Uwe Ludewig

Phosphorus (P) is an essential plant nutrient with low bioavailability in soils, and its uptake and that of nitrogen (N) are crucial for optimal growth. Here, diverse maize genotypes with distinct breeding histories, including doubled haploids from landraces, were grown under two P levels. The genotypes showed substantial variation in internal P concentrations and in performance under low P. Several lines maintained adequate N regardless of P supply; however, two lines from the heterotic dent pool performed poorly under low P and exhibited N deficiency. Only these lines lacked a robust transcriptional phosphate starvation response (PSR) in roots. Although N-related gene expression varied among lines, it did not explain differences in N acquisition. Inositol pyrophosphates (PP-InsPs), potential key regulators of P status signalling, were similarly diminished in contrasting lines under P deficiency, suggesting that they may be required, but not sufficient for PSR responses in roots. Overall, the results reveal large genotypic variation in low-P responses, line-specific N–P interactions at the post-transcriptional level and a critical role of shoot-root communication, which are discussed in the context of differential responses to P availability.

Background: Plant beneficial microorganisms as inoculants can improve crop performance, but factors affecting their impact on plant performance under field conditions remain unclear, thereby limiting their use in farming. Here, we investigated how farming practices (e.g., tillage and N-fertilization intensity) and growing seasons influenced the impact of a beneficial microorganism consortium (BMc: Trichoderma, Bacillus, and Pseudomonas strains) in maize and affected the rhizosphere competence of each BMc strain. In addition, we tested whether the consortium affects the resident rhizosphere microbiome and crop performance. In two growing seasons (2020 and 2021), we assessed how BMc inoculation affects maize growth, nutritional status, gene expression, and rhizosphere microbiome under different farming practices at the flowering stage. Results: Inoculated strains successfully colonized the maize rhizosphere independently of farming practice. BMc inoculation improved plant growth and iron uptake in 2020, regardless of farming practice. These effects co-occurred with lower precipitation levels in 2020 compared to 2021. BMc inoculation reduced the expression of several stress-related genes in maize in 2020 under drought. An increased iron uptake by the BMc-inoculated plants was observed in 2020 and was associated with the upregulation of the gene ZmNAS3, which is linked to iron uptake. Therefore, BMc inoculation mitigated the drought impact on maize. The microbial rhizosphere communities were altered by BMc inoculation in both years, but patterns of responder taxa differed between seasons. Metagenome analysis revealed that more genes (e.g., genes encoding biosurfactants and siderophores) were enriched in the rhizosphere of BMc-inoculated plants in 2020 than in 2021. Moreover, we identified bacterial and fungal taxa positively associated with maize iron uptake. The relative abundance of these iron uptake-associated bacterial and fungal taxa significantly increased due to BMc inoculation in 2020, while they showed overall higher relative abundances in 2021, independently of BMc inoculation. We mapped the sequences of these iron-associated taxa to publicly available genomes and verified the occurrence of various plant beneficial traits in several mapped genomes. Conclusions: Overall, we show that the growing season determined the effect of BMc inoculation on maize plants by shaping microbiome composition and function in the maize rhizosphere more than farming practice. These findings highlight the importance of the complex interplay between microbial inoculants and the resident rhizosphere microorganisms under abiotic stress conditions.

The race-specific resistance gene Lr14a in wheat confers a unique type of heterogenous resistance reaction. It encodes an ankyrin-repeat transmembrane domain protein that confers immunity against the fungal pathogen Puccinia triticina . Here, we show that Lr14a functions as a calcium-permeable channel, mediating cytoplasmic Ca²⁺ influx that is crucial for leaf rust resistance in wheat. Infection with avirulent isolates induced Lr14a expression predominantly in mesophyll cells while triggering cell death in guard cells in wheat. This study revealed a mechanism by which the product of an R gene regulates stomatal immunity non-cell autonomously through the mediation of calcium signaling.

Due to shallow root systems, potato is a particularly drought-sensitive crop. To counteract these limitations, the application of plant growth-promoting microorganisms (PGPMs) is discussed as a strategy to improve nutrient acquisition and biotic and abiotic stress resilience. However, initial root colonization by PGPMs, in particular, can be affected by stress factors that negatively impact root growth and activity or the survival of PGPMs in the rhizosphere. In this study, perspectives for the use of commercial silicate-based soil conditioners (SCs) supposed to improve soil water retention were investigated. The SC products were based on combinations with lignocellulose polysaccharides (Sanoplant^® = SP) or polyacrylate (Geohumus^® = GH). It was hypothesized that SC applications would support beneficial plant–inoculant interactions (arbuscular mycorrhiza, AM: Rhizophagus irregularis MUCL41833, and Pseudomonas brassicacearum 3Re2-7) on a silty loam soil–sand mixture under water-deficit conditions (6–12 weeks at 15–20% substrate water-holding capacity, WHC). Although no significant SC effects on WHC and total plant biomass were detectable, the SC-inoculant combinations increased the proportion of leaf biomass not affected by drought stress symptoms (chlorosis, necrosis) by 66% (SP) and 91% (GH). Accordingly, osmotic adjustment (proline, glycine betaine accumulation) and ROS detoxification (ascorbate peroxidase, total antioxidants) were increased. This was associated with elevated levels of phytohormones involved in stress adaptations (abscisic, jasmonic, salicylic acids, IAA) and reduced ROS (H₂O₂) accumulation in the leaf tissue. In contrast to GH, the SP treatments additionally stimulated AM root colonization. Finally, the SP-inoculant combination significantly increased tuber biomass (82%) under well-watered conditions, and a similar trend was observed under drought stress, reaching 81% of the well-watered control. The P status was sufficient for all treatments, and no treatment differences were observed for stress-protective nutrients, such as Zn, Mn, or Si. By contrast, GH treatments had negative effects on tuber biomass, associated with excess accumulation of Mn and Fe in the leaf tissue close to toxicity levels. The findings suggest that inoculation with the PGPMs in combination with SC products (SP) can promote physiological stress adaptations and AM colonization to improve potato tuber yield, independent of effects on soil water retention. However, this does not apply to SC products in general.

This study investigated the drought protection effects of six fungal and bacterial inoculants and ten consortia thereof on vegetative growth, nutritional status, and tuberization of potato under controlled and field conditions. It was hypothesized that microbial consortia offer improved drought protection as compared with single strains, due to complementary or synergistic effects, with differential impacts also of N fertilization management. Under NO₃⁻ fertilization, a 70% reduction in water supply over six weeks reduced shoot and tuber biomass of non-inoculated plants by 30% and 50%, respectively, and induced phosphate (P) limitation compared to the well-watered control. The P nutritional status was significantly increased above the deficiency threshold by three single-strain inoculants and eight consortia. This was associated with the presence of the arbuscular mycorrhizal fungus (AMF) inoculant Rhizophagus irregularis MUCL41833 (five cases) and stimulation of root growth (five cases). Additionally, Bacillus amyloliquefaciens FZB42 and AMF + Pseudomonas brassicacearum 3Re2-7 significantly reduced irreversible drought-induced leaf damage after recovery to well-watered conditions. However, the microbial inoculants did not mitigate drought-induced reductions in tuber biomass, neither in greenhouse nor in field experiments. By contrast, NH₄⁺-dominated fertilization significantly increased tuber biomass under drought stress (534%), which was further increased by additional AMF inoculation (951%). This coincided with (i) improved enzymatic detoxification of drought-induced reactive oxygen species (ROS), (ii) improved osmotic adjustment in the shoot tissue (glycine betaine accumulation), (iii) increased shoot concentrations of ABA, jasmonic acid, and indole acetic acid, involved in drought stress signaling and tuberization, and (iv) reduced irreversible drought-induced leaf damage. Additional application of bacterial inoculants further improved ROS detoxification by increasing the production of antioxidants but stimulated biomass allocation towards shoot growth at the expense of tuber development. The results demonstrated that microbial consortia could increase the probability of drought protection effects influenced by the form of N supply. However, protective effects on vegetative growth do not necessarily translate into yield benefits, which can be achieved by adequate combination of inoculants and fertilizers.

White lupin (lupinus albus L.) forms special bottlebrush‐like root structures called cluster roots (CR) when phosphorus is low, to remobilise sparingly soluble phosphates in the soil. The molecular mechanisms that control the CR formation remain unknown. Root development in other plants is regulated by CLE (CLAVATA3/ EMBRYO SURROUNDING REGION (ESR)‐RELATED) peptides, which provide more precise control mechanisms than common phytohormones. This makes these peptides interesting candidates to be involved in CR formation, where fine tuning to environmental factors is required. In this study we present an analysis of CLE peptides in white lupin. The peptides LaCLE35 (RGV_HyPSGANPLHN) and LaCLE55 (RRV_HyPSC_HyPDPLHN) reduced root growth and altered CR in hydroponically cultured white lupins. We demonstrate that rootlet density and rootlet length were locally, but not systemically, impaired by exogenously applied CLE35. The peptide was identified in the xylem sap. The inhibitory effect of CLE35 on root growth was attributed to arrested cell elongation in root tips. Taken together, CLE peptides affect both rootlet density and rootlet length, which are two critical factors for CR formation, and may be involved in fine tuning this peculiar root structure that is present in a few crops and many Proteaceae species, under low phosphorus availability.

Microbiome-based solutions are regarded key for sustainable agroecosystems. However, it is unclear how agricultural practices affect the rhizosphere microbiome, plant–microorganism interactions and crop performance under field conditions. Therefore, we installed root observation windows in a winter wheat field cultivated either under long-term mouldboard plough (MP) or cultivator tillage (CT). Each tillage practice was also compared at two nitrogen (N) fertilization intensities, intensive (recommended N-supply with pesticides/growth regulators) or extensive (reduced N-supply, no fungicides/growth regulators). Shoot biomass, root exudates and rhizosphere metabolites, physiological stress indicators, and gene expression were analyzed together with the rhizosphere microbiome (bacterial/archaeal 16S rRNA gene, fungal ITS amplicon, and shotgun metagenome sequencing) shortly before flowering. Compared to MP, the rhizosphere of CT winter wheat contained more primary and secondary metabolites, especially benzoxazinoid derivatives. Potential copiotrophic and plant-beneficial taxa (e.g. Bacillus, Devosia, and Trichoderma) as well as functional genes (e.g. siderophore production, trehalose synthase, and ACC deaminase) were enriched in the CT rhizosphere, suggesting that tillage affected belowground plant–microorganism interactions. In addition, physiological stress markers were suppressed in CT winter wheat compared to MP. In summary, tillage practice was a major driver of crop performance, root deposits, and rhizosphere microbiome interactions, while the N-fertilization intensity was also relevant, but less important.

Conventional chemical crop protection with pesticides is increasingly seen as being critical, because of pesticide residues in food and the environment. Integrated alternative management strategies such as crop rotations and soil management might also involve the targeted use of certain mineral fertilizers with benefits for plant health. A key element required for healthy crops is nitrogen, which is applied at differing dosages in various chemical forms, all with distinct effects on crop physiology and plant growth. Here, we review classical and more recent evidence for the crop disease-protective effects of nitrogen and various chemical nitrogen forms. We conclude that simple general statements concerning disease-protective roles in agricultural environments remain elusive, although complex plant-soil microbial interaction networks are becoming increasingly understood. The health of modern varieties might be substantially improved by certain chemical nitrogen fertilizer forms, particularly when the disease-causing fungal species are known.

Phosphorus (P) is an essential nutrient for plant growth and development, as well as an important factor limiting sustainable maize production. Targeted nitrogen (N) fertilization in the form of ammonium has been shown to positively affect Pi uptake under P-deficient conditions compared to nitrate. Nevertheless, its profound effects on root traits, P uptake, and soil microbial composition are still largely unknown. In this study, two maize genotypes F160 and F7 with different P sensitivity were used to investigate phosphorus-related root traits such as root hair length, root diameter, AMF association, and multiple P efficiencies under P limitation when fertilized either with ammonium or nitrate. Ammonium application improved phosphorous acquisition efficiency in the F7 genotype but not in F160, suggesting that the genotype plays an important role in how a particular N form affects P uptake in maize. Additionally, metabarcoding data showed that young maize roots were able to promote distinct microbial taxa, such as arbuscular mycorrhizal fungi, when fertilized with ammonium. Overall, the results suggest that the form of chemical nitrogen fertilizer can be instrumental in selecting beneficial microbial communities associated with phosphorus uptake and maize plant fitness.