Expression of Ethylene Biosynthetic Genes in the Gynoecium and Receptacle Associated With Sepal Abscission During Senescence in Delphinium Grandiflorum

Delphinium flowers are highly sensitive to ethylene and its sepals abscise during senescence, which is associated with increases in 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) and ACC oxidase (ACO) activities and ethylene production in gynoecium and receptacle. Three ACS genes (DgACS1, DgACS2, and DgACS3) and three ACO genes (DgACO1, DgACO2, and DgACO3) were cloned from Delphinium grandiflorum cv. Super Grand Blue. To investigate the contribution of these genes to ethylene production, their expression was analyzed in these genes in the gynoecium and receptacle during natural senescence and following ethylene exposure and pollination. Ethylene production in the gynoecium and receptacle increased during natural flower senescence. The transcript levels of the ACS and ACO genes in these organs, excluding DgACS2 in the receptacle, increased during senescence. Exposure to ethylene accelerated sepal abscission and more strongly increased ethylene production in the receptacle than in the gynoecium. DgACS1 transcript levels in the gynoecium and DgACS2 and DgACO3 transcript levels in the receptacle were increased by ethylene exposure. Pollination accelerated sepal abscission and increased ethylene production in the gynoecium and receptacle. Pollination slightly affected ACS and ACO transcript levels in the gynoecium, whereas DgACO3 transcript level in the receptacle were markedly increased. These results reveal that ACS and ACO gene expression is differently regulated in the gynoecium and receptacle, and some of these genes are more strongly upregulated by ethylene exposure and pollination in the receptacle than in the gynoecium, suggesting the significance of the receptacle to sepal abscission.

Activation of 1-Aminocyclopropane-1-Carboxylic Acid Synthases Sets Stomatal Density and Clustered Ratio on Leaf Epidermis of Arabidopsis in Response to Drought

The adjustment of stomatal density and clustered ratio on the epidermis is the important strategy for plants to respond to drought, because the stoma-based water loss is directly related to plant growth and survival under drought conditions. But the relevant adjustment mechanism still needs to be explored. 1-Aminocyclopropane-1-carboxylate (ACC) is disclosed to promote stomatal development, while in vivo ACC levels depend on activation of ACC synthase (ACS) family members. Based on the findings of ACS expression involving in drought response and several ACS activity inhibitors reducing stomatal density and cluster in drought response, here we examined how ACS activation is involved in the establishment of stomatal density and cluster on the epidermis under drought conditions. Preliminary data indicated that activation of ACS2 and/or ACS6 (ACS2/6) increased stomatal density and clustered ratio on the Arabidopsis leaf epidermis by accumulating ACC under moderate drought, and raised the survival risk of seedlings under escalated drought. Further exploration indicated that, in Arabidopsis seedlings stressed by drought, the transcription factor SPEECHLESS (SPCH), the initiator of stomatal development, activates ACS2/6 expression and ACC production; and that ACC accumulation induces Ca2+ deficiency in stomatal lineage; this deficiency inactivates a subtilisin-like protease STOMATAL DENSITY AND DISTRIBUTION 1 (SDD1) by stabilizing the inhibition of the transcription factor GT-2 Like 1 (GTL1) on SDD1 expression, resulting in an increases of stomatal density and cluster ratio on the leaf epidermis. This work provides a novel evidence that ACS2/6 activation plays a key role in the establishment of stomatal density and cluster on the leaf epidermis of Arabidopsis in response to drought.

Auxin Treatment Enhances Anthocyanin Production in the Non-Climacteric Sweet Cherry (Prunus avium L.)

Abscisic acid (ABA) is a key signaling molecule promoting ripening of non-climacteric fruits such as sweet cherry (Prunus avium L.). To shed light on the role of other hormones on fruit development, ripening and anthocyanin production, the synthetic auxin 1-naphthaleneacetic acid (NAA) was applied to sweet cherry trees during the straw-color stage of fruit development. NAA-treated fruits exhibited higher concentrations of 1-aminocyclopropane-1-carboxylic acid (ACC) and ABA-glucose ester (ABA-GE), which are a precursor of ethylene and a primary storage form of ABA, respectively. Consistent with these observations, transcript levels of genes encoding ACC synthase and ACC oxidase, both involved in ethylene biosynthesis, were increased after 6 days of NAA treatment, and both ABA concentration and expression of the regulator gene of ABA biosynthesis (NCED1 encoding 9-cis-epoxycarotenoid dioxygenase) were highest during early fruit ripening. In addition, transcript levels of key anthocyanin regulatory, biosynthetic and transport genes were significantly upregulated upon fruit exposure to NAA. This was accompanied by an increased anthocyanin concentration and fruit weight whilst fruit firmness and cracking index decreased. Altogether our data suggest that NAA treatment alters ethylene production, which in turn induces ripening in sweet cherry and enhanced anthocyanin production, possibly through ABA metabolism. The results from our study highlight the potential to use a single NAA treatment for manipulation of cherry ripening.

Reciprocal antagonistic regulation of E3 ligases controls ACC synthase stability and responses to stress

Ethylene influences plant growth, development, and stress responses via crosstalk with other phytohormones; however, the underlying molecular mechanisms are still unclear. Here, we describe a mechanistic link between the brassinosteroid (BR) and ethylene biosynthesis, which regulates cellular protein homeostasis and stress responses. We demonstrate that as a scaffold, 1-aminocyclopropane-1-carboxylic acid (ACC) synthases (ACS), a rate-limiting enzyme in ethylene biosynthesis, promote the interaction between Seven-in-Absentia of Arabidopsis (SINAT), a RING-domain containing E3 ligase involved in stress response, and ETHYLENE OVERPRODUCER 1 (ETO1) and ETO1-like (EOL) proteins, the E3 ligase adaptors that target a subset of ACS isoforms. Each E3 ligase promotes the degradation of the other, and this reciprocally antagonistic interaction affects the protein stability of ACS. Furthermore, 14–3-3, a phosphoprotein-binding protein, interacts with SINAT in a BR-dependent manner, thus activating reciprocal degradation. Disrupted reciprocal degradation between the E3 ligases compromises the survival of plants in carbon-deficient conditions. Our study reveals a mechanism by which plants respond to stress by modulating the homeostasis of ACS and its cognate E3 ligases.

Transcriptional and Post-transcriptional Regulation of Ethylene Biosynthesis by Salicyclic Acid in Kiwifruit

Levels of ethylene, implicated in a diverse array of plants for inducing fruit ripening, is influenced by genetic and environmental factors, such as the other plant hormones. Among these, salicylic acid (SA) has been demonstrated to inhibit ethylene biosynthesis in fruit, yet the underlying regulatory mechanisms remains elusive. Here, we showed that treatment with exogenous ASA (acetylsalicylic acid) dramatically reduced ethylene production, as well as activities of ACC synthase (ACS) and ACC oxidase (ACO), in kiwifruit tissues. Comparative transcriptome analysis indicated the differential expression of ethylene biosynthetic genes (AdACS1/2 and AdACO5). A screen of transcription factors indicated that AdERF105L and AdWRKY29 were ASA-responsive regulators of AdACS1/2 and AdACO5, respectively. In addition to these genes, AdACS3 and AdACO3 were abundantly expressed in both ASA-treated and control tissues. AdACS3 protein was phosphorylated and stabilized by AdMPK16, a mitogen-activated protein kinase; while AdACO3 activity was enhanced by AdAP, an aspartic peptidase. Exogenous ASA down-regulated AdMPK16 and AdAP, thereby influencing ethylene biosynthesis at a post-transcriptional level. These findings propose a multidimensional system for SA-inhibition on ethylene biosynthesis, inducing differential expression of some ethylene biosynthesis genes, as well as differential effects on protein activity on other targets.

Phomopsis Liquidambaris Contributes to the Decease of Ethylene Biosynthesis in Rice under Salt Stress via Inhibiting the Activity of 1-Aminocyclopropane-1-Carboxylate Deaminase

The endophytic fungus Phomopsis liquidambaris is characterized as a plant growth-promoting agent under salt stress, but its mechanism is unknown. Herein, 1-Aminocyclopropane-1-Carboxylate Deaminase (ACCD) from the strain was confirmed that it had the ability of utilizing 1-Aminocyclopropane-1-Carboxylate as the sole nitrogen source. The full-length ACCD gene was 1,152 bp, which encodes a mature protein of 384 amino acids with a molecular mass of 41.53 kDa. The ACCD activity was 3.9-fold in 3 mmol L− 1 ACC by qRT-PCR under salt stress comparing with no salt tress. Ethylene production was increased to 34.55–70.60% and reduced the growth of rice by 23–69.73% under salt stress. Inoculation of P. liquidambaris increased root-shoot length, fresh and dry weight, and overall growth of stressed rice seedlings. ACC accumulation, ACC synthase and ACC oxidase activities increased in salt-treated rice seedlings, while they were significantly reduced when P. liquidambaris was inoculated into Rice by qRT-PCR. It therefore can be concluded that P. liquidambaris can be used as a plant growth promoting fungus against salt stress and other biotic or abiotic stresses.

Formation Mechanism and Anatomical Structure of the Knee Roots of Taxodium Ascendens

Aims: Flooding seriously limits the growth and distribution of plants. Taxodium ascendens is a typical tree species with high flood tolerance, and it can generate knee roots in the wetlands. This study was conducted to understand the formation mechanism of the knee roots.Methods: The number and size of knee roots and soil flooding conditions were investigated in this study. Furthermore, physiology, biochemical responses, and the anatomical structure of knee roots and underground roots were measured at different developmental stages. Results: The results show that the formation of knee roots was significantly affected by the soil water table (P < 0.05). Moreover, knee root formation was affected by ethylene and indole-3-acetic acid (IAA) concentrations in the roots. The 1-aminocyclopropane-1-carboxylic acid (ACC) content and ACC synthase activity were significantly lower in the knee roots than in the underground roots. The ethylene release rate was significantly higher in the knee roots than in the underground roots (P < 0.05), and IAA content first increased and then decreased with knee root development. The cells of the periderm at the apex of the knee roots were dead and had a large number of intercellular spaces, which was beneficial for the growth of T. ascendens. Conclusions: Seasonal flooding induced the production of endogenous hormones, resulting in the formation of knee roots, which improved root respiration and ventilation. The results obtained can gain a basis for the formation mechanism of knee roots and provide scientific evidence for the afforestation and management under wetland conditions.

Activation of ACC synthase 2/6 increases stomatal density and cluster on the Arabidopsis leaf epidermis during drought

It is known that the transcription factor SPEECHLESS (SPCH) drives entry of epidermal cells into stomatal lineage, and that the activation of subtilisin-like protease SDD1 reduces stomatal density and cluster on the epidermis. However, there is still a big gap in our understanding of the relationship between stomatal development and the establishment of stomatal density and pattern, especially during drought. Interestingly, 1-aminocyclopropane-1-carboxylic acid (ACC) not only promotes stomatal development, but also is involved in the establishment of stomatal density and pattern. ACC generation comes from the activity of ACC synthase (ACS), while ACS activity could be mediated by drought. This work showed that the Arabidopsis SPCH activated ACS2/6 expression and ACC-dependent stomatal generation with an increase of stomatal density and cluster under drought conditions; and the possible mechanisms were that ACC-induced Ca2+ shortage in stomatal lineage reduced the inhibition of the transcription factor GT-2 Like 1 (GTL1) on SDD1 expression. These suggest that ACS2/6-dependent ACC accumulation integrated stomatal development with the establishment of stomatal density and pattern by mediating Ca2+ levels in stomatal lineage cells on the leaf epidermis, and this integration is directly related to the growth or survival of plants under escalated drought stress.HighlightACC synthase ACS2/6 activation integrated stomatal individual development with space setting between stomata by mediating Ca2+ levels in stomatal lineage on the leaf epidermis in response to drought.