A Novel Two-Step Method for Screening Shade Tolerant Mutant Plants via Dwarfism

EMS-induced mutagenesis in Choy sum (Brassica chinensis var. parachinensis) and selection for low light tolerance using abiotic stress indices

BACKGROUND

Choy Sum (Brassica rapa ssp. chinensis var. parachinensis), grown in a controlled environment, is vulnerable to changes in indoor light quality and displays distinct photo-morphogenesis responses. The scarcity of Choy Sum germplasm for indoor cultivation necessitates the development of new cultivars. Hence, this study attempted to develop mutants through chemical mutagenesis and select low-light-tolerant mutants by using abiotic stress tolerance indices.

RESULTS

A mutant population of Choy Sum created using 1.5% ethyl methane sulfonate (EMS) at 4 h was manually pollinated to obtain the M2 generation. 154 mutants with reduced hypocotyl length were initially isolated from 3600 M2 seedlings screened under low light (R: FR = 0.5). Five mutants that showed reduced plant height at mature stages were selected and screened directly for shade tolerance in the M3 generation. Principal component analysis based on phenotypic data distinguished the M3 mutants from the wild type. Abiotic stress tolerance indices such as relative stress index (RSI), stress tolerance index (STI), geometric mean productivity (GMP), yield stability index (YSI), and stress resistance index (SRI) showed significant (P < 0.05), and positive associations with leaf yield under shade. M3-12-2 was selected as a shade-tolerant mutant based on high values of STI, YSI, and SRI with low values for tolerance (TOL) and stress susceptibility index (SSI).

CONCLUSIONS

The results demonstrate that mutation breeding can be used to create dominant mutants in Choy Sum. Furthermore, we show that screening for low light and selection based on abiotic tolerance indices allowed the identification of mutants with high resilience under shade. This method should apply to developing new cultivars in other crop plants that can be suitable for controlled environments with stable yield performance.

24-Epibrassinolide alleviates drought effects in young Carapa guianensis plants, improving the hydraulic safety margin, gas exchange and antioxidant defence

Climate change is increasing the frequency of extreme events such as droughts, limiting plant growth and productivity. Exogenous application of plant growth regulators, such as 24-epibrassinolide (EBR), might be a solution as this molecule is organic, eco-friendly, and biodegradable. This is the first research to examine possible roles of EBR on the hydraulic safety margin, physiological behaviour, and metabolism in Carapa guianensis Aubl. (Meliaceae) exposed to drought. C. guianensis is a widely distributed tree in tropical forests of the Amazon. The objective was to determine whether EBR can improve tolerance to water deficit in young C. guianensis by measuring hydraulic traits, nutritional, biochemical and physiological responses, and biomass. The experiment had four randomized treatments: two water conditions (control and water deficit) and two concentrations of EBR (0 and 100 nM EBR). EBR increased the water potential and hydraulic safety margin, increased CO2 fixation, and improved stomatal performance. EBR also stimulated antioxidant defences (SOD, CAT, APX, and POX). Overall, tretreatment with EBR improved drought tolerance of young C. guianensis plants.

Hydrogen sulfide improves tall fescue photosynthesis response to low-light stress by regulating chlorophyll and carotenoid metabolisms

Hydrogen sulfide (H₂S), as a gaseous messenger molecule, plays critical roles in signal transduction and biological modulation. In the present study, the roles of H₂S in regulating chlorophyll (Chl) and carotenoid (Car) contents to improve photosynthesis in tall fescue were investigated under low-light (LL) stress. Compared to control conditions, LL stress significantly reduced total biomass, net photosynthetic rate (P_n), maximal quantum yield of photosystem II (PSII) photochemistry (F_v/F_m), and the contents of Chl and Car. Under exogenous sodium hydrosulfide (NaHS, H₂S donor) application, these parameters were enhanced, ultimately increasing photosynthesis. Moreover, exogenous H₂S up-regulated the expression of chlorophyll biosynthesis genes while down-regulated chlorophyll degradation genes, resulting in increases in chlorophyll precursors. Components of carotenoids and expression of genes encoding biosynthesis and degradation enzymes varied similarly. Additionally, application exogenous H₂S up-regulated expression of FaDES1 and FaDCD. Thus, it enhanced L-cysteine desulfhydrase 1 (DES1, EC 4.4.1.1) and D-cysteine desulfhydrase (DCD, EC 4.4.1.15) activities leading to elevated endogenous H₂S. However, these responses were reversed by treatment with hypotaurine (HT, H₂S scavenger). These results suggested that H₂S is involved in regulating photosynthesis to improve LL tolerance via modulating Chl and Car metabolisms in tall fescue.

Methyl jasmonate-induced resistance to Delia platura (Diptera: Anthomyiidae) in Lupinus mutabilis

BACKGROUND

Andean lupin (Lupinus mutabilis Sweet) is an important leguminous crop from South America with a high protein content. In Ecuador, lupin yields are severely affected by the infestation of Delia platura larvae on germinating seeds. The application of elicitor molecules with activity against herbivorous insects to control D. platura infestation constitutes an unexplored and promising alternative for chemical insecticides. In this study, methyl jasmonate (MeJA), hexanoic acid, menadione sodium bisulfite, and DL-β-aminobutyric acid were evaluated for their ability to induce resistance against D. platura in three commercial lupin cultivars.

RESULTS

Only seeds pretreated with MeJA significantly impaired insect performance during choice and no-choice assays. Additionally, fitness indicators such as seed germination and growth were not affected by MeJA treatment. To investigate the molecular mechanisms behind the MeJA-mediated resistance, RT-qPCR assays were performed. First, RT-qPCR reference genes were validated, showing that LmUBC was the most stable reference gene. Next, expression analysis over time revealed that MeJA application up-regulated the activity of the jasmonic acid biosynthetic genes LmLOX2 and LmAOS, together with other jasmonate-related defense genes, such as LmTPS1, LmTPS4, LmPI2, LmMBL, LmL/ODC, LmCSD1, and LmPOD.

CONCLUSION

This study indicates that MeJA can be used as an environmentally friendly elicitor molecule to protect Andean lupin from D. platura attack without fitness cost. MeJA application induces plant defense responses to insects in Andean lupin that may be modulated by the onset of terpenoid biosynthesis, proteinase inhibitors, lectins, polyamines, and antioxidative enzymes. © 2021 Society of Chemical Industry.

Auxin and its role in plant development: structure, signalling, regulation and response mechanisms

Auxins are plant hormones that play a central role in controlling plant growth and development across different environmental conditions. Even at low concentrations, auxins can regulate gene expression through specific transcription factors and proteins that are modulated to environmental responses in the signalling cascade. Auxins are synthesized in tissues with high cell division activity and distributed by specific transmembrane proteins that regulate efflux and influx. This review presents recent advances in understanding the biosynthetic pathways, both dependent and independent of tryptophan, highlighting the intermediate indole compounds (indole-3-acetamide, indole-3-acetaldoxime, indole-3-pyruvic acid and tryptamine) and the key enzymes for auxin biosynthesis, such as YUCs and TAAs. In relation to the signalling cascade, it has been shown that auxins influence gene expression regulation by the connection between synthesis and distribution. Moreover, the molecular action of the auxin response factors and auxin/indole-3-acetic acid transcription factors with the F-box TIR1/AFB auxin receptors regulates gene expression. In addition, the importance of microRNAs in the auxin signalling pathway and their influence on plant plasticity to environmental fluctuations is also demonstrated. Finally, this review describes the chemical and biological processes involving auxins in plants.

Expression and processing of polycistronic artificial microRNAs and trans-acting siRNAs from transiently introduced transgenes in Solanum lycopersicum and Nicotiana benthamiana

Targeted gene silencing using small regulatory RNAs is a widely used technique for genetic studies in plants. Artificial microRNAs are one common approach, as they have the advantage of producing just a single functional small RNA, which can be designed for high target specificity and low off-target effects. Simultaneous silencing of multiple targets with artificial microRNAs can be achieved by producing polycistronic microRNA precursors. Alternatively, specialized trans-acting short interfering RNA (tasiRNA) precursors can be designed to produce several specific tasiRNAs at once. Here we tested several artificial microRNA- and tasiRNA-based methods for multiplexed gene silencing in Solanum lycopersicum (tomato) and Nicotiana benthamiana. All analyses used transiently expressed transgenes delivered by infiltration of leaves with Agrobacterium tumefacians. Small RNA sequencing analyses revealed that many previously described approaches resulted in poor small RNA processing. The 5'-most microRNA precursor hairpins on polycistronic artificial microRNA precursors were generally processed more accurately than precursors at the 3'-end. Polycistronic artificial microRNAs where the hairpin precursors were separated by transfer RNAs had the best processing precision. Strikingly, artificial tasiRNA precursors failed to be processed in the expected phased manner in our system. These results highlight the need for further development of multiplexed artificial microRNA and tasiRNA strategies. The importance of small RNA sequencing, as opposed to single-target assays such as RNA blots or real-time polymerase chain reaction, is also discussed.

Brassinosteroids trigger tolerance to iron toxicity in rice

Iron (Fe) toxicity is one of the most frequent abiotic stresses in rice, as it affects from 15% to 30% of the total production. Brassinosteroids (BRs), including 24-epibrassinolide (EBR), regulate ion homeostasis and improve the antioxidant system. The aim of this research was to determine whether EBR can contribute to the tolerance of rice plants exposed to Fe toxicity and to evaluate the possible effect on anatomical characteristics, nutrient concentrations, the antioxidant system, and gas exchange. The experiment was randomized with four treatments, two with different concentrations of Fe (250 and 6250 μM, control and toxicity, respectively) and these were either supplied with EBR or not (0 and 10 nM EBR, described as -EBR and +EBR, respectively). Treating plants grown under Fe toxic conditions with EBR caused an 70% increase in root aerenchyma area, compared to plants without steroid treatment. Our results revealed that EBR treatment could mitigate the deleterious effects of Fe toxicity in rice plants, by modulating the aerenchyma area, which contributes to the formation of an oxidative barrier and reduce the Fe mobilization at the root surface. Plants that were exposed to Fe toxic concentrations and treated with EBR showed (1) an increase in the enzyme activities of superoxide dismutase, catalase, ascorbate peroxidase and peroxidase, (2) mitigation of oxidative damage and (3) increased scavenging of reactive oxygen species. Finally, EBR alleviated the negative impacts induced by excess Fe on the net photosynthetic rate and the instantaneous carboxylation efficiency. These benefits were directly related to higher electron transport and stomatal density and indirectly linked to the protection mechanism exercised by the antioxidant enzymes on photosynthetic machinery. We conclude that EBR is able to confer tolerance to Fe toxicity in rice plants.

Field evidences for the positive effects of aerosols on tree growth

Theoretical and eddy covariance studies demonstrate that aerosol-loading stimulates canopy photosynthesis, but field evidence for the aerosol effect on tree growth is limited. Here, we measured in situ daily stem growth rates of aspen trees under a wide range of aerosol-loading in China. The results showed that daily stem growth rates were positively correlated with aerosol-loading, even at exceptionally high aerosol levels. Using structural equation modeling analysis, we showed that variations in stem growth rates can be largely attributed to two environmental variables covarying with aerosol loading: diffuse fraction of radiation and vapor pressure deficit (VPD). Furthermore, we found that these two factors influence stem growth by influencing photosynthesis from different parts of canopy. Using field observations and a mechanistic photosynthesis model, we demonstrate that photosynthetic rates of both sun and shade leaves increased under high aerosol-loading conditions but for different reasons. For sun leaves, the photosynthetic increase was primarily attributed to the concurrent lower VPD; for shade leaves, the positive aerosol effect was tightly connected with increased diffuse light. Overall, our study provides the first field evidence of increased tree growth under high aerosol loading. We highlight the importance of understanding biophysical mechanisms of aerosol-meteorology interactions, and incorporating the different pathways of aerosol effects into earth system models to improve the prediction of large-scale aerosol impacts, and the associated vegetation-mediated climate feedbacks.

Organ-specific phytohormone synthesis in two Geranium species with antithetical responses to far-red light enrichment

Plants growing in high densities experience a reduced red (R) to far-red (FR) light ratio and shade-intolerant species respond with accelerated elongation growth to reach the top of the canopy: the shade avoidance syndrome (SAS). FR-enriched light inactivates phytochrome photoreceptors, which results in subsequent action of several plant hormones regulating growth. SAS is adaptive for shade-intolerant plants, but is suppressed in shade-tolerant plant species. Inspired by a previously published transcriptome analysis, we use two species of the genus Geranium here to study the involvement of auxin, brassinosteroids (BRs), and gibberellins (GAs) in supplemental FR-induced elongation growth. G. pyrenaicum, a shade-avoiding species, strongly induces auxin and gibberellin levels, but not BR, in elongating petioles. We show that, in this species, FR light perception, hormone synthesis, and growth are local and restricted to the petiole, and not the leaf lamina. Using chemical hormone inhibitors, we confirm the essential role of auxin and GAs in supplemental FR-induced elongation growth. Shade-tolerant G. robertianum does not display the change in hormone levels upon FR light enrichment, resulting in the lack of a shade avoidance response.

Auxin-to-Gibberellin Ratio as a Signal for Light Intensity and Quality in Regulating Soybean Growth and Matter Partitioning

The intensity and quality (red to far-red (R/Fr) ratio) of light directly affect growth of plant under shading. Gibberellins (GAs) and auxin [indole-3-acetic acid (IAA)] play important roles in mediating the shading adaptive responses of plants. Thus, the intensity and quality of the uncoupling light from shading were assessed to identify the influence of each component on the morphology and matter distribution of the leaf, stem, and petiole. This assessment was based on the changes in endogenous Gibberellin 1 (GA1) and IAA levels. Soybean plants were grown in a growth chamber with four treatments [normal (N), N+Fr, low (L), and L+Fr light]. Results revealed that the reductions in photosynthetically active radiation (PAR) and R/Fr ratio equally increased height and stem mass fractions (SMFs) of the soybean seedling. The light intensity significantly influenced the dry mass per unit area and mass fraction of soybean leaves, whereas the light quality regulated the petiole elongation and mass fraction. Low R/Fr ratio (high Fr light) increased the soybean biomass by improving the photosynthetic assimilation rate and quantum yield of photosystem II. In addition, the IAA and GA1 levels in the leaf, stem, and petiole did not reflect the growth response trends of each tissue toward light intensity and quality; however, trends of the IAA-to-GA1 content ratios were similar to those of the growth and matter allocation of each soybean tissue under different light environments. Therefore, the response of growth and matter allocation of soybean to light intensity and quality may be regulated by the IAA-to-GA1 content ratio in the tissues of the soybean plant.

Transcriptome Analysis Reveals Differential Gene Expression and a Possible Role of Gibberellins in a Shade-Tolerant Mutant of Perennial Ryegrass

The molecular basis behind shade tolerance in plants is not fully understood. Previously, we have shown that a connection may exist between shade tolerance and dwarfism, however, the mechanism connecting these phenotypes is not well understood. In order to clarify this connection, we analyzed the transcriptome of a previously identified shade-tolerant mutant of perennial ryegrass (Lolium perenne L.) called shadow-1. shadow-1 mutant plants are dwarf, and are significantly tolerant to shade in a number of environments compared to wild-type controls. In this study, we treated shadow-1 and wild-type plants with 95% shade for 2 weeks and compared the transcriptomes of these shade-treated individuals with both genotypes exposed to full light. We identified 2,200 differentially expressed genes (DEGs) (1,096 up-regulated and 1,104 down-regulated) in shadow-1 mutants, compared to wild type, following exposure to shade stress. Of these DEGs, 329 were unique to shadow-1 plants kept under shade and were not found in any other comparisons that we made. We found 2,245 DEGs (1,153 up-regulated and 1,092 down-regulated) in shadow-1 plants, compared to wild-type, under light, with 485 DEGs unique to shadow-1 plants under light. We examined the expression of gibberellin (GA) biosynthesis genes and found that they were down-regulated in shadow-1 plants compared to wild type, notably gibberellin 20 oxidase (GA20ox), which was down-regulated to 3.3% (96.7% reduction) of the wild-type expression level under shade conditions. One GA response gene, lipid transfer protein 3 (LTP3), was also down-regulated to 41.5% in shadow-1 plants under shade conditions when compared to the expression level in the wild type. These data provide valuable insight into a role that GA plays in dwarfism and shade tolerance, as exemplified by shadow-1 plants, and could serve as a guide for plant breeders interested in developing new cultivars with either of these traits.