Nitrate Regulates Maize Root Transcriptome through Nitric Oxide Dependent and Independent Mechanisms

Maize root responds to nitrate by modulating its development through the coordinated action of many interacting players. Nitric oxide is produced in primary root early after the nitrate provision, thus inducing root elongation. In this study, RNA sequencing was applied to discover the main molecular signatures distinguishing the response of maize root to nitrate according to their dependency on, or independency of, nitric oxide, thus discriminating the signaling pathways regulated by nitrate through nitric oxide from those regulated by nitrate itself of by further downstream factors. A set of subsequent detailed functional annotation tools (Gene Ontology enrichment, MapMan, KEGG reconstruction pathway, transcription factors detection) were used to gain further information and the lateral root density was measured both in the presence of nitrate and in the presence of nitrate plus cPTIO, a specific NO scavenger, and compared to that observed for N-depleted roots. Our results led us to identify six clusters of transcripts according to their responsiveness to nitric oxide and to their regulation by nitrate provision. In general, shared and specific features for the six clusters were identified, allowing us to determine the overall root response to nitrate according to its dependency on nitric oxide.

Natural variation in Arabidopsis ISR1 affects iron localization and induced systemic resistance

Beneficial root-associated bacteria can induce systemic resistance (ISR) to foliar pathogens and there is known transcriptional and genetic overlap in the root response to iron deficiency and ISR. A previous study found that there is natural variation in ISR among Arabidopsis accessions. The Ws accession is deficient in ISR, and the responsible recessive genetic locus, named ISR1, was mapped to chromosome 3. To find candidate genes that may underlie ISR deficiency in Ws, we identified genes that are induced in response to the ISR-triggering bacterium Pseudomonas simiae WCS417 and to iron stress and that have non-synonymous mutations in the Ws genome with respect to the ISR-responsive Col-0. We identified a kelch-domain containing protein encoded by At3g07720 that has a genomic rearrangement in Ws. We found that overexpression of Col-0 At3g07720 restores ISR to Ws, indicating that At3g07720 encodes ISR1. Isr1 loss of function mutants do not affect plant growth under iron limiting conditions but have increased levels of apoplastic iron. We found that iron supplementation, P. simiae WCS417, or a loss of isr1 enhance ROS production in a non-additive manner, suggesting they work through the same mechanism to enhance resistance. Our findings show that ISR1 is required for iron localization, immunity, and ISR, and suggest that increased iron uptake induced by ISR-eliciting bacteria may directly contribute to immunity through increased reactive oxygen production.

Root Biomass Distribution of Populus sibirica and Ulmus pumila Afforestation Stands Is Affected by Watering Regimes and Fertilization in the Mongolian Semi-arid Steppe

Desertification of the semi-arid steppe of Mongolia is advancing very rapidly, motivating afforestation efforts. The “Green Belt” joint project (Government of Mongolia and Republic of Korea), which aims to mitigate soil degradation and develop agroforestry activities through the planting of a forest shelterbelt, is one such response. In these plantations, tree growth has been supported by different watering regimes (no watering, 2, 4, and 8 L h−1) and by two types of soil fertilization (NPK and Compost). The present paper analyses the effect of these techniques on soil chemistry and root biomass partitioning of Populus sibirica (Horth ex Tausch) and Ulmus pumila (L.) tree species. In July 2019, at the plantation site in Lun Soum, Tuv province (Mongolia), six trees were excavated by hand in each treatment, the root system was divided into taproot and five diameter classes (0–2; 2–5; 5–10; 10–20; > 20 mm), and the biomass was measured. Soil organic matter, macronutrients, and pH were also measured. The addition of fertilizers in the long-term did not enhance the soil chemical properties. The build-up of root biomass in both species correlated positively with increasing levels of the watering, while the application of fertilizers led to root growth suppression. For most of the root classes and both species, an irrigation level of 4 L h−1 was sufficient to yield the highest biomass and could be recommended for afforesting the semi-arid steppe of Mongolia. The root biomass of P. sibirica was more dependent on the watering regimes and of U. pumila was more negatively influenced by the application of fertilizers, indicating that U. pumila, due to the its lower water need, could be suitable for afforesting semi-arid environments. Our experiments suggest that afforestation practices in the semi-arid steppe of Mongolia should be supported by a prior analysis of plants' needs, soil type, dose, and type of fertilizers to be applied. Knowledge of the root response to the supporting techniques is necessary for choosing the best one for the plantation and, thus, to develop a sustainable and successful strategy to restore these degraded lands.

Mechanisms of Root Response to Manganese Stress in Wheat Seedlings and Significance of Selenium Supplementation – Biochemical and Cytological Studies

In recent years cultivated soils have been increasingly supplemented with nutrients that at low doses are necessary for proper plant functioning but become toxic at high doses. New methods are needed to prevent these destructive actions, and for this reason we studied the effects of two elements – Mn treated as a stressor and Se treated as a potential defense in two wheat cultivars. The intensity of stress was manifested in tissue browning and weight reduction and was determined by an increase in lipid peroxidation and quantitative analysis of hydrogen peroxide levels. It was found that the excess of Mn in the substrate caused more intense changes in these indicators in the root system than in the leaves, and that Se presence partly eliminated the stress evoked effects. Moreover, Mn-treatment was accompanied by a greater absorption of this element by the roots, and a reduced uptake of other elements (K, Fe, S, P), with the exception of Ca, an increase in which was observed especially in the additional presence of Se. It was suggested that the rise in Ca level can lead to modification of cell differentiations and may be one of the steps in defense mechanisms. The change in the direction of cell differentiation in the apical part of the root was observed microscopically under Mn stress and was accompanied by a quantitative increase in 5-met C. Based on DNA methylation profiles detected by MSAP we concluded that various types of methylation sites may be activated under Mn treatment in roots.

Strigolactones And Auxin Cooperate To Regulate Maize Root Development and Response to Nitrate

In maize, nitrate regulates root development thanks to the coordinated action of many players. In this study, the involvement of SLs and auxin as putative components of the nitrate regulation of lateral root was investigated. To this aim, the endogenous SL content of maize root in response to nitrate was assessed by LC-MS/MS and measurements of lateral root density in the presence of analogues or inhibitors of auxin and strigolactones were performed. Furthermore, an untargeted RNA-seq based approach was used to better characterize the participation of auxin and strigolactones to the transcriptional signature of maize root response to nitrate. Our results suggested that N deprivation induces zealactone and carlactonoic acid biosynthesis in root, to a higher extent if compared to P-deprived roots. Moreover, data on lateral root density led to hypothesise that the induction of LR development early occurring upon nitrate supply involves the inhibition of SL biosynthesis, but that the downstream target of SL shutdown, beside auxin, includes also additional unknown players. Furthermore, RNA-seq results provided a set of putative markers for the auxin- or SL-dependent action of nitrate, meanwhile allowing to identify also novel components of the molecular regulation of maize root response to nitrate. Globally the existence of at least four different pathways was hypothesised, one dependent on auxin, a second one mediated by SLs, a third deriving from the SL-auxin interplay and one last attributable to nitrate itself through further downstream signals. Further work will be necessary to better assess the reliability of the model proposed.

Arabidopsis Sucrose Transporter 4 (AtSUC4) is Involved in Root Response of High Content Sucrose-Growth Via IAA- and ABA- Dependent Manner

Source-to-sink transport of sucrose mediated by sucrose transporters (SUCs) is one of the major determinants of plant growth. However, the role of AtSUC4, the only member of Group IV sucrose transporter in Arabidopsis, was undervalued in sink organ during seedling period. In our study, the primary root length of the atsuc4 mutants was significantly longer than that of the wild-type (WT) under exogenous 4% and 6% sucrose treatment. But this phenotype could not be imitated by external application of glucose or mannitol. It means that the atsuc4 mutants were insensitive to high sucrose stress compared with WT. Meanwhile, HPLC-MS/MS results showed that the root of atsuc4 mutants accumulated less sucrose and ABA and more IAA content compared with WT on 4% and 6% sucrose supplementation. Transcriptome analysis showed that many key genes involved in IAA and ABA signals were respective stimulated and repressed in the atsuc4 mutants, respectively. Taken together, we concluded that the deficiency of AtSUC4, not only reducing the transported and uptaked of sucrose, but also as a signal, may be collaborated with IAA and ABA to regulate root growth under high sucrose stress. This study confirmed the new function of AtSUC4, and provided an promising candidate gene for improving tolerance to high sucrose stress.

Advances in understanding plant root response to nematode attack

Plant parasitic nematodes are damaging pests on all crops grown across the world. They exploit plants using a range of strategies, ranging from simple browsing ectoparasitism to highly complex biotrophic endoparasites. Some nematodes induce the formation of complex feeding structures in the roots of their hosts that require extensive reprogramming of host gene expression. These changes include changes in fundamentally important plant processes, including the cell cycle. Natural resistance can be used to control plant nematodes, and great progress has been made in mapping and identifying resistance genes against nematodes. Recent work has shown that the dependence of nematodes on a feeding structure has allowed plants to evolve new mechanisms of resistance that target this structure with a toxic response.

Advances in understanding plant root response to weedy root parasites

This chapter addresses advances in understanding plant root responses to weedy root parasites. It begins by reviewing host-parasitic weed interactions, focusing specifically on seed dispersal and germination and the possibility of host infection as a consequence of germination. The chapter then moves on to discuss host plant pre-penetration and post-penetration defence mechanisms. It concludes by emphasising the importance of developing management strategies for parasitic weed management.