An investigation into the beneficial effects and molecular mechanisms of humic acid on foxtail millet under drought conditions

Does humic acid foliar application affect growth and nutrient status of water-stressed maize?

Maize (Zea mays L.) is one of the world's most important crops, but its productivity is at high risk as climate change increases the risk of water stress. Therefore, the development of mitigation strategies to combat water stress in agriculture is fundamental to ensure food security. Humic acids are known to have a positive effect on drought tolerance, but data on their efficacy under waterlogging are lacking. This study aimed to elucidate the effect of a new humic acid product, a by-product of Ukrainian bentonite mining, on maize growth and nutrient status under waterlogging. Maize was grown for 9 weeks and three water stress treatments, which were applied for 14 days: waterlogging, alternating waterlogging and drought, and drought. On the day of stress application, the humic acid product (1% v/v) was applied to the leaves. Soil Plant Analysis Development (SPAD) values were recorded during the stress treatments. Plants were harvested after stressing ceased and fresh weight and P and Zn status were analyzed. Drought reduced shoot fresh weight, while it was unaffected under waterlogging. This is in contrast to SPAD readings, which showed a significant decrease over time under submergence, but not under drought. Under alternating stress, although SPAD values declined under waterlogging but stabilized when switched to drought, no growth reduction was apparent. Application of the humic acid product was ineffective in all cases. Although anthocyanin discoloration occurred under waterlogging stress, P deficiency, which is usually the main factor driving anthocyanin formation, was not the reason. Interestingly, Zn concentration decreased under waterlogging but not under the other stresses, which was alleviated by humic acid application. However, no effect of foliar-applied humic acids was observed under alternating and drought stress. It can be concluded that the tested humic acid product has the potential to improve the Zn status of maize under waterlogging.

Biostimulants: A sufficiently effective tool for sustainable agriculture in the era of climate change?

Climate change is currently considered as one of the main concerns of the agriculture sector, as it limits crop production and quality. Furthermore, the current context of global crisis with international political instability and war conflicts over the world is pushing the agriculture sector even more to urgently boost productivity and yield and doing so in a sustainable way in the current frame of climate change. Biostimulants can be an effective tool in alleviating the negative effects of environmental stresses to which plants are exposed, such as drought, salinity, heavy metals and extreme temperatures etc. Biostimulants act through multiple mechanisms, modifying gene expression, metabolism and phytohormone production, promoting the accumulation of compatible solutes and antioxidants and mitigating oxidative stress. However, it is important to keep in mind that the use and effect of biostimulants has limitations and must be accompanied by other techniques to ensure crop yield and quality in the current frame of climate change, such as proper crop management and the use of other sustainable resources. Here, we will not only highlight the potential use of biostimulants to face future agricultural challenges, but also take a critical look at their limitations, underlining the importance of a broad vision of sustainable agriculture in the context of climate change.

Improving grape fruit quality through soil conditioner: Insights from RNA-seq analysis of Cabernet Sauvignon roots

The application of fertilizers and soil quality are crucial for grape fruit quality. However, the molecular data linking different fertilizer (or soil conditioner [SC]) treatments with grape fruit quality is still lacking. In this study, we investigated three soil treatments, namely inorganic fertilizer (NPK, 343.5 kg/hm2 urea [N ≥ 46%]; 166.5 kg/hm2 P2O5 [P2O5 ≥ 64%]; 318 kg/hm2 K2O [K2O ≥ 50%]), organic fertilizer (Org, 9 t/hm2 [organic matter content ≥ 35%, N + P2O5 + K2O ≥ 13%]), and SC (SC, 3 t/hm2 [humic acid ≥ 38.5%; C, 56.1%; H, 3.7%; N, 1.5%; O, 38%; S, 0.6%]), on 4-year-old Cabernet Sauvignon grapevines. Compared with the NPK- and Org-treated groups, the SC significantly improved the levels of soluble solids, tannins, anthocyanins, and total phenols in the grape berries, which are important biochemical indicators that affect wine quality. Furthermore, we conducted RNA-seq analysis on the grapevine roots from each of the three treatments and used weighted gene co-expression network analysis to identify five hub genes that were associated with the biochemical indicators of the grape berries. Furthermore, we validated the expression levels of three hub genes (ERF, JP, and SF3B) and five selected genes related to anthocyanin biosynthesis (UFGT1, UFGT2, UFGT3, GST, and AT) by using quantitative reverse transcription-polymerase chain reaction. Compared to the NPK and Org treatment groups, the SC treatment resulted in a significant increase in the transcription levels of three hub genes as well as VvUFGT1, VvUFGT3, VvGST, and VvAT. These results suggest that the SC can improve grape fruit quality by altering gene transcription patterns in grapevine roots and further influence the biochemical indices of grape fruits, particularly anthocyanin content. This study reveals that the application of SC can serve as an important measure for enhancing vineyard SC and elevating grape quality.

Humic acid improves wheat growth by modulating auxin and cytokinin biosynthesis pathways

Humic acids have been widely used for centuries to enhance plant growth and productivity. The beneficial effects of humic acids have been attributed to different functional groups and phytohormone-like compounds enclosed in macrostructure. However, the mechanisms underlying the plant growth-promoting effects of humic acids are only partially understood. We hypothesize that the bio-stimulatory effect of humic acids is mainly due to the modulation of innate pathways of auxin and cytokinin biosynthesis in treated plants. A physiological investigation along with molecular characterization was carried out to understand the mechanism of bio-stimulatory effects of humic acid. A gene expression analysis was performed for the genes involved in auxin and cytokinin biosynthesis pathways in wheat seedlings. Furthermore, Arabidopsis thaliana transgenic lines generated by fusing the auxin-responsive DR5 and cytokinin-responsive ARR5 promoter to ß-glucuronidase (GUS) reporter were used to study the GUS expression analysis in humic acid treated seedlings. This study demonstrates that humic acid treatment improved the shoot and root growth of wheat seedlings. The expression of several genes involved in auxin (Tryptophan Aminotransferase of Arabidopsis and Gretchen Hagen 3.2) and cytokinin (Lonely Guy3) biosynthesis pathways were up-regulated in humic acid-treated seedlings compared to the control. Furthermore, GUS expression analysis showed that bioactive compounds of humic acid stimulate endogenous auxin and cytokinin-like activities. This study is the first report in which using ARR5:GUS lines we demonstrate the biostimulants activity of humic acid.

Milletdb: a multi-omics database to accelerate the research of functional genomics and molecular breeding of millets

Millets are a class of nutrient-rich coarse cereals with high resistance to abiotic stress; thus, they guarantee food security for people living in areas with extreme climatic conditions and provide stress-related genetic resources for other crops. However, no platform is available to provide a comprehensive and systematic multi-omics analysis for millets, which seriously hinders the mining of stress-related genes and the molecular breeding of millets. Here, a free, web-accessible, user-friendly millets multi-omics database platform (Milletdb, http://milletdb.novogene.com) has been developed. The Milletdb contains six millets and their one related species genomes, graph-based pan-genomics of pearl millet, and stress-related multi-omics data, which enable Milletdb to be the most complete millets multi-omics database available. We stored GWAS (genome-wide association study) results of 20 yield-related trait data obtained under three environmental conditions [field (no stress), early drought and late drought] for 2 years in the database, allowing users to identify stress-related genes that support yield improvement. Milletdb can simplify the functional genomics analysis of millets by providing users with 20 different tools (e.g., 'Gene mapping', 'Co-expression', 'KEGG/GO Enrichment' analysis, etc.). On the Milletdb platform, a gene PMA1G03779.1 was identified through 'GWAS', which has the potential to modulate yield and respond to different environmental stresses. Using the tools provided by Milletdb, we found that the stress-related PLATZs TFs (transcription factors) family expands in 87.5% of millet accessions and contributes to vegetative growth and abiotic stress responses. Milletdb can effectively serve researchers in the mining of key genes, genome editing and molecular breeding of millets.

Transcriptomic and Metabolomic Analysis of Soybean Nodule Number Improvements with the Use of Water-Soluble Humic Materials

Water-soluble humic materials (WSHMs) can enhance the nodule numbers of soybean plants. In this study, targeted metabolomics and transcriptomics were used to understand this mechanism. Results showed that 500 mg/L WSHM increased the adsorption and colonization of rhizobia in soybean roots. High-performance liquid chromatography and targeted metabolomics showed that WSHMs could regulate the content and distribution of endogenous hormones of soybean plants at the initial stage of soybean nodulation. Transcriptomic analysis showed a total of 2406 differentially expressed genes (DEGs) by the 25th day, accounting for 4.89% of total annotation genes (49159). These DEGs were found to contribute primarily to the MAPK signaling pathway, glycolysis/gluconeogenesis, and plant hormone signal transduction according to the -log 10 (Padjust) value in the KEGG pathway. Subsequently, DEGs related to these hormones were selected for verification using quantity-PCR. The WSHM increased the number of nodules by regulating the expression of endogenous hormones in soybean plants.

Effects of the Dark Septate Endophyte (DSE) Exophiala pisciphila on the Growth of Root Cell Wall Polysaccharides and the Cadmium Content of Zea mays L. under Cadmium Stress

This paper aims to investigate the mechanism by which dark septate endophytes (DSEs) enhance cadmium (Cd) tolerance in there host plants. Maize (Zea mays L.) was inoculated with a DSE, Exophiala pisciphila, under Cd stress at different concentrations (0, 5, 10, and 20 mg·kg⁻¹). The results show that, under 20 mg/kg Cd stress, DSE significantly increased maize biomass and plant height, indicating that DSE colonization can be utilized to increase the Cd tolerance of host plants. More Cd was retained in DSE-inoculated roots, especially that fixed in the root cell wall (RCW). The capability of DSE to induce a higher Cd holding capacity in the RCW is caused by modulation of the total sugar and uronic acid of DSE-colonized RCW, mainly the pectin and hemicellulose fractions. The fourier-transform spectroscopy analysis results show that carboxyl, hydroxyl, and acidic groups are involved in Cd retention in the DSE-inoculated RCW. The promotion of the growth of maize and improvement in its tolerance to Cd due to DSEs are related to restriction of the translocation of Cd from roots to shoots; resistance of Cd uptake Cd inside cells; and the increase in RCW-integrated Cd through modulating RCW polysaccharide components.

Transcriptomics of Biostimulation of Plants Under Abiotic Stress

Plant biostimulants are compounds, living microorganisms, or their constituent parts that alter plant development programs. The impact of biostimulants is manifested in several ways: via morphological, physiological, biochemical, epigenomic, proteomic, and transcriptomic changes. For each of these, a response and alteration occur, and these alterations in turn improve metabolic and adaptive performance in the environment. Many studies have been conducted on the effects of different biotic and abiotic stimulants on plants, including many crop species. However, as far as we know, there are no reviews available that describe the impact of biostimulants for a specific field such as transcriptomics, which is the objective of this review. For the commercial registration process of products for agricultural use, it is necessary to distinguish the specific impact of biostimulants from that of other legal categories of products used in agriculture, such as fertilizers and plant hormones. For the chemical or biological classification of biostimulants, the classification is seen as a complex issue, given the great diversity of compounds and organisms that cause biostimulation. However, with an approach focused on the impact on a particular field such as transcriptomics, it is perhaps possible to obtain a criterion that allows biostimulants to be grouped considering their effects on living systems, as well as the overlap of the impact on metabolism, physiology, and morphology occurring between fertilizers, hormones, and biostimulants.