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Maintenance of root growth is essential for plant adaptation to soil drying. Here, we tested the hypothesis that auxin transport is involved in mediating ABA's modulation by activating proton secretion in the root tip to maintain root growth under moderate water stress. Rice and Arabidopsis plants were raised under a hydroponic system and subjected to moderate water stress (−0.47 MPa) with polyethylene glycol (PEG). ABA accumulation, auxin transport and plasma membrane H+-ATPase activity at the root tip were monitored in addition to the primary root elongation and root hair density. We found that moderate water stress increases ABA accumulation and auxin transport in the root apex. Additionally, ABA modulation is involved in the regulation of auxin transport in the root tip. The transported auxin activates the plasma membrane H+-ATPase to release more protons along the root tip in its adaption to moderate water stress. The proton secretion in the root tip is essential in maintaining or promoting primary root elongation and root hair development under moderate water stress. These results suggest that ABA accumulation modulates auxin transport in the root tip, which enhances proton secretion for maintaining root growth under moderate water stress.

Water stress is one of the major environmental stresses causing growth retardation and yield loss of plants. In the past decades, osmotic adjustment, antioxidant protection, and stomatal movement have been extensively studied, but much less attention has been paid to the study of root system reprogramming to maximize water absorption and survival under water stress. Here, it is shown that polyethylene glycol (PEG)-simulated mild and moderate osmotic stress induced premature differentiation of the root apical meristem (RAM). It is demonstrated that RAM premature differentiation is a conserved adaptive mechanism that is widely adopted by various plants to cope with osmotic stress simulated by PEG 8000, and the occurrence of RAM premature differentiation is directly related to stress tolerance of plants. It is shown that the osmotic stress-induced premature differentiation caused growth cessation of primary roots allowing outgrowth of lateral roots. This work has uncovered a key mechanism for controlling the plastic development of the root system by which plants are capable of survival, growth, or reproduction under water stress.

Plant root border cells (RBCs) prevent the colonization of plant growth‐promoting rhizobacteria (PGPR) at the root tip, rendering the PGPR unable to effectively control pathogens infecting the root tip. In this study, we engineered four strains of Pseudomonas sp. UW4, a typical PGPR strain, each carrying an enhanced green fluorescent protein (EGFP)‐expressing plasmid. The UW4E strain harboured only the plasmid, whereas the UW4E‐flg22 strain expressed a secreted EGFP‐Flg22 fusion protein, the UW4E‐Flg(flg22) strain expressed a non‐secreted Flg22, and the UW4E‐flg22‐D strain expressed a secreted Flg22‐DNase fusion protein. UW4E‐flg22 and UW4E‐flg22‐D, which secreted Flg22, induced an immune response in wheat RBCs and colonized wheat root tips, whereas the other strains, which did not secrete Flg22, failed to elicit this response and did not colonize wheat root tips. The immune response revealed that wheat RBCs synthesized mucilage, extracellular DNA, and reactive oxygen species. Furthermore, the Flg22‐secreting strains showed a 33.8%–93.8% higher colonization of wheat root tips and reduced the root rot incidence caused by Rhizoctonia solani and Fusarium pseudograminearum by 24.6%–35.7% compared to the non‐Flg22‐secreting strains in pot trials. There was a negative correlation between the incidence of wheat root rot and colonization of wheat root tips by these strains. In contrast, wheat root length and dry weight were positively correlated with the colonization of wheat root tips by these strains. These results demonstrate that engineered secretion of Flg22 by PGPR is an effective strategy for controlling root rot and improving plant growth. The PGPR strain Pseudomonas sp. UW4, expressing secreted Flg22, induced an immune response in wheat root border cells, colonized wheat root tips, effectively controlled root rot, and enhanced wheat growth.

The root tip zone is regarded as the principal action site for iron (Fe) toxicity and is more sensitive than other root zones, but the mechanism underpinning this remains largely unknown. We explored the mechanism underpinning the higher sensitivity at the Arabidopsis root tip and elucidated the role of nitric oxide (NO) using NO-related mutants and pharmacological methods. Higher Fe sensitivity of the root tip is associated with reduced potassium (K+) retention. NO in root tips is increased significantly above levels elsewhere in the root and is involved in the arrest of primary root tip zone growth under excess Fe, at least in part related to NO-induced K+ loss via SNO1 (sensitive to nitric oxide 1)/SOS4 (salt overly sensitive 4) and reduced root tip zone cell viability. Moreover, ethylene can antagonize excess Fe-inhibited root growth and K+ efflux, in part by the control of root tip NO levels. We conclude that excess Fe attenuates root growth by effecting an increase in root tip zone NO, and that this attenuation is related to NO-mediated alterations in K+ homeostasis, partly via SNO1/SOS4.

Aim Rising air temperature and changing precipitation patterns already strongly influence forest ecosystems, yet large‐scale patterns of belowground root trait variation and their underlying drivers are poorly understood. Here, we investigated general patterns of root tip adjustments within fine‐root systems and the potential ecological implications of these patterns. Location Global. Methods We synthesize key fine‐root traits related to resource acquisition and determined their responses along climate and edaphic gradients. We specifically identified patterns of root tip abundance (number of root tips per dry biomass of fine roots ≤2 mm in diameter), and root tip density (number of root tips per soil volume) among angiosperm and gymnosperm trees to climate, edaphic gradients and stand properties. Results We found that angiosperm trees, which were more common in warmer, sometimes drier climates with more fertile soil, formed more root tips (higher root tip abundance, root tip density and higher slope of root tip density vs. fine‐root biomass) than gymnosperm trees, which lived in cooler, wetter climates with poor soil. Angiosperm and gymnosperm trees exhibited opposing trends in response to gradients in climate as gymnosperm trees tended to decrease root tip abundance and root tip density but alternatively increase mycorrhizal mycelial biomass with increasing MAT/MAP (ratio of mean annual temperature to mean annual precipitation), while angiosperm trees tended to increase root tip abundance and root tip density with increasing MAT/MAP. However, the individual trends of root tip abundance and root tip density for angiosperm and gymnosperm trees to MAT or MAP were more similar and often non‐significant. Main conclusions These results suggest disparate carbon or biomass adjustment strategies within gymnosperm and angiosperm tree fine‐root systems along climate gradients. Differences in angiosperm and gymnosperm tree adjustments in their fine‐root systems to changing environments have implications for how these plant groups are likely to perform in different environments and how their responses to future climate change should be modelled.

Aims Metacaspases are cysteine-dependent proteases, which play essential roles in programmed cell death (PCD), and caspase-3-like protease is the crucial executioner. However, its response mechanism to aluminum (Al)-induced PCD is still elusive. Methods Here, the type I metacaspase gene in peanut ( Arachis hypoganea L.), AhMC1 , was cloned from the Al-sensitive cultivar ZH2. Physiological and biochemical methods, as well as gene expression analyses, were employed to explore its function in Al-induced PCD in peanut root tips. Results AhMC1 had a 1068-bp open reading frame, encoding a peptide of 355 amino acids, and the purified protein exhibited a high caspase-3-like protease activity. Its expression levels in different tissues of peanut varieties ZH2 and 99–1507 (Al-tolerant) varied under Al-stress conditions. The subcellular localization indicated that AhMC1 was transferred from mitochondria into the cytoplasm. Furthermore, overexpressing AhMC1 reduced the resistance to Al stress. Sense transgenic plants showed a low relative root growth rate, and reduced superoxide dismutase, peroxidase, and catalase activities, compared with wild-type and antisense transgenic plants under Al-stress conditions, but had a high root-cell death rate, and increased Al and maleic dialdehyde contents. Conclusions The data suggest that metacaspase AhMC1 is a positive factor in Al-induced PCD in peanut root tips.

Plants are expected to play a critical role in the biological life support systems of crewed spaceflight missions, including in the context of upcoming missions targeting the Moon and Mars. Therefore, understanding the response of plants to spaceflight is essential for improving the selection and engineering of plants and spaceflight conditions. In particular, understanding the root-tip’s response to spaceflight is of importance as it is the center of orchestrating the development of the root, the primary organ for the absorption of nutrients and anchorage. GLDS-120 is a pioneering study by Paul et al. that used transcriptomics to evaluate the spaceflight response of the root-tip of the model plant Arabidopsis thaliana in dark and light through separate analyses of three genotype groups (Wassilewskija, Columbia-0, and Columbia-0 PhyD) and comparison of genotype responses. Here, we provide a complementary analysis of this dataset through a combined analysis of all samples while controlling for the genotypes in a paired analysis. We identified a robust transcriptional response to spaceflight with 622 DEGs in light and 200 DEGs in dark conditions. Gene enrichment analysis identified 37 and 13 significantly enriched terms from biological processes in light and dark conditions, respectively. Prominent enrichment for hypoxia-related terms in both conditions suggests hypoxia is a key stressor for root development during spaceflight. Additional enriched terms in light conditions include the circadian cycle, light response, and terms for the metabolism of flavonoid and indole-containing compounds. These results further our understanding of plants’ responses to the spaceflight environment.Key messageWe report a complementary analysis of GLDS-120 dataset, identifying a robust transcriptional response and prominent enrichment for hypoxia-related pathways by Arabidopsis thaliana’s root-tip during spaceflight.

Background and aims The adaptation of plants to land ecosystems involves complex rhizosphere interactions between organic matter and microbial communities. Border cells (BC) constitute the first living boundary in plant-soil ecosystems and play an important role in environmental sensing and signaling in response to different biotic and abiotic conditions. In this study, we evaluate the effect of humic acid on the release of BCs and its impact on the colonization of Herbaspirillum seropedicae at maize root tips. Methods Maize seedlings (1.0 ± 0.05 cm root length) were immersed for 48 h in solutions with different concentrations of humic acid (0, 12, 42, 143 and 500 mg L−1). Light and scanning electron microscopy were used to evaluate the structural interaction between border cells and H. seropedicae at the root tips. Results The release of BCs from root tips was significantly increased by humic acid (HA) application and exhibited a bell-shaped dose-response curve; the highest release of BCs occurred at 143 mg HA L−1 and was confirmed by microscopy. The colonization of roots by H. seropedicae strain RAM10 (tagged with green fluorescent protein, GFP) was monitored by epifluorescence microscopy with and without exogenous humic acid (143 mg L−1). Increased BC release resulted in a high density of diazotrophic bacteria at root tips, and bacteria sometimes aggregated with mucilage and humic acid particles, thus enhancing their viability. Increased BC numbers in response to humic acid might explain previous studies showing a concomitant increase in H. seropedicae populations in the rhizosphere, rhizoplane, and endosphere of grasses. Conclusions The population of H. seropedicae strain RAM10 colonizing root caps and BCs increased in response to exogenous humic acids.

Soil nitrogen (N) and phosphorus (P) shortages limit the growth of shrubs, and P shortage limit the growth of trees in karst ecosystems. Changes in fine root functional traits are the important strategies for plants to respond to such nutrient shortages. However, such responses in karst ecosystems are poorly known. To determine the responses of fine root functional traits to soil N and P changes and define their resource-use strategies in the ecosystem, we tested the specific root length (SRL), root tips over the root biomass (RT/RB), and N concentration (Nroot) in the fine roots of four plant species (two shrubs (Alchornea trewioides and Ligustrum sinense) and two trees (Celtis biondii and Pteroceltis tatarinowii)) during the dry (January) and the wet (July) season. The results showed that the SRL, RT/RB, and Nroot in the fine roots of shrub species were lower than those of tree species, and the three parameters were higher in the wet season than in the dry season. Linear regression models revealed that the SRL, RT/RB, and Nroot of overall species increased with increasing soil N and P concentrations and availabilities, and were positively correlated with increasing rhizosphere soil oxalic acid, microbial biomass carbon (C), and the activities of hydrolytic enzymes. In addition, the individual plant species had unique patterns of the three fine root traits that resulted affected by the change of soil nutrients and biochemistry. Thus, the specific root length, root tips over the root biomass, and N concentrations of fine roots were species-specific, affected by seasonal change, and correlated with soil nutrients and biochemistry. Our findings suggests that fine root functional traits increase the ability of plant species to tolerate nutrient shortage in karst ecosystems, and possibly indicated that a P-exploitative strategy in tree species and an N-conservative strategy in shrub species were exhibited.

Roots of Allium cepa were exposed to six CuO NPs suspensions (0, 5, 10, 20, 40, 80 mg L −1 ) in this study. Results revealed that with the increase of CuO NPs concentration, the Cu content in roots increased significantly. Compared to control, onion roots treated with CuO NPs (except 5 mg L −1 suspension) grew slowly after 24 h. The surface of the root cap and meristematic zone were obviously damaged. The apical meristem of roots treated by 10 mg L −1 and above concentrations stopped division. The nucleus of meristematic cells deformed, and nucleoli number increased. The plasmolysis occurred, and the cell membrane and nuclear membrane fractured. With the increase of CuO NPs concentration, the MDA content increased, and the root activity decreased. When dealt with 80 mg L −1 CuO NPs for 72 h, onion roots appeared to be corroded.