Abscisic acid alleviates iron deficiency by promoting root iron reutilization and transport from root to shoot in Arabidopsis

Abscisic acid (ABA) has been demonstrated to be involved in iron (Fe) homeostasis, but the underlying mechanism is largely unknown. Here, we found that Fe deficiency induced ABA accumulation rapidly (within 6 h) in the roots of Arabidopsis. Exogenous ABA at 0.5 μM decreased the amount of root apoplastic Fe bound to pectin and hemicellulose, and increased the shoot Fe content significantly, thus alleviating Fe deficiency‐induced chlorosis. Exogenous ABA promoted the secretion of phenolics to release apoplastic Fe and up‐regulated the expression of AtNRAMP3 to enhance reutilization of Fe stored in the vacuoles, leading to a higher level of soluble Fe and lower ferric–chelate reductase (FCR) activity in roots. Treatment with ABA also led to increased Fe concentrations in the xylem sap, partially because of the up‐regulation of AtFRD3, AtYSL2 and AtNAS1, genes related to long‐distance transport of Fe. Exogenous ABA could not alleviate the chlorosis of abi5 mutant resulting from the significantly low expression of AtYSL2 and low transport of Fe from root to shoot. Taken together, our data support the conclusion that ABA is involved in the reutilization and transport of Fe from root to shoot under Fe deficiency conditions in Arabidopsis.     

Glutathione plays an essential role in nitric oxide‐mediated iron‐deficiency signaling and iron‐deficiency tolerance in Arabidopsis

Iron (Fe) deficiency is a common agricultural problem that affects both the productivity and nutritional quality of plants. Thus, identifying the key factors involved in the tolerance of Fe deficiency is important. In the present study, the zir1 mutant, which is glutathione deficient, was found to be more sensitive to Fe deficiency than the wild type, and grew poorly in alkaline soil. Other glutathione‐deficient mutants also showed various degrees of sensitivity to Fe‐limited conditions. Interestingly, we found that the glutathione level was increased under Fe deficiency in the wild type. By contrast, blocking glutathione biosynthesis led to increased physiological sensitivity to Fe deficiency. On the other hand, overexpressing glutathione enhanced the tolerance to Fe deficiency. Under Fe‐limited conditions, glutathione‐deficient mutants, zir1, pad2 and cad2 accumulated lower levels of Fe than the wild type. The key genes involved in Fe uptake, including IRT1, FRO2 and FIT, are expressed at low levels in zir1; however, a split‐root experiment suggested that the systemic signals that govern the expression of Fe uptake‐related genes are still active in zir1. Furthermore, we found that zir1 had a lower accumulation of nitric oxide (NO) and NO reservoir S‐nitrosoglutathione (GSNO). Although NO is a signaling molecule involved in the induction of Fe uptake‐related genes during Fe deficiency, the NO‐mediated induction of Fe‐uptake genes is dependent on glutathione supply in the zir1 mutant. These results provide direct evidence that glutathione plays an essential role in Fe‐deficiency tolerance and NO‐mediated Fe‐deficiency signaling in Arabidopsis.     

Analysis of IIId, IIIe and IVa group basic-helix-loop-helix proteins expressed in Arabidopsis root epidermis

Differentiation of Arabidopsis epidermal cells into root hairs and trichomes is a functional model system for understanding plant cell development. Previous studies showed that one of the Arabidopsis basic-helix-loop-helix (AtbHLH) proteins, GLABRA3 (GL3), is involved in root-hair and trichome differentiation. We analyzed 11 additional AtbHLH genes with homology to GL3. Estimation of the phylogeny based on amino acid sequences of the bHLH region suggests that 11 AtbHLH genes used in this study evolved by duplications of a single common GL3 ancestor. Promoter-GUS analysis showed that AtbHLH006, AtbHLH013, AtbHLH017 and AtbHLH020 were expressed in roots. Among them, AtbHLH006 and AtbHLH020 were preferentially expressed in root epidermal non-hair cells. Consistent with the expression patterns from promoter-GUS analysis, GFP fluorescence was observed in the nuclei of root epidermal non-hair cells of AtbHLH006p::AtbHLH006:GFP and AtbHLH020p::AtbHLH020:GFP transgenic plants. However, AtbHLH006 and AtbHLH0020 proteins did not interact with epidermis-specific MYB proteins and TTG1. Taken together, AtbHLH006 and AtbHLH020 may function in root epidermal cells, but other GL3-like bHLH proteins may have evolved to regulate different processes.     

Arabidopsis ROOT HAIR DEFECTIVE3 is involved in nitrogen starvation-induced anthocyanin accumulation

Anthocyanin accumulation is a common phenom-enon seen in plants under environmental stress. In this study, we identified a new allele of ROOT HAIR DEFECTIVE3 (RHD3) showing an anthocyanin overaccumulat...     

Hypoxia‐responsive ERFs involved in postdeastringency softening of persimmon fruit

Removal of astringency by endogenously formed acetaldehyde, achieved by postharvest anaerobic treatment, is of critical importance for many types of persimmon fruit. Although an anaerobic environment accelerates de‐astringency, it also has the deleterious effect of promoting excessive softening, reducing shelf life and marketability. Some hypoxia‐responsive ethylene response factors (ERFs) participate in anaerobic de‐astringency, but their role in accelerated softening was unclear. Undesirable rapid softening induced by high CO₂ (95%) was ameliorated by adding the ethylene inhibitor 1‐MCP (1 μL/L), resulting in reduced astringency while maintaining firmness, suggesting that CO₂‐induced softening involves ethylene signalling. Among the hypoxia‐responsive genes, expression of eight involved in fruit cell wall metabolism (Dkβ‐gal1/4, DkEGase1, DkPE1/2, DkPG1, DkXTH9/10) and three ethylene response factor genes (DkERF8/16/19) showed significant correlations with postdeastringency fruit softening. Dual‐luciferase assay indicated that DkERF8/16/19 could trans‐activate the DkXTH9 promoter and this interaction was abolished by a mutation introduced into the C‐repeat/dehydration‐responsive element of the DkXTH9 promoter, supporting the conclusion that these DkERFs bind directly to the DkXTH9 promoter and regulate this gene, which encodes an important cell wall metabolism enzyme. Some hypoxia‐responsive ERF genes are involved in deastringency and softening, and this linkage was uncoupled by 1‐MCP. Fruit of the Japanese cultivar ‘Tonewase’ provide a model for altered anaerobic response, as they lost astringency yet maintained firmness after CO₂ treatment without 1‐MCP and changes in cell wall enzymes and ERFs did not occur.     

Arabidopsis CPR5 regulates ethylene signaling via molecular association with the ETR1 receptor

The plant hormone ethylene plays various functions in plant growth, development and response to environmental stress. Ethylene is perceived by membrane‐bound ethylene receptors, and among the homologous receptors in Arabidopsis, the ETR1 ethylene receptor plays a major role. The present study provides evidence demonstrating that Arabidopsis CPR5 functions as a novel ETR1 receptor‐interacting protein in regulating ethylene response and signaling. Yeast split ubiquitin assays and bi‐fluorescence complementation studies in plant cells indicated that CPR5 directly interacts with the ETR1 receptor. Genetic analyses indicated that mutant alleles of cpr5 can suppress ethylene insensitivity in both etr1‐1 and etr1‐2, but not in other dominant ethylene receptor mutants. Overexpression of Arabidopsis CPR5 either in transgenic Arabidopsis plants, or ectopically in tobacco, significantly enhanced ethylene sensitivity. These findings indicate that CPR5 plays a critical role in regulating ethylene signaling. CPR5 is localized to endomembrane structures and the nucleus, and is involved in various regulatory pathways, including pathogenesis, leaf senescence, and spontaneous cell death. This study provides evidence for a novel regulatory function played by CPR5 in the ethylene receptor signaling pathway in Arabidopsis.     

Nitric oxide is essential for the development of aerenchyma in wheat roots under hypoxic stress

In response to flooding/waterlogging, plants develop various anatomical changes including the formation of lysigenous aerenchyma for the delivery of oxygen to roots. Under hypoxia, plants produce high levels of nitric oxide (NO) but the role of this molecule in plant‐adaptive response to hypoxia is not known. Here, we investigated whether ethylene‐induced aerenchyma requires hypoxia‐induced NO. Under hypoxic conditions, wheat roots produced NO apparently via nitrate reductase and scavenging of NO led to a marked reduction in aerenchyma formation. Interestingly, we found that hypoxically induced NO is important for induction of the ethylene biosynthetic genes encoding ACC synthase and ACC oxidase. Hypoxia‐induced NO accelerated production of reactive oxygen species, lipid peroxidation, and protein tyrosine nitration. Other events related to cell death such as increased conductivity, increased cellulase activity, DNA fragmentation, and cytoplasmic streaming occurred under hypoxia, and opposing effects were observed by scavenging NO. The NO scavenger cPTIO (2‐(4‐carboxyphenyl)‐4,4,5,5‐tetramethylimidazoline‐1‐oxyl‐3‐oxide potassium salt) and ethylene biosynthetic inhibitor CoCl₂ both led to reduced induction of genes involved in signal transduction such as phospholipase C, G protein alpha subunit, calcium‐dependent protein kinase family genes CDPK, CDPK2, CDPK 4, Ca‐CAMK, inositol 1,4,5‐trisphosphate 5‐phosphatase 1, and protein kinase suggesting that hypoxically induced NO is essential for the development of aerenchyma.     

Differential expression of genes encoding 1-aminocyclopropane-1-carboxylate synthase in <Emphasis Type="BoldItalic">Arabidopsis</Emphasis> during hypoxia

Ethylene plays an essential role in response to hypoxic stress in plants. In most plant species, 1-aminocyclopropane-1-carboxylate synthase (ACS) is the key enzyme that regulates the production of ethylene. We examined the expression of ACS genes in Arabidopsis during hypoxia. Our data showed that the expression of 4 of the 12 Arabidopsis ACS genes, ACS2, ACS6, ACS7, and ACS9, is induced during hypoxia with three distinct patterns. The hypoxic induction of ACS9 is inhibited by aminooxy acetic acid, an inhibitor of ethylene biosynthesis. In addition, the hypoxic induction of ACS9 is also reduced in etr1-1 and ein2-1, two ethylene insensitive mutants in ethylene-signaling pathways, whereas the addition of 1-aminocyclopropane-1-carboxylic acid, a direct precursor of ethylene, does not induce ACS9 under normoxic conditions. These results indicate that ethylene is needed, but not sufficient, for the induction of ACS9 during hypoxia. This pattern of regulation is similar to that of ADH that encodes alcohol dehydrogenase, which we have reported previously. In contrast, the increased ethylene production during hypoxia has an inhibitory effect on ACS2 induction in roots, whereas ethylene has no effect on the hypoxic induction of ACS6 and ACS7. Based on these results, we propose that two signaling pathways are triggered during hypoxia. One pathway leads to the activation of ACS2, ACS6, and ACS7, whereas the other pathway leads to the activation of ADH and ACS9.     

Defence responses regulated by jasmonate and delayed senescence caused by ethylene receptor mutation contribute to the tolerance of petunia to Botrytis cinerea

Ethylene and jasmonate (JA) have powerful effects when plants are challenged by pathogens. The inducible promoter‐regulated expression of the Arabidopsis ethylene receptor mutant ethylene‐insensitive1‐1 (etr1‐1) causes ethylene insensitivity in petunia. To investigate the molecular mechanisms involved in transgenic petunia responses to Botrytis cinerea related to the ethylene and JA pathways, etr1‐1‐expressing petunia plants were inoculated with Botrytis cinerea. The induced expression of etr1‐1 by a chemical inducer dexamethasone resulted in retarded senescence and reduced disease symptoms on detached leaves and flowers or intact plants. The extent of decreased disease symptoms correlated positively with etr1‐1 expression. The JA pathway, independent of the ethylene pathway, activated petunia ethylene response factor (PhERF) expression and consequent defence‐related gene expression. These results demonstrate that ethylene induced by biotic stress influences senescence, and that JA in combination with delayed senescence by etr1‐1 expression alters tolerance to pathogens.