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...     

Arabidopsis sucrose transporter AtSUC1 is important for pollen germination and sucrose-induced anthocyanin accumulation

The Arabidopsis (Arabidopsis thaliana) sucrose transporter AtSUC1 (At1g71880) is highly expressed in pollen; however, its function has remained unknown. Here, we show that suc1 mutant pollen is defective in vivo, as evidenced by segregation distortion, and also has low rates of germination in vitro. AtSUC1-green fluorescent protein was localized to the plasma membrane in pollen tubes. AtSUC1 is also expressed in roots and external application of sucrose increased AtSUC1 expression in roots. AtSUC1 is important for sucrose-dependent signaling leading to anthocyanin accumulation in seedlings. suc1 mutants accumulated less anthocyanins in response to exogenous sucrose or maltose and microarray analysis revealed reduced expression of many genes important for anthocyanin biosynthesis. The results indicate that AtSUC1 is important for sugar signaling in vegetative tissue and for normal male gametophyte function.     

Root Growth of Arabidopsis thaliana Is Regulated by Ethylene and Abscisic Acid Signaling Interaction in Response to HrpNEa, a Bacterial Protein of Harpin Group

HrpN_Ea is a harpin protein from Erwinia amylovora, a bacterial pathogen that causes fire blight in rosaceous plants. Treating plants with HrpN_Ea stimulates ethylene and abscisic acid (ABA) to induce plant growth and drought tolerance, respectively. Herein, we report that both growth hormones cooperate to mediate the role of HrpN_Ea in promoting root growth of Arabidopsis thaliana seedlings. Root growth is promoted coordinately with elevation in levels of ABA and ethylene subsequent to soaking of germinating seeds of wild-type (WT) Arabidopsis in a solution of HrpN_Ea. However, these responses are arrested by inhibiting WT roots from synthesizing ethylene as well as sensing of ABA and ethylene. The effects of HrpN_Ea on roots are also nullified in ethylene-insensitive etr1-1 and ein5-1 mutants and in the ABA-insensitive mutant abi2-1 of Arabidopsis. These results provide evidence for presence of a relationship between root growth enhancement and signaling by ABA and ethylene in response to HrpN_Ea. Nevertheless, when HrpN_Ea is applied to leaves, ethylene signaling is active in the absence of ABA signaling to promote plant growth. This suggests the presence of a different signaling mechanism in leaves from that in roots. X. Ren and F. Liu contributed equally to this study and are regarded as joint first authors     

拟南芥转录因子Ethylene-insensitive3（EIN3）抑制花青素的合成

Ethylene-insensitive3（EIN3）和 EIN3-like1（EIL1）蛋白是乙烯信号转导途径中一类重要的核转录因子。花青素是植物体中的一类水溶性天然色素,在植物的许多生理过程中起重要作用。本研究以拟南芥双突变体ein3-1eil1-3为研究材料,通过RT-PCR技术确定了拟南芥双突变体ein3-1eil1-3中EIN3和EIL1基因均已被敲除,单突变体ein3-1中的EIN3基因被敲除。通过肉眼定性观察发现突变体ein3-1eil1-3的种子和叶片内均呈紫色。通过紫外分光光度计定量分析发现,花青素积累量也明显比突变体ein3-1和野生型多。通过GUS染色发现EIN3启动子主要在花、柱头、成熟花粉、种子胚和果荚等组织中有较强的表达。这与突变体ein3-1eil1-3的种子和叶片内均呈紫色并花青素含量增高一致。因此,拟南芥转录因子EIN3可能与EIL1共同参与抑制花青素的合成。     

Ethylene and shoot regeneration: hookless1 modulates de novo shoot organogenesis in Arabidopsis thaliana

We have investigated the role of ethylene in shoot regeneration from cotyledon explants of Arabidopsis thaliana. We examined the ethylene sensitivity of five ecotypes representing both poor and prolific shoot regenerators and identified Dijon-G, a poor regenerator, as an ecotype with dramatically enhanced ethylene sensitivity. However, inhibiting ethylene action with silver nitrate generally reduced shoot organogenesis in ecotypes capable of regeneration. In ecotype Col-0, we found that ethylene-insensitive mutants (etr1-1, ein2-1, ein4, ein7) exhibited reduced shoot regeneration rates, whereas constitutive ethylene response mutants (ctr1-1, ctr1-12) increased the proportion of explants producing shoots. Our experiments with ethylene over-production mutants (eto1, eto2 and eto3) indicate that the ethylene biosynthesis inhibitor gene, ETO1, can act as an inhibitor of shoot regeneration. Pharmacological elevation of ethylene levels was also found to significantly increase the proportion of explants regenerating shoots. We determined that the hookless1 (hls1-1) mutant, a suppressor of the ethylene response phenotypes of ctr1 and eto1 mutants, is capable of dramatically enhancing shoot organogenesis. The effects of ACC and loss of HLS1 function on shoot organogenesis were found to be largely additive.     

Concomitant activation of jasmonate and ethylene response pathways is required for induction of a plant defensin gene in Arabidopsis.

Activation of the plant defensin gene PDF1.2 in Arabidopsis by pathogens has been shown previously to be blocked in the ethylene response mutant ein2-1 and the jasmonate response mutant coi1-1. In this work, we have further investigated the interactions between the ethylene and jasmonate signal pathways for the induction of this defense response. Inoculation of wild-type Arabidopsis plants with the fungus Alternaria brassicicola led to a marked increase in production of jasmonic acid, and this response was not blocked in the ein2-1 mutant. Likewise, A. brassicicola infection caused stimulated emission of ethylene both in wild-type plants and in coi1-1 mutants. However, treatment of either ein2-1 or coi1-1 mutants with methyl jasmonate or ethylene did not induce PDF1.2, as it did in wild-type plants. We conclude from these experiments that both the ethylene and jasmonate signaling pathways need to be triggered concomitantly, and not sequentially, to activate PDF1.2 upon pathogen infection. In support of this idea, we observed a marked synergy between ethylene and methyl jasmonate for the induction of PDF1.2 in plants grown under sterile conditions. In contrast to the clear interdependence of the ethylene and jasmonate pathways for pathogen-induced activation of PDF1.2, functional ethylene and jasmonate signaling pathways are not required for growth responses induced by jasmonate and ethylene, respectively.     

Genetic and transformation studies reveal negative regulation of ERS1 ethylene receptor signaling in Arabidopsis

Background  

Ethylene receptor single mutants of Arabidopsis do not display a visibly prominent phenotype, but mutants defective in multiple ethylene receptors exhibit a constitutive ethylene response phenotype. It is inferred that ethylene responses in Arabidopsis are negatively regulated by five functionally redundant ethylene receptors. However, genetic redundancy limits further study of individual receptors and possible receptor interactions. Here, we examined the ethylene response phenotype in two quadruple receptor knockout mutants, (ETR1) ers1 etr2 ein4 ers2 and (ERS1) etr1 etr2 ein4 ers2, to unravel the functions of ETR1 and ERS1. Their functions were also reciprocally inferred from phenotypes of mutants lacking ETR1 or ERS1. Receptor protein levels are correlated with receptor gene expression. Expression levels of the remaining wild-type receptor genes were examined to estimate the receptor amount in each receptor mutant, and to evaluate if effects of ers1 mutations on the ethylene response phenotype were due to receptor functional compensation. As ers1 and ers2 are in the Wassilewskija (Ws) ecotype and etr1, etr2, and ein4 are in the Columbia (Col-0) ecotype, possible effects of ecotype mixture on ethylene responses were also investigated.     

Interactions between abscisic acid and ethylene signaling cascades

We screened for mutations that either enhanced or suppressed the abscisic acid (ABA)-resistant seed germination phenotype of the Arabidopsis abi1-1 mutant. Alleles of the constitutive ethylene response mutant ctr1 and ethylene-insensitive mutant ein2 were recovered as enhancer and suppressor mutations, respectively. Using these and other ethylene response mutants, we showed that the ethylene signaling cascade defined by the ETR1, CTR1, and EIN2 genes inhibits ABA signaling in seeds. Furthermore, epistasis analysis between ethylene- and ABA-insensitive mutations indicated that endogenous ethylene promotes seed germination by decreasing sensitivity to endogenous ABA. In marked contrast to the situation in seeds, ein2 and etr1-1 roots were resistant to both ABA and ethylene. Our data indicate that ABA inhibition of root growth requires a functional ethylene signaling cascade, although this inhibition is apparently not mediated by an increase in ethylene biosynthesis. These results are discussed in the context of the other hormonal regulations controlling seed germination and root growth.     

Loss-of-function mutation of EIN2 in Arabidopsis exaggerates oxidative stress induced by salinity

Accumulation of reactive oxygen species (ROS) causes oxidative stress under adverse environmental conditions, such as salinity. Ethylene decreases accumulation of ROS induced by salinity, but the mechanism is still unclear. To examine the interactions between salinity and ROS accumulation and the possible role of ethylene metabolism in regulation, we used mutant ein2-5 in Arabidopsis with loss of function in EIN2. The mutant is compared to the wild-type Col-0, completely insensitivity to ethylene at the morphological, physiological and molecular levels. The oxidative responses of the wild type and mutant to salinity were compared. Loss-of-function of EIN2 enhanced sensitivity to salinity, implying that EIN2 is required for plant response to salinity. Furthermore, salinity resulted in accumulation of large amounts of ROS in ein2-5 seedlings when compared with Col-0, suggesting that the loss-of-function of EIN2 exaggerates oxidative stress induced by salinity. Activities of the antioxidant enzymes SOD, POD and CAT decreased significantly in ein2-5 under salinity when compared with Col-0 plants. The expression profiles of the genes Fe-SOD, PODs and CAT1, which code for ROS scavenging enzymes were severely decreased in ein2-5 under salinity compared with Col-0, suggesting that EIN2 was involved in regulating expression of these genes. Taken together, our results demonstrate that loss-of-function of EIN2 increased oxidative stress induced by salinity and that EIN2 is involved in modulating ROS accumulation, at least in part, by decreasing activities of ROS-scavenging enzymes.     

Enhanced oxidative stress in the ethylene-insensitive (ein3-1) mutant of Arabidopsis thaliana exposed to salt stress

To better understand the role of ethylene signaling in plant stress tolerance, salt-induced changes in gene expression levels of ethylene biosynthesis, perception and signaling genes were measured in Arabidopsis thaliana plants exposed to 15 days of salinity. Among the genes analyzed, EIN3 showed the highest expression level increase under salt stress, suggesting a key role for this ethylene-signaling component in response to salt stress. Therefore, we analyzed the salt stress response over 15 days (by adding 100 mM NaCl to the nutrient solution) in the ein3-1 mutant compared to the wild-type (Col-0) in terms of growth, oxidative stress markers (lipid peroxidation, foliar pigments and low-molecular-weight antioxidants) and levels of growth- and stress-related phytohormones (including cytokinins, auxins, gibberellins, abscisic acid, jasmonic acid and salicylic acid). The ein3-1 mutant grew similarly to wild-type plants both under control and salt stress conditions, which was associated with a differential time course evolution in the levels of the cytokinins zeatin and zeatin riboside, and the auxin indole-3-acetic acid between the ein3-1 mutant and the wild-type. Despite showing no signs of physiological deterioration under salt stress (in terms of rosette biomass, leaf water and pigment contents, and PSII efficiency) the ein3-1 mutant showed enhanced lipid peroxidation under salt stress, as indicated by 2.4-fold increase in both malondialdehyde and jasmonic acid contents compared to the wild-type. We conclude that, at moderate doses of salinity, partial insensitivity to ethylene might be compensated by changes in endogenous levels of other phytohormones and lipid peroxidation-derived signals in the ein3-1 mutant exposed to salt stress, but at the same time, this mutant shows higher oxidative stress under salinity than the wild-type.