Towards a generic approach to remote non-invasive estimation of foliar carotenoid-to-chlorophyll ratio

Plant optics: underlying mechanisms in remotely sensed signals for phenotyping applications

Optical-based remote sensing offers great potential for phenotyping vegetation traits and functions for a range of applications including vegetation monitoring and assessment. A key strength of optical-based approaches is the underlying mechanistic link to vegetation physiology, biochemistry, and structure that influences a spectral signal. By exploiting spectral variation driven by plant physiological response to environment, remotely sensed products can be used to estimate vegetation traits and functions. However, oftentimes these products are proxies based on covariance, which can lead to misinterpretation and decoupling under certain scenarios. This viewpoint will discuss (i) the optical properties of vegetation, (ii) applications of vegetation indices, solar-induced fluorescence, and machine-learning approaches, and (iii) how covariance can lead to good empirical proximation of plant traits and functions. Understanding and acknowledging the underlying mechanistic basis of plant optics must be considered as remotely sensed data availability and applications continue to grow. Doing so will enable appropriate application and consideration of limitations for the use of optical-based remote sensing for phenotyping applications.

Copper stress-induced phytotoxicity associated with photosynthetic characteristics and lignin metabolism in wheat seedlings

Copper (Cu) pollution is one of environmental problems that adversely affects the growth and development of plants. However, knowledge of lignin metabolism associated with Cu-induced phytotoxicity mechanism is insufficient. The objective of this study was to reveal the mechanisms underlying Cu-induced phytotoxicity by evaluating changes in the photosynthetic characteristics and lignin metabolism in the seedlings of wheat cultivar 'Longchun 30'. Treatment with varying concentrations of Cu clearly retarded seedling growth, as demonstrated by a reduction in the growth parameters. Cu exposure reduced the photosynthetic pigment content, gas exchange parameters, and chlorophyll fluorescence parameters, including the maximum photosynthetic efficiency, potential efficiency of photosystem II (PS II), photochemical efficiency of PS II in light, photochemical quenching, actual photochemical efficiency, quantum yield of PS II electron transport, and electron transport rate, but notably increased the nonphotochemical quenching and quantum yield of regulatory energy dissipation. Additionally, a significant increase was observed in the amount of cell wall lignin in wheat leaves and roots under Cu exposure. This increase was positively associated with the up-regulation of enzymes related to lignin synthesis, such as phenylalanine ammonia-lyase, 4-coumarate:CoA ligase, cinnamyl alcohol dehydrogenase, laccase, cell wall bound (CW-bound) guaiacol peroxidase, and CW-bound conifer alcohol peroxidase, and TaPAL, Ta4CL, TaCAD, and TaLAC expression. Correlation analysis revealed that lignin levels in the cell wall were negatively correlated with the growth of wheat leaves and roots. Taken together, Cu exposure inhibited photosynthesis in wheat seedlings, resulting from a reduction in photosynthetic pigment content, light energy conversion, and photosynthetic electron transport in the leaves of Cu-stressed seedlings, and the Cu-inhibitory effect on seedling growth was related to the inhibition of photosynthesis and an increase in cell wall lignification.

Drought-tolerance of transgenic winter wheat with partial suppression of the proline dehydrogenase gene

The global climate changes and the consequent increase in the number of soil and air droughts during the vegetation period of grain crops require the development of new strategies to adapt plants to those yield-decreasing stressors. A relevant way of increasing drought-tolerance of cereals is the use of biotechnological methods, particularly RNA interference, which can down-regulate the activity of plants’ genes and increase concentration of stress metabolites that perform osmoprotective functions during drought. We studied the tolerance to soil moisture shortage in transgenic plants of winter wheat with partial suppression of the proline dehydrogenase gene, obtained using the technology of short interfering RNAs. We analyzed physiological and biochemical parameters and structural elements of yield productivity of 4 wild genotypes and their transgenic lines with reduced activity of proline dehydrogenase in the conditions of 7-day drought during the late booting–ear emergence. We determined that the presence of double-stranded RNA suppressor of the proline dehydrogenase gene in transgenic lines led to increase in the level of accumulation of free proline in flag leaves. At the same time, its concentration in transgenic lines was higher than in untransformed plants of the wild genotypes in both drought conditions and conditions of sufficient moisture. We found that against the background of water deficiency, the total chlorophyll content in leaves of plants of transgenic lines was significantly higher, and the ratio of carotenoids to chlorophyll was lower than in plants of the wild genotypes, suggesting mitigation of the negative impact of drought on the plants of transgenic lines. Lacking soil moisture, genetically altered lines of wheat had significantly higher parameters of the structure of grain yield compared with untransformed genotypes. At the same time, we observed genotypic difference according to grain productivity in biotechnological plants. Therefore, the results we obtained confirm the perspectives of using the technology of short interfering RNAs to increase tolerance of winter wheat to water deficiency.

Multi-sensor spectral synergies for crop stress detection and monitoring in the optical domain: A review

Remote detection and monitoring of the vegetation responses to stress became relevant for sustainable agriculture. Ongoing developments in optical remote sensing technologies have provided tools to increase our understanding of stress-related physiological processes. Therefore, this study aimed to provide an overview of the main spectral technologies and retrieval approaches for detecting crop stress in agriculture. Firstly, we present integrated views on: i) biotic and abiotic stress factors, the phases of stress, and respective plant responses, and ii) the affected traits, appropriate spectral domains and corresponding methods for measuring traits remotely. Secondly, representative results of a systematic literature analysis are highlighted, identifying the current status and possible future trends in stress detection and monitoring. Distinct plant responses occurring under shortterm, medium-term or severe chronic stress exposure can be captured with remote sensing due to specific light interaction processes, such as absorption and scattering manifested in the reflected radiance, i.e. visible (VIS), near infrared (NIR), shortwave infrared, and emitted radiance, i.e. solar-induced fluorescence and thermal infrared (TIR). From the analysis of 96 research papers, the following trends can be observed: increasing usage of satellite and unmanned aerial vehicle data in parallel with a shift in methods from simpler parametric approaches towards more advanced physically-based and hybrid models. Most study designs were largely driven by sensor availability and practical economic reasons, leading to the common usage of VIS-NIR-TIR sensor combinations. The majority of reviewed studies compared stress proxies calculated from single-source sensor domains rather than using data in a synergistic way. We identified new ways forward as guidance for improved synergistic usage of spectral domains for stress detection: (1) combined acquisition of data from multiple sensors for analysing multiple stress responses simultaneously (holistic view); (2) simultaneous retrieval of plant traits combining multi-domain radiative transfer models and machine learning methods; (3) assimilation of estimated plant traits from distinct spectral domains into integrated crop growth models. As a future outlook, we recommend combining multiple remote sensing data streams into crop model assimilation schemes to build up Digital Twins of agroecosystems, which may provide the most efficient way to detect the diversity of environmental and biotic stresses and thus enable respective management decisions.

A Metabolomic Approach to Assess the Toxicity of the Olive Tree Endophyte Bacillus sp. PTA13 Lipopeptides to the Aquatic Macrophyte Lemna minor L

Pesticides represent a major human input into the ecosystem, posing a serious risk to non-target organisms. Therefore, there is pressure toward the reduction in their use and the discovery of alternative sources of bioactivity. Endophytic microorganisms represent a source of bioactivity, whose potential for plant protection has been recently established. In this context, an olive tree endophytic Bacillus sp. was isolated, exhibiting superior antifungal activity, mainly attributed to its major surfactin, iturin, and fengycin and the minor gageotetrin and bacilotetrin groups of lipopeptides (LP). Based on the potential of LP and the lack of information on their toxicity to aquatic organisms, we have investigated the toxicity of an LP extract to the model macrophyte Lemna minor L. The extract exhibited low phytotoxicity (EC₅₀ = 419 μg·mL^-1), and for the investigation of its effect on the plant, GC/EI/MS metabolomics was applied following exposure to sub-lethal doses (EC₂₅ and EC₅₀). Results revealed a general disturbance of plants' biosynthetic capacity in response to LP treatments, with substantial effect on the amino acid pool and the defense mechanism regulated by jasmonate. There are no previous reports on the phytotoxicity of LP to L. minor, with evidence supporting their improved toxicological profile and potential in plant protection.

Transcriptomics and metabolomics of engineered Synechococcus elongatus during photomixotrophic growth

Background

Converting carbon dioxide (CO₂) into value-added chemicals using engineered cyanobacteria is a promising strategy to tackle the global warming and energy shortage issues. However, most cyanobacteria are autotrophic and use CO₂ as a sole carbon source, which makes it hard to compete with heterotrophic hosts in either growth or productivity. One strategy to overcome this bottleneck is to introduce sugar utilization pathways to enable photomixotrophic growth with CO₂ and sugar (e.g., glucose and xylose). Advances in engineering mixotrophic cyanobacteria have been obtained, while a systematic interrogation of these engineered strains is missing. This work aimed to fill the gap at omics level.

Results

We first constructed two engineered Synechococcus elongatus YQ2-gal and YQ3-xyl capable of utilizing glucose and xylose, respectively. To investigate the metabolic mechanism, transcriptomic and metabolomic analysis were then performed in the engineered photomixotrophic strains YQ2-gal and YQ3-xyl. Transcriptome and metabolome of wild-type S. elongatus were set as baselines. Increased abundance of metabolites in glycolysis or pentose phosphate pathway indicated that efficient sugar utilization significantly enhanced carbon flux in S. elongatus as expected. However, carbon flux was redirected in strain YQ2-gal as more flowed into fatty acids biosynthesis but less into amino acids. In strain YQ3-xyl, more carbon flux was directed into synthesis of sucrose, glucosamine and acetaldehyde, while less into fatty acids and amino acids. Moreover, photosynthesis and bicarbonate transport could be affected by upregulated genes, while nitrogen transport and assimilation were regulated by less transcript abundance of related genes in strain YQ3-xyl with utilization of xylose.

Conclusions

Our work identified metabolic mechanism in engineered S. elongatus during photomixotrophic growth, where regulations of fatty acids metabolism, photosynthesis, bicarbonate transport, nitrogen assimilation and transport are dependent on different sugar utilization. Since photomixotrophic cyanobacteria is regarded as a promising cell factory for bioproduction, this comprehensive understanding of metabolic mechanism of engineered S. elongatus during photomixotrophic growth would shed light on the engineering of more efficient and controllable bioproduction systems based on this potential chassis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01760-1.

A Comparative Metabolomic Analysis Reveals Difference Manufacture Suitability in "Yinghong 9" and "Huangyu" Teas (Camellia sinensis)

"Yinghong 9" is a widely cultivated large-leaf variety in South China, and the black tea made from it has a high aroma and strong sweet flavor. "Huangyu" is a light-sensitive tea variety with yellow leaves. It was cultivated from the bud-mutation of "Yinghong 9" and has a very low level of chlorophyll during young shoot development. Due to chlorophyll being involved in carbon fixation and assimilation, the changes in photosynthesis might potentially affect the accumulation of flavor metabolites, as well as the quality of "Huangyu" tea. Although "Huangyu" has a golden yellow color and high amino acid content, the mechanism underlying the formation of leaf color and drinking value remains unclear. The widely targeted metabolomics and GC-MS analysis were performed to reveal the differences of key metabolites in fresh and fermented leaves between "Yinghong 9" and "Huangyu." The results showed that tea polyphenols, total chlorophyll, and carotenoids were more abundant in "Yinghong 9." Targeted metabolomics analysis indicated that kaempferol-3-glycoside was more abundant in "Yinghong 9," while "Huangyu" had a higher ratio of kaempferol-3-glucoside to kaempferol-3-galactoside. Compared with "Yinghong 9" fresh leaves, the contents of zeaxanthin and zeaxanthin palmitate were significantly higher in "Huangyu." The contents of α-farnesene, β-cyclocitral, nerolidol, and trans-geranylacetone, which were from carotenoid degradation and involved in flowery-fruity-like flavor in "Huangyu" fermented leaves, were higher than those of "Yinghong 9." Our results indicated that "Huangyu" was suitable for manufacturing non-fermented tea because of its yellow leaf and flowery-fruity-like compounds from carotenoid degradation.

Linking remote sensing parameters to CO2 assimilation rates at a leaf scale

Solar-induced chlorophyll fluorescence (SIF) and photochemical reflectance index (PRI) are expected to be useful for remote sensing of photosynthetic activity at various spatial scales. This review discusses how chlorophyll fluorescence and PRI are related to the CO₂ assimilation rate at a leaf scale. Light energy absorbed by photosystem II chlorophylls is allocated to photochemistry, fluorescence, and heat dissipation evaluated as non-photochemical quenching (NPQ). PRI is correlated with NPQ because it reflects the composition of xanthophylls, which are involved in heat dissipation. Assuming that NPQ is uniquely related to the photochemical efficiency (quantum yield of photochemistry), photochemical efficiencies can be assessed from either chlorophyll fluorescence or PRI. However, this assumption may not be held under some conditions such as low temperatures and photoinhibitory environments. Even in such cases, photosynthesis may be estimated more accurately if both chlorophyll fluorescence and PRI are determined simultaneously. To convert from photochemical efficiency to CO₂ assimilation, environmental responses in stomatal conductance also need to be considered. Models linking chlorophyll fluorescence and PRI with CO₂ assimilation rates will contribute to understanding and future prediction of the global carbon cycle.