhanced Sensitivity to Abscisic Acid

Abscisic acid (ABA) acts as an important plant hormone in regulating various aspects of plant growth and developmental processes particularly under abiotic stress conditions. An increased ABA level in plant cells inhibits DNA replication and cell division, causing plant growth retardation. In this study, we have investigated the effects of ABA on the growth responses of some major loss-of-function mutants of DNA double-stand break (DSB) repair genes in Arabidopsis during seed germination and early stages of seedling growth for understanding the role of ABA in the induction of genome instability in plants. A comparative analysis of ABA sensitivity of wild-type Arabidopsis and the knockout mutant lines related to DSB sensors, including atatm, atatr, the non-homologous end joining (NHEJ) pathway genes, and mutants related to homologous recombination (HR) pathway genes showed relatively enhanced sensitivity of atatr and HR-related mutants to ABA treatment. The expression levels of HR-related genes were increased in wild-type Arabidopsis (Col-0) during seed germination and early stages of seedling growth. Immunoblotting experiments detected phosphorylation of histone H2AX in wild-type (Col-0) and DSB repair gene mutants after ABA treatment, indicating the activation of DNA damage response due to ABA treatment. Analyses of DSB repair kinetics using comet assay under neutral condition have revealed comparatively slower DSB repair activity in HR mutants. Overall, our results have provided comprehensive information on the possible effect of ABA on DNA repair machinery in plants and also indicated potential functional involvement of HR pathway in repairing ABA induced DNA damage in Arabidopsis.

Interactions between Arabidopsis DNA repair genes UVH6, DDB1A, and DDB2 during abiotic stress tolerance and floral development

Plants must protect themselves from a spectrum of abiotic stresses. For example, the sun is a source of heat, intense light, and DNA-damaging ultraviolet (UV) rays. Damaged DNA binding protein 1A (DDB1A), DDB2, and UV hypersensitive 6 (UVH6)/XPD are all involved in the repair of UV-damaged DNA - DDB1A and DDB2 in the initial damage recognition stage, while the UVH6/XPD helicase unwinds the damaged strand. We find that, as predicted, Arabidopsis ddb1a and ddb2 mutants do not affect uvh6/xpd UV tolerance. In addition, uvh6 is heat sensitive, and ddb1a and ddb2 weakly enhance this trait. The uvh6 ddb1a and uvh6 ddb2 double mutants also exhibit sensitivity to oxidative stress, suggesting a role for DDB1 complexes in heat and oxidative stress tolerance. Finally, we describe a new uvh6 phenotype, the low penetrance production of flowers with five petals and five sepals. ddb1a and ddb2 suppress this phenotype in uvh6 mutants. Interestingly, heat treatment also induces five-petalled flowers in the ddb1a and ddb2 single mutants. Thus UVH6, DDB1A, and DDB2 all contribute to UV tolerance, heat tolerance and floral patterning.

Plant hormone signaling and modulation of DNA repair under stressful conditions

The role played by phytohormone signaling in the modulation of DNA repair gene and the resulting effects on plant adaptation to genotoxic stress are poorly investigated. Information has been gathered using the Arabidopsis ABA (abscisic acid) overly sensitive mutant abo4-1, defective in the DNA polymerase ε function that is required for DNA repair and recombination. Similarly, phytohormone-mediated regulation of the Ku genes, encoding the Ku heterodimer protein involved in DNA repair, cell cycle control and telomere homeostasis has been demonstrated, highlighting a scenario in which hormones might affect genome stability by modulating the frequency of homologous recombination, favoring plant adaptation to genotoxic stress. Within this context, the characterisation of Arabidopsis AtKu mutants allowed disclosing novel connections between DNA repair and phytohormone networks. Another intriguing aspect deals with the emerging correlation between plant defense response and the mechanisms responsible for genome stability. There is increasing evidence that systemic acquired resistance (SAR) and homologous recombination share common elements represented by proteins involved in DNA repair and chromatin remodeling. This hypothesis is supported by the finding that volatile compounds, such as methyl salicylate (MeSA) and methyl jasmonate (MeJA), participating in the plant-to-plant communication can trigger genome instability in response to genotoxic stress agents. Phytohormone-mediated control of genome stability involves also chromatin remodeling, thus expanding the range of molecular targets. The present review describes the most significant advances in this specific research field, in the attempt to provide a better comprehension of how plant hormones modulate DNA repair proteins as a function of stress.

Chlorophyll b reductase plays an essential role in maturation and storability of Arabidopsis seeds

Although seeds are a sink organ, chlorophyll synthesis and degradation occurs during embryogenesis and in a manner similar to that observed in photosynthetic leaves. Some mutants retain chlorophyll after seed maturation, and they are disturbed in seed storability. To elucidate the effects of chlorophyll retention on the seed storability of Arabidopsis (Arabidopsis thaliana), we examined the non-yellow coloring1 (nyc1)/nyc1-like (nol) mutants that do not degrade chlorophyll properly. Approximately 10 times more chlorophyll was retained in the dry seeds of the nyc1/nol mutant than in the wild-type seeds. The germination rates rapidly decreased during storage, with most of the mutant seeds failing to germinate after storage for 23 months, whereas 75% of the wild-type seeds germinated after 42 months. These results indicate that chlorophyll retention in the seeds affects seed longevity. Electron microscopic studies indicated that many small oil bodies appeared in the embryonic cotyledons of the nyc1/nol mutant; this finding indicates that the retention of chlorophyll affects the development of organelles in embryonic cells. A sequence analysis of the NYC1 promoter identified a potential abscisic acid (ABA)-responsive element. An electrophoretic mobility shift assay confirmed the binding of an ABA-responsive transcriptional factor to the NYC1 promoter DNA fragment, thus suggesting that NYC1 expression is regulated by ABA. Furthermore, NYC1 expression was repressed in the ABA-insensitive mutants during embryogenesis. These data indicate that chlorophyll degradation is induced by ABA during seed maturation to produce storable seeds.

Genetic interactions between Arabidopsis DET1 and UVH6 during development and abiotic stress response

Plants must adapt to a variety of abiotic inputs, including visible light, ultraviolet (UV) light, and heat. In Arabidopsis thaliana, DE-ETIOLATED 1 (DET1) plays a role in visible light signaling, UV tolerance, and development. UV-HYPERSENSITIVE 6 (UVH6) mutants are UV and heat sensitive, as well as dwarf and pale, like det1. In this study, we examine the genetic interactions between these two genes. In dark-grown seedlings, uvh6 exhibits a weak de-etiolated phenotype but does not affect the stronger de-etiolated phenotype of det1. In the light, det1 is epistatic to uvh6 with regard to chlorophyll level, but their effect on all size parameters is additive and therefore independent. With regard to UV tolerance, det1 UV resistance is epistatic to uvh6 UV sensitivity. In heat stress experiments, det1 enhances heat-induced tissue damage in the uvh6 background but suppresses heat-induced growth inhibition. Thus, det1 acts epistatically to uvh6 with respect to de-etiolation, chlorophyll level, UV tolerance, and heat-induced growth inhibition, whereas det1 and uvh6 act additively to regulate plant size and heat-induced cell death. These data provide insight into interplay between light and heat signaling.

Ovate family protein 1 as a plant Ku70 interacting protein involving in DNA double-strand break repair

The Ku heterodimer, a DNA repair protein complex consisting of 70- and 80-kDa subunits, is involved in the non-homologous end-joining (NHEJ) pathway. Plants are thought to use the NHEJ pathway primarily for the repair of DNA double-strand breaks (DSBs). The Ku70/80 protein has been identified in many plants and been shown to possess several similar functions to its counter protein complex in mammals. In the present study, ovate family protein 1 (AtOFP1) was demonstrated to be a plant Ku-interacting protein by yeast two-hybrid screening and the GST pull-down assay. Truncation analysis revealed that the C-terminal domain of AtKu70 contains interacting sites for AtOFP1. The electrophoretic mobility shift assay (EMSA) indicated that AtOFP1 is also a DNA binding protein with its binding domain at the N-terminus. In 3-week-old seedlings, expression of the AtOFP1 gene increased after exposure to DNA-damaging agents (such as methyl methanesulfonate (MMS) and menadione) in a time dependent manner. Seedlings lacking the AtOFP1 protein were more sensitive to MMS and menadione as compared with wild-type. Furthermore, similar to AtKu70(-/-) and AtKu80(-/-), the AtOFP1(-/-) mutant showed relatively lower NHEJ activity in vivo. Taken together, these results suggest that AtOFP1 may play a role in DNA repair through the NHEJ pathway accompanying with the AtKu protein.