Differential effects of methyl jasmonate on growth and division of etiolated zucchini cotyledons

Exogenous methyl jasmonate and cytokinin antagonistically regulate lignin biosynthesis by mediating CsHCT expression in Camellia sinensis

Tea plant, an important beverage crop, is cultivated worldwide. Lignification can improve the hardness of tea plant, which is of great significance for tea quality. Jasmonates (JAs) and cytokinin are plant hormones that control processes of plant development and secondary metabolite accumulation. Hydroxycinnamoyl-CoA shikimate/quinate hydroxycinnamoyl transferase (HCT) is primarily involved in lignin biosynthesis. The effects of exogenous application of JAs and cytokinin on lignin biosynthesis and related HCT gene expression profiles in tea plants are still unclear. In order to investigate the effects of exogenous JAs and cytokinin on lignin accumulation, anatomical structures, and CsHCT gene profiles in tea plants, we treated tea plants with methyl jasmonate (MeJA) and cytokinin (6-BA). MeJA and 6-BA treatments triggered the lignification at 6 and 12 d in tea leaves. The combined treatment resulted in an increase in lignin content at 6 d, which was 1.32 times of that at 0 d for 'Mengshan 9.' The CsHCTs in clade 2 (CsHCT5, CsHCT6, CsHCT7, and CsHCT8) were mainly expressed in leaves. We found that exogenous MeJA and cytokinin might be able to antagonistically regulate tea plant lignin accumulation through the mediation of CsHCT expression. This study revealed that HCTs play potential important roles involved in lignin biosynthesis of tea plant development and hormonal stimuli.

Quantitative proteomics analysis reveals the response mechanism of peanut (Arachis hypogaea L.) to imbibitional chilling stress

Imbibitional chilling stress inhibits normal seed germination and seedling establishment and leads to large losses in peanut production. This is a major limiting factor when sowing peanut earlier and further north. To reveal the response mechanism of peanut to imbibitional chilling stress, a Tandem Mass Tag (TMT)-based quantitative proteomics analysis was conducted to identify differentially accumulated proteins (DAPs) under imbibitional chilling stress. Hormone profiling and transcriptional analysis were performed to confirm the proteomics data. Further seed priming analysis with exogenous cytokinins was conducted to validate the role of cytokinins in alleviating imbibitional chilling injury. A total of 5029 proteins were identified and quantified in all of the experimental groups. Among these, 104 proteins were DAPs as compared with the control. Enrichment analysis revealed that these DAPs were significant in various molecular functional and biological processes, especially for biosynthesis and metabolism of plant hormones. Hormone profiling and transcription analysis suggested that the reduced abundance of cytokinin oxidase may be caused by down-regulation of gene expression of the corresponding genes and leads to an elevated content of cytokinins under chilling stress. Seed priming analysis suggested that exogenous application of cytokinins may alleviate injury caused by imbibitional chilling. Our study provides a comprehensive proteomics analysis of peanut under imbibitional chilling stress, suggesting the role of plant hormones in the response mechanism. The results provide a better understanding of the imbibitional chilling stress response mechanism in peanut that will aid in peanut production.

Jasmonates and Plant Salt Stress: Molecular Players, Physiological Effects, and Improving Tolerance by Using Genome-Associated Tools

Soil salinity is one of the most limiting stresses for crop productivity and quality worldwide. In this sense, jasmonates (JAs) have emerged as phytohormones that play essential roles in mediating plant response to abiotic stresses, including salt stress. Here, we reviewed the mechanisms underlying the activation and response of the JA-biosynthesis and JA-signaling pathways under saline conditions in Arabidopsis and several crops. In this sense, molecular components of JA-signaling such as MYC2 transcription factor and JASMONATE ZIM-DOMAIN (JAZ) repressors are key players for the JA-associated response. Moreover, we review the antagonist and synergistic effects between JA and other hormones such as abscisic acid (ABA). From an applied point of view, several reports have shown that exogenous JA applications increase the antioxidant response in plants to alleviate salt stress. Finally, we discuss the latest advances in genomic techniques for the improvement of crop tolerance to salt stress with a focus on jasmonates.

Regulation of water transport in Arabidopsis by methyl jasmonate

Following a stress event, jasmonate-dependent signaling pathway triggers a shift from growth to defense responses that are accompanied by the cessation of growth in many plants. However, the processes leading to this growth inhibition remain obscure. In this study, we provide evidence for a rapid inhibition of cell hydraulic conductivity (L_p) by methyl jasmonate (MeJA) in the roots of wild-type Arabidopsis within 0.5 h of 20 and 50 μM MeJA treatments. We also demonstrate that MeJA did not affect L_p in fad3-2 and fad7-2 Arabidopsis mutants that are deficient in jasmonate precursor, linolenic acid. The reductions of L_p in wild-type plants were accompanied by the down-regulation of several plasma membrane intrinsic protein (PIP) isoforms, and dephosphorylation. Treatments with HgCl₂ did not further reduce L_p in the wild-type plants, but significantly reduced L_p in the fad3-2 and fad7-2 that had been first treated with MeJA. Continuous prolonged exposure to exogenous 50 μM MeJA inhibited the relative growth rates (RGR) of shoots and net photosynthesis (P_n) in the Arabidopsis wild-type and fad7-2 plants, but had no effect on the RGR of roots. The results demonstrated that a reduction of aquaporin (AQP)-mediated water transport was the initial target of MeJA exposure, and may contribute to the processes of growth inhibition by MeJA.

The bacterial volatile dimethyl-hexa-decylamine reveals an antagonistic interaction between jasmonic acid and cytokinin in controlling primary root growth of Arabidopsis seedlings

Chemical communication underlies major adaptive traits in plants and shapes the root microbiome. An increasing number of diffusible and/or volatile organic compounds released by bacteria have been identified, which play phytostimulant or protective functions, including dimethyl-hexa-decylamine (DMHDA), a volatile biosynthesized by Arthrobacter agilis UMCV2 that induces jasmonic acid (JA) signaling in Arabidopsis. Here, he found that the growth repressing effects of both DMHDA and JA are antagonized by kinetin and correlated with an inhibition of cytokinin-related ARR5::GUS and TCS::GFP expression in Arabidopsis primary roots. Moreover, we demonstrate that shoot supplementation of JA triggers JAZ1 expression both locally and systemically and represses cytokinin-dependent promoter activity in roots. A similar effect was observed after cotyledon wounding, in which an increase of JA-inducible LOX2:GUS expression represses root growth, which correlates with the loss of TCS::GFP detection at the very root tip. Our data demonstrate that the bacterial volatile DMHDA crosstalks with cytokinin signaling and reveals the downstream antagonistic interaction between JA and cytokinin in controlling root growth.

Exogenous methyl jasmonate regulates cytokinin content by modulating cytokinin oxidase activity in wheat seedlings under salinity

The treatment of 4-days-old wheat seedlings with methyl jasmonate (MeJA) in concentration optimal for their growth (0.1 μM) resulted in a rapid transient almost two-fold increase in the level of cytokinins (CKs). MeJA-induced accumulation of CKs was due to inhibition of both cytokinin oxidase (CKX) (cytokinin oxidase/dehydrogenase, EC 1.5.99.12) gene expression and activity of this enzyme. Pretreatment of wheat seedlings with MeJA decreased the growth-retarding effect of sodium chloride salinity and accelerated growth recovery after withdrawal of NaCl from the incubation medium. We speculate that this protective effect of the hormone might be due to MeJA's ability to prevent the salinity-induced decline in CK concentration that was caused by inhibition of gene expression and activity of CKX in wheat seedlings. The data might indicate an important role for endogenous cytokinins in the implementation of growth-promoting and protective effects of exogenous MeJA application on wheat plants.

Exploring Jasmonates in the Hormonal Network of Drought and Salinity Responses

Present and future food security is a critical issue compounded by the consequences of climate change on agriculture. Stress perception and signal transduction in plants causes changes in gene or protein expression which lead to metabolic and physiological responses. Phytohormones play a central role in the integration of different upstream signals into different adaptive outputs such as changes in the activity of ion-channels, protein modifications, protein degradation, and gene expression. Phytohormone biosynthesis and signaling, and recently also phytohormone crosstalk have been investigated intensively, but the function of jasmonates under abiotic stress is still only partially understood. Although most aspects of jasmonate biosynthesis, crosstalk and signal transduction appear to be similar for biotic and abiotic stress, novel aspects have emerged that seem to be unique for the abiotic stress response. Here, we review the knowledge on the role of jasmonates under drought and salinity. The crosstalk of jasmonate biosynthesis and signal transduction pathways with those of abscisic acid (ABA) is particularly taken into account due to the well-established, central role of ABA under abiotic stress. Likewise, the accumulating evidence of crosstalk of jasmonate signaling with other phytohormones is considered as important element of an integrated phytohormonal response. Finally, protein post-translational modification, which can also occur without de novo transcription, is treated with respect to its implications for phytohormone biosynthesis, signaling and crosstalk. To breed climate-resilient crop varieties, integrated understanding of the molecular processes is required to modulate and tailor particular nodes of the network to positively affect stress tolerance.

Cytokinin cross-talking during biotic and abiotic stress responses

As sessile organisms, plants have to be able to adapt to a continuously changing environment. Plants that perceive some of these changes as stress signals activate signaling pathways to modulate their development and to enable them to survive. The complex responses to environmental cues are to a large extent mediated by plant hormones that together orchestrate the final plant response. The phytohormone cytokinin is involved in many plant developmental processes. Recently, it has been established that cytokinin plays an important role in stress responses, but does not act alone. Indeed, the hormonal control of plant development and stress adaptation is the outcome of a complex network of multiple synergistic and antagonistic interactions between various hormones. Here, we review the recent findings on the cytokinin function as part of this hormonal network. We focus on the importance of the crosstalk between cytokinin and other hormones, such as abscisic acid, jasmonate, salicylic acid, ethylene, and auxin in the modulation of plant development and stress adaptation. Finally, the impact of the current research in the biotechnological industry will be discussed.

Differences in cytokinin control on cellular dynamics of zucchini cotyledons cultivated in two experimental systems

The effect of endogenous cytokinins on the pattern of palisade cell division post-germination does not depend on the conditions of cotyledon development -in planta (attached to seedlings) or in vitro (isolated from dry zucchini seeds and cultured on water). In cotyledons originating from 4-day-old seedlings (experimental system 1), exogenous cytokinin temporarily (in the first 2 day of cultivation) enhanced post-mitotic cell enlargement of palisade cells, mainly due to enhanced water uptake and use of cell storage compounds, all of which lead to cotyledon senescence. Cytokinin is not able to resume the completed palisade cell division on day 5. As a result, the number of cells and the final areas of treated and control cotyledons are quite similar. By contrast, the effects of cytokinin on cotyledons isolated from dry seeds (experimental system 2) are better expressed, promoting an increase in number of palisade cells accompanied by additional cotyledon area enlargement. However, the prolonged post-mitotic cell expansion in control cotyledons compensates for the reduced speed of cell growth and division activity and decreases differences in final cotyledon area between treatments. The results define cell division as the primary target of cytokinin stimulation in cotyledon tissues competent for division, and determine the temporal patterns of palisade cell cycling related to cotyledon age. This knowledge permits a better choice of experimental system to study effects on cell proliferation and cell growth, as well as cell enlargement and senescence-related events using physiologically homogeneous material.

Adventitious rooting is enhanced by methyl jasmonate in tobacco thin cell layers

Adventitious roots (ARs) are induced by auxins. Jasmonic acid (JA) and methyl jasmonate (MeJA) are also plant growth regulators with many effects on development, but their role on ARs needs investigation. To this aim, we analyzed AR formation in tobacco thin cell layers (TCLs) cultured with 0.01-10 microM MeJA, either under root-inductive conditions, i.e., on medium containing 10 microM indole-3-butyric acid (IBA) and 0.1 microM kinetin, or without hormones. The explants were excised from the cultivars Samsun, Xanthii and Petite Havana, and from genotypes with altered AR-forming ability in response to auxin, namely the non-rooting rac mutant and the over-rooting Agrobacterium rhizogenes rolB transgenic line. Results show that NtRNR1 (G1/S) and Ntcyc29 (G2/M) gene activity, cell proliferation and meristemoid formation were stimulated in hormone-cultured TCLs by submicromolar MeJA concentrations. The meristemoids developed either into ARs and xylogenic nodules, or into xylogenic nodules only (rac TCLs). MeJA-induced meristemoid over-production characterized rolB TCLs. No rooting or xylogenesis occurred under hormone-free conditions, independently of MeJA and genotype. Endogenous JA progressively (days 1-4) increased in hormone-cultured TCLs in the absence of MeJA. JA levels were enhanced by 0.1 microM MeJA, on both days 1 and 4. Endogenous IBA was the only auxin detected, both in the free form and as IBA-glucose. Free IBA increased up to day 2, remaining constant thereafter (day 4). Its level was enhanced by 0.1 microM MeJA only on day 1, while IBA conjugation was not affected by MeJA. Taken together, these results show that an interplay between jasmonates and auxins regulates AR formation and xylogenesis in tobacco TCLs.