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Main conclusion In response to low nitrogen stress, multiple hormones together with nitric oxide signaling pathways work synergistically and antagonistically in crop root elongation. Changing root morphology allows plants to adapt to soil nutrient availability. Nitrogen is the most important essential nutrient for plant growth. An important adaptive strategy for crops responding to nitrogen deficiency is root elongation, thereby accessing increased soil space and nitrogen resources. Multiple signaling pathways are involved in this regulatory network, working together to fine-tune root elongation in response to soil nitrogen availability. Based on existing research, we propose a model to explain how different signaling pathways interact to regulate root elongation in response to low nitrogen stress. In response to a low shoot nitrogen status signal, auxin transport from the shoot to the root increases. High auxin levels in the root tip stimulate the production of nitric oxide, which promotes the synthesis of strigolactones to accelerate cell division. In this process, cytokinin, ethylene, and abscisic acid play an antagonistic role, while brassinosteroids and auxin play a synergistic role in regulating root elongation. Further study is required to identify the QTLs, genes, and favorable alleles which control the root elongation response to low nitrogen stress in crops.

Aims This paper aims to investigate the adaptation of maize root system architecture (RSA) in response to increasing planting densities. Methods A three-year field study was conducted with three planting densities (40,000, 70,000, and 90,000 plants per ha, which are abbreviated as D40000, D70000 and D90000, respectively). The dynamic change of root morphological traits and the 3-dimensional RSA were quantified. Results The grain yield per ha increased with increasing plant density from D40000 to D70000, and then decreased at D90000. Compared to D70000, high planting density of D90000 did not changed the total root biomass per ha but increased shoot biomass per ha by 4 to 8% in two of the three experimental years. Grain yield per plant and plant NPK concentration decreased with increasing planting density. Total accumulation of P and K per ha also decreased at D90000 compared to D70000. Root to shoot ratio was reduced at high planting density beginning 50 days after emergence. Compared to the control (D70000), total root length (TRL) per plant was reduced by 18 to 30% at D90000 and increased by 43 to 56% at D40000, root biomass per plant was reduced by 23 to 34% at D90000 and increased by 66 to 75% at D40000. High plant density reduced the number of nodal roots, lateral root density (LRD) and the average lateral root (LR) length, but with less effect on the length of axial roots. The RSA is characteristic of “intra-row contraction and inter-row extension”. Vertically, root growth in top soil layer (0- to 36- cm) was enhanced under supra-optimal plant density, but had a negligible effect in deep soil layers (36- to 60- cm). Conclusions To adapt to the limited photosynthesis capacity in the roots under high planting density, maize plants tend to reduce nodal root number and inhibit lateral root growth. They maintain nodal root length to explore a larger soil space, and adjust root growth in the intra-row and inter-row direction to avoid root-to-root competition.

Aims Arbuscular mycorrhizal fungi play important roles in plant phosphorus (P) accumulation. The aim of this study was to uncover how and to what extent soil plant-available P levels and maize genotypes influence the contribution of mycorrhizal P uptake pathway to plant P nutrition. Methods We selected an old genotype HMY and a modern genotype XY335, combined with 32 P labeling and qPCR to quantify P uptake efficiency of the direct pathway (DP) and the mycorrhizal pathway (MP) at three Olsen-P levels: 4.5 (low), 8 (medium) and 50 (high) mg kg −1 . Results The P uptake efficiency ratio PAE-MP/PAE-DP was highest in the treatment with medium Olsen-P, and was correlated positively with MP contribution. The traits of arbuscular mycorrhizal fungi, such as percent colonization, hyphal length density, P uptake per unit hyphae length, and the expression of the mycorrhiza-specific P transporter ZmPT1;6 were higher in XY335 than HMY in high-P soil, which was in accordance with the importance of the MP contribution. Conclusions Greater mycorrhizal responsiveness in the modern maize genotype than the old genotype under high P soil condition is related to higher P uptake efficiency of MP than DP; the inherent potential of MP can be maximized by managing soil plant P availability to achieve optimal P supply in intensive farming.

Nitrogen (N) plays a vital role in photosynthesis and crop productivity. Maize plants may be able to increase physiological N utilization efficiency (NUtE) under low-N stress by increasing photosynthetic rate (P n) per unit leaf N, that is, photosynthetic N-use efficiency (PNUE). In this study, we analyzed the relationship between PNUE and N allocation in maize ear-leaves during the grain-filling stage under low N (no N application) and high N (180 kg N ha(-1)) in a 2-year field experiment. Under low N, grain yield decreased while NUtE increased. Low-N treatment reduced the specific N content of ear leaves by 38% without significant influencing P n, thereby increasing PNUE by 54%. Under low-N stress, maize plants tended to invest relatively more N into bioenergetics to sustain electron transport. In contrast, N allocated to chlorophyll and light-harvesting proteins was reduced to control excess electron production. Soluble proteins were reduced to shrink the N storage reservoir. We conclude that optimization of N allocation within leaves is a key adaptive mechanism to maximize P n and crop productivity when N is limited during the grain-filling stage in maize under low-N conditions.

Aims To investigate genotypic differences in the plasticity of root system architecture in response to increasing planting density and understand how this plastic response affects grain yield. Methods A two-year field study was conducted with eight maize hybrids and three planting densities (60,000, 75,000, and 90,000 plants per ha). High-throughput imaging system and an automatic analysis method with Root Estimator for Shovelomics Traits (REST) software were adopted to study root architecture. The coefficient of variation (CV) was determined to reflect the plastic response of the root traits at different planting densities. Results Root size and root architecture varied with increasing plant density and among the genotypes. With increasing planting density, root biomass and root length per plant decreased. The average root opening angle in inter-row and intra-row directions (RA), average root maximal width in inter-row direction and intra-row directions (RMW), ratio of root opening angle between intra-row and inter-row directions (RatioRA), and ratio of root maximal width between intra-row and inter-row directions (RatioRMW) were also reduced. These results suggest that plants growing under high planting density have narrower root extension width, steeper root angle, and greater root distribution in inter-row direction. The CV of all the root traits between the neighboring plants increased with increasing plant density. Although significant genotype × planting density interactions occurred with most of the root traits, there was a quadratic correlation between grain yield and most of the root traits, especially at high planting density (R 2  = 0.17 ~ 0.48). There was a negative linear relationship between grain yield and the CV of root biomass, root length, RA, RMW, RatioRA, and RatioRMW (R 2  = 0.21 ~ 0.37). Among the eight hybrids, JQ202 had medium root size, more inter-row root distribution, and the smallest CV in root traits across three planting densities, and the highest grain yield. LY99, SR999 and DY39 had largest CV for root traits and the smallest grain yield. Conclusions Genotypes with less variation in root size, medium root size, medium broad root system and more inter-row root distribution help to reduce root-to-root competition and tend to have higher yield at high planting density.

Main conclusion Glutamine (Gln) is an efficient nitrogen source in promoting aboveground nitrogen and biomass accumulation in ZD958 (an elite maize hybrid with great potential for further genetic improvement) seedlings when conditioning a smaller but adequate root system. Amino acids account for a significant part of nitrogen (N) resources in the soil. However, how amino acid-N affects crop growth remains to be further investigated. Here, glutamine (Gln) application (80% NH 4 NO 3  + 20% Gln; mixed N) enhanced shoot growth of the maize hybrid ZD958. N concentration in the shoot increased, which is associated with favorable increases in SPAD values, GS/GOGAT activities, and accumulation of glutamate, asparagine, total free amino acids and soluble proteins in the shoot under mixed N. On the other hand, root growth was reduced when exposed to Gln as indicated by the significantly lower dry weight, root/shoot ratio, and primary, seminal, crown, and total root lengths, as well as unfavorable physiological alterations. Up-regulation of expression of ZmAMT1.3 , ZmNRT2.1 , and ZmAAP2 in the root and that of ZmAMT1.1 , ZmAMT1.3 , and ZmLHT1 in the shoot preconditioned N over-accumulation in the shoot and facilitated shoot growth, presumably via enhancing N translocation to the shoot, when Gln was supplied. Together, Gln is an efficient N source in promoting aboveground N and biomass accumulation in ZD958 seedlings when conditioning a smaller but adequate root system. Notably, ZD958′s parental lines Z58 and Chang7-2 displayed a wide range of variations in Gln responses, which may be partially attributed to single nucleotide polymorphisms (SNPs) in cis -elements and coding regions revealed in this study and much larger quantities of unidentified genetic variations between Z58 and Chang7-2. Extensive genetic divergence of these two elite inbred lines implied large potentials for further genetic improvement of ZD958 in relation to organic N use efficiency.

This study investigates the incipient motion and scouring of sediments around simulated submarine cables in a controlled flume experiment, focusing on five distinct grain sizes in an experimental pool. The measured incipient velocity values were compared with predictions from three established formulas, leading to a modification of the Sun Zhilin formula for improved accuracy. By incrementally increasing flow velocity, the scour depth and scour duration were measured required to expose cables buried at varying depths for different sediment sizes, and the relationships between scour rate, relative flow rate, and Froude number were analyzed. The results indicate that as the Froude number increases, both the relative flow velocity and scour rate increase, thereby enhancing the erosion of sediment. The modified formula demonstrated a higher consistency with observed scour depths, providing a reliable tool for assessing submarine cable exposure risks. These findings offer valuable insights for developing effective protection strategies to enhance cable stability in complex marine environments. This research highlights the importance of understanding sediment dynamics and their impact on submarine cable stability, contributing to the development of more effective protection strategies for submarine cables in dynamic seabed conditions.

The inhibitory effect of ammonium on primary root growth has been well documented; however the underlying physiological and molecular mechanisms are still controversial. To avoid ammonium toxicity to shoot growth, we used a vertical two-layer split plate system, in which the upper layer contained nitrate and the lower layer contained ammonium. In this way, nitrogen status was maintained and only the apical part of the root system was exposed to ammonium. Using a kinematic approach, we show here that 1 mM ammonium reduces primary root growth, decreasing both elemental expansion and cell production. Ammonium inhibits the length of elongation zone and the maximum elemental expansion rate. Ammonium also decreases the apparent length of the meristem as well as the number of dividing cells without affecting cell division rate. Moreover, ammonium reduces the number of root cap cells but appears to affect neither the status of root stem cell niche nor the distal auxin maximum at the quiescent center. Ammonium also inhibits root gravitropism and concomitantly down-regulates the expression of two pivotal auxin transporters, AUX1 and PIN2. Insofar as ammonium inhibits root growth rate in AUX1 and PIN2 loss-of-function mutants almost as strongly as in wild type, we conclude that ammonium inhibits root growth and gravitropism by largely distinct pathways.

Although the remobilization of vegetative nitrogen (N) and post-silking N both contribute to grain N in maize (Zea mays L.), their regulation by grain sink strength is poorly understood. Here we use 15N labeling to analyze the dynamic behaviors of both pre- and post-silking N in relation to source and sink manipulation in maize plants. The results showed that the remobilization of pre-silking N started immediately after silking and the remobilized pre-silking N had a greater contribution to grain N during early grain filling, with post-silking N importance increasing during the later filling stage. The amount of post-silking N uptake was largely driven by post-silking dry matter accumulation in both grain as well as vegetative organs. Prevention of pollination during silking had less effect on post-silking N uptake, as a consequence of compensatory growth of stems, husk + cob and roots. Also, leaves continuously export N even though grain sink was removed. The remobilization efficiency of N in the leaf and stem increased with increasing grain yield (hence N requirement). It is suggested that the remobilization of N in the leaf is controlled by sink strength but not the leaf per se. Enhancing post-silking N uptake rather than N remobilization is more likely to increase grain N accumulation.

In maize, shoot-borne roots dominate the whole root system and play essential roles in water and nutrient acquisition and lodging tolerance. Shoot-borne roots initiate at shoot nodes, including crown roots from the belowground nodes and brace roots from aboveground nodes. In contrast to crown roots, few genes for brace roots development have been identified. Here, we characterized a maize AP2/ERF transcription factor, ZmRAP2.7, to be involved in brace roots development. expressed in all types of roots, and the encoded protein localized in the nucleus with transcriptional activation activity. A maize transposon insert mutant defective in expression revealed a decreased number of brace roots but not crown roots. Maize mutant, which showed an elevated expression of , however, revealed an increased number of brace roots. The -based association analysis in a maize panel further identified a SNP marker at the fifth exon of gene to be associated with number of brace roots. These results uncovered a function of ZmRAP2.7 in brace roots development and provided the valuable gene and allele for genetic improvement of maize root systems.