Root Cortical Aerenchyma Formation Research Articles

Transcription factor bHLH121 regulates root cortical aerenchyma formation in maize

Root anatomical phenotypes present a promising yet underexploited avenue to deliver major improvements in yield and climate resilience of crops by improving water and nutrient uptake. For instance, the formation of root cortical aerenchyma (RCA) significantly increases soil exploration and resource capture by reducing the metabolic costs of root tissue. A key bottleneck in studying such phenotypes has been the lack of robust high-throughput anatomical phenotyping platforms. We exploited a phenotyping approach based on laser ablation tomography, termed Anatomics, to quantify variation in RCA formation of 436 diverse maize lines in the field. Results revealed a significant and heritable variation for RCA formation. Genome-wide association studies identified a single-nucleotide polymorphism mapping to a root cortex-expressed gene-encoding transcription factor bHLH121. Functional studies identified that the bHLH121 Mu transposon mutant line and CRISPR/Cas9 loss-of-function mutant line showed reduced RCA formation, whereas an overexpression line exhibited significantly greater RCA formation when compared to the wild-type line. Characterization of these lines under suboptimal water and nitrogen availability in multiple soil environments revealed that bHLH121 is required for RCA formation developmentally as well as under studied abiotic stress. Overall functional validation of the bHLH121 gene's importance in RCA formation provides a functional marker to select varieties with improved soil exploration and thus yield under suboptimal conditions.

Genome-wide association study reveals quantitative trait loci for waterlogging-triggered adventitious roots and aerenchyma formation in common wheat.

Waterlogging severely affects wheat growth and development. Limited availability of oxygen in the root zone negatively affects the metabolism of plants. The formation of adventitious roots (ARs) and root cortical aerenchyma (RCA) are the most important adaptive trait contributing to plants' ability to survive in waterlogged soil conditions. This study used a genome-wide association study (GWAS) approach with 90K single nucleotide polymorphisms (SNPs) in a panel of 329 wheat genotypes, to reveal quantitative trait loci (QTL) conferring ARs and RCA. The wheat genotypes exposed to waterlogging were evaluated for ARs and RCA in both field and glasshouse over two consecutive years. Six and five significant marker-trait associations (MTAs) were identified for ARs and RCA formation under waterlogging, respectively. The most significant MTA for AR and RCA was found on chromosome 4B. Two wheat cultivars with contrasting waterlogging tolerance (tolerant: H-242, sensitive: H-195) were chosen to compare the development and regulation of aerenchyma in waterlogged conditions using staining methods. Results showed that under waterlogging conditions, H2O2 signal generated before aerenchyma formation in both sensitive and tolerant varieties with the tolerant variety accumulating more H2O2 and in a quicker manner compared to the sensitive one. Several genotypes which performed consistently well under different conditions can be used in breeding programs to develop waterlogging-tolerant wheat varieties.

Impacts of barley root cortical aerenchyma on growth, physiology, yield components, and grain quality under field waterlogging conditions

Waterlogging is a major ecological threat that restricts crop growth and yield in high rainfall zones across the globe. Waterlogging tolerance is conferred by several complimentary mechanisms, with root cortical aerenchyma (RCA) formation being one of the major ones. In this study, three pairs of near-isogenic lines (NILs) of commercial barley varieties which we introgressed a quantitative trait locus (QTL) for RCA formation, and eight other genotypes contrasting in waterlogging tolerance were subjected to growth under waterlogged and control (non-waterlogged) conditions. Field trials were conducted in 2019 and 2020. The waterlogging treatment began at the two-three leaf stage and continued for two months. The water was subsequently drained, and the crops allowed to recover for final grain yield. Under waterlogging conditions, genotypes with RCA-QTL had significantly higher aerenchyma, white adventitious roots, leaf normalised difference vegetation index (NDVI), higher shoot biomass, tiller number, compared with genotypes without RCA-QTL. Across the three commercial varieties in waterlogged conditions, the average boost to yields from the introgression was 1.8 t/ha. The genotype YF225 showed a degree of waterlogging tolerance but had no RCA-QTL. This appears to be an alternate waterlogging tolerance mechanism that is not yet understood. Importantly, the addition of the RCA-QTL to three commercial varieties showed no significant negative effects on plant growth, yield, and grain quality attributes under control conditions. We conclude that the introgression of an RCA-QTL into other varieties is a promising breeding target for mitigating losses caused by waterlogging. • The formation of aerenchyma can significantly increase root survival and final grain yield under waterlogging conditions. • The addition of the QTL for aerenchyma formation showed no significant effects on plant growth under control conditions. • Pyramiding RCA-QTL with other tolerance mechanisms could create varieties with significantly better waterlogging tolerance.

Genome-Wide Association Study Reveals Marker Trait Associations (MTA) for Waterlogging-Triggered Adventitious Roots and Aerenchyma Formation in Barley.

Waterlogging is an environmental stress, which severely affects barley growth and development. Limited availability of oxygen in the root zone negatively affects the metabolism of the whole plant. Adventitious roots (AR) and root cortical aerenchyma (RCA) formation are the most important adaptive traits that contribute to a plant’s ability to survive in waterlogged soil conditions. This study used a genome-wide association (GWAS) approach using 18,132 single nucleotide polymorphisms (SNPs) in a panel of 697 barley genotypes to reveal marker trait associations (MTA) conferring the above adaptive traits. Experiments were conducted over two consecutive years in tanks filled with soil and then validated in field experiments. GWAS analysis was conducted using general linear models (GLM), mixed linear models (MLM), and fixed and random model circulating probability unification models (FarmCPU model), with the FarmCPU showing to be the best suited model. Six and five significant (approximately −log10 (p) ≥ 5.5) MTA were identified for AR and RCA formation under waterlogged conditions, respectively. The highest −log10 (p) MTA for adventitious root and aerenchyma formation were approximately 9 and 8 on chromosome 2H and 4H, respectively. The combination of different MTA showed to be more effective in forming RCA and producing more AR under waterlogging stress. Genes from major facilitator superfamily (MFS) transporter and leucine-rich repeat (LRR) families for AR formation, and ethylene responsive factor (ERF) family genes and potassium transporter family genes for RCA formation were the potential candidate genes involved under waterlogging conditions. Several genotypes, which performed consistently well under different conditions, can be used in breeding programs to develop waterlogging-tolerant varieties.

The role of reactive oxygen species and nitric oxide in the formation of root cortical aerenchyma under cadmium contamination.

The present study aimed to evaluate root cortical aerenchyma formation in response to Cd-driven hydrogen peroxide (H2 O2 ) production and the role of nitric oxide (NO) in the alleviation of Cd oxidative stress in maize roots and its effects on aerenchyma development. Maize plants were subjected to continuous flooding for 30 days, and the following treatments were applied weekly: Cd(NO3 )2 at 0, 10, and 50 μM and Na2 [Fe(CN)5 NO]·2H2 O (an NO donor) at 0.5, 0.1, and 0.2μM. The root biometrics; oxidative stress indicators H2 O2 and malondialdehyde (MDA); and activities of catalase (CAT), superoxide dismutase (SOD), and ascorbate peroxidase (APX) were analyzed. The root dry and fresh masses decreased at higher concentrations of NO and Cd. H2 O2 also decreased at higher NO concentrations; however, MDA increased only at higher Cd levels. SOD activity decreased at higher concentrations of NO, but CAT activity increased. Aerenchyma development decreased in response to NO. Consequently, NO acts as an antagonist to Cd, decreasing the concentration of H2 O2 by reducing SOD activity and increasing CAT activity. Although H2 O2 is directly linked to aerenchyma formation, increased H2 O2 concentrations are necessary for root cortical aerenchyma development.

Rice plants overexpressing OsEPF1 show reduced stomatal density and increased root cortical aerenchyma formation

Stomata are adjustable pores in the aerial epidermis of plants. The role of stomata is usually described in terms of the trade-off between CO2 uptake and water loss. Little consideration has been given to their interaction with below-ground development or diffusion of other gases. We overexpressed the rice EPIDERMAL PATTERNING FACTOR1 (OsEPF1) to produce rice plants with reduced stomatal densities, resulting in lowered leaf stomatal conductance and enhanced water use efficiency. Surprisingly, we found that root cortical aerenchyma (RCA) is formed constitutively in OsEPF1OE lines regardless of tissue age and position. Aerenchyma is tissue containing air-spaces that can develop in the plant root during stressful conditions, e.g. oxygen deficiency when it functions to increase O2 diffusion from shoot to root. The relationship with stomata is unknown. We conclude that RCA development and stomatal development are linked by two possible mechanisms: first that reduced stomatal conductance inhibits the diffusion of oxygen to the root, creating an oxygen deficit and stimulating the formation of RCA, second that an unknown EPF signalling pathway may be involved. Our observations have fundamental implications for the understanding of whole plant gas diffusion and root-to-shoot signalling events.

Does phosphorus deficiency induce formation of root cortical aerenchyma maintaining growth of Cunninghamia lanceolata?

Phosphorous (P) deficiency induces root cortical aerenchyma (RCA) formation in Chinese fir seedlings with marked genotypic variation, and the adaptive role of RCA is mainly related to root P reduction. Root cortical aerenchyma (RCA) formation in response to phosphorus (P) deficiency and its adaptive role are largely unexplored in trees. We hypothesized that RCA formation in response to P deficiency varies with its severity and genotype, and that P-stressed seedlings maintain similar seedling growth to non-stressed seedlings by reducing the P requirements of roots and metabolic cost of soil exploration. Seedlings of two half-sib families (nos. 25 and 36) of Cunninghamia lanceolata (Chinese fir) were grown in pots with three levels of P supply for 3 months. Significant differences in RCA formation in 40–45 mm root segments from the root tip were observed between families and P supply levels. Severe P deficiency induced more RCA formation in family no. 36 than in family no. 25, whereas moderate P deficiency induced RCA formation in family no. 36 only. Family no. 36 had significantly higher height increment, needle biomass, and P concentration in stems and needles, but lower root biomass and root P concentration, than family 25. A significantly negative relationship was found between RCA and root P concentration, increments in average root diameter, and root surface area, whereas significantly positive correlations were observed between some root morphological traits and biomass and P concentration in the organs of P-stressed seedlings of family 36. In conclusion, severe P deficiency induces formation of RCA in Chinese fir seedlings with marked genotypic variation, and the adaptive role of RCA formation during P deficiency is mainly related to the reduction of root P concentration.

Aerenchyma development in different root zones of maize genotypes under water limitation and different phosphorus nutrition

Root cortical aerenchyma (RCA) is suggested to reduce metabolic cost for root growth, but it might lower water uptake by plants. The objective of this work was to evaluate the effects of drought and phosphorus on the RCA development along the root axis and to elucidate its role in water stress tolerance of two maize genotypes. Plants of drought-tolerant DKB390 and drought-sensitive BRS1010 genotypes were grown in Vermiculite at field capacity of 100, 75, 50, and 25 % and supplied with 0.1, 0.4, and 0.8 mM phosphorus. Growth parameters, RCA, and plant P content were evaluated for all plants. Higher RCA development was observed in DKB390 than in BRS1010. Drought reduced the percentage of RCA in the root-hair zone of both genotypes but increased its development in the root maturation zone. Phosphorus limitation enhanced RCA development only in the DKB390. Under drought stress, DKB390 showed resilient growth whereas growth was inhibited in BRS1010. Higher root P content was related to its higher supply. Therefore, RCA formation was induced either by drought or by phosphorus limitation, while no interaction was evident. The RCA development varied along the root axis in order to balance water and phosphorus uptake and the drought response was genotype dependent.

Ethylene modulates root cortical senescence in barley.

Root cortical senescence (RCS) is a poorly understood phenomenon with implications for adaptation to edaphic stress. It was hypothesized that RCS in barley (Hordeum vulgare L.) is (1) accelerated by exogenous ethylene exposure; (2) accompanied by differential expression of ethylene synthesis and signalling genes; and (3) associated with differential expression of programmed cell death (PCD) genes. Gene expression of root segments from four barley genotypes with and without RCS was evaluated using quantitative real-time PCR (qRT-PCR). The progression of RCS was manipulated with root zone ethylene and ethylene inhibitor applications. The results demonstrate that ethylene modulates RCS. Four genes related to ethylene synthesis and signalling were upregulated during RCS in optimal, low nitrogen and low phosphorus nutrient regimes. RCS was accelerated by root zone ethylene treatment, and this effect was reversed by an ethylene action inhibitor. Roots treated with exogenous ethylene had 35 and 46 % more cortical senescence compared with the control aeration treatment in seminal and nodal roots, respectively. RCS was correlated with expression of two genes related to programmed cell death (PCD). The development of RCS is similar to root cortical aerenchyma formation with respect to ethylene modulation of the PCD process.

Nitrate increases ethylene production and aerenchyma formation in roots of lowland rice plants under water stress.

Ethylene increases root cortical aerenchyma formation in maize (Zea mays L.), rice (Oryza sativa L.) and other species. To further investigate the effects of nitrate, ammonium and water stress on ethylene production and aerenchyma formation in roots, two lowland rice cultivars (Shanyou 63, hybrid indica, and Yangdao 6, inbred indica) were cultured hydroponically with 10% (w/v) polyethylene glycol to simulate water stress. Water stress decreased shoot biomass, stomatal conductivity and leaf water potential in cultivars fed with nitrate but not with ammonium. Water stress induced more aerenchyma formation in cultivars fed with nitrate rather than ammonium, and increased cortical aerenchyma was found in Yangdao 6. Endogenous ethylene production by roots increased significantly under water stress in plants fed with nitrate rather than ammonium. Exogenous ethylene stimulated root cortical aerenchyma formation. Expression of the ethylene biosynthesis gene 1-aminocyclo-propane-1-carboxylic acid (ACC) synthase (ACS5) was greater in roots fed with nitrate rather than ammonium in the presence and absence of water stress. The expression of ethylene signalling pathway genes involved in programmed cell death (lesion-simulating disease (L.S.D.)1.1 and L.S.D.2; enhanced disease susceptibility (EDS) and phytoalexin-deficient (PAD4)) were regulated by the N form and water stress. In plants of cultivars fed with ammonium, L.S.D.1.1 expression increased under water stress, whereas L.S.D.2, EDS and PAD4 expression decreased. In conclusion, nitrate increases ethylene production and cortical aerenchyma formation in roots of water-stressed lowland rice. However, ammonium increased L.S.D.1.1 expression in water-stressed roots, and decreased ACS5, EDS and PAD4 expression, which would inhibit ethylene production and aerenchyma formation.

Utility of root cortical aerenchyma under water limited conditions in tropical maize (Zea mays L.)

It has recently been shown that root cortical aerenchyma (RCA) formation substantially reduces the metabolic costs of soil exploration, thereby improving rooting depth, water capture, and plant growth under drought. The objectives of this study were to evaluate phenotypic variation of RCA formation in Malawi maize germplasm and confirm the utility of RCA formation in different agroecologies at field stations and on smallholder farms in Malawi. Forty maize accessions collected across Malawi were grown under rainfed field conditions, and a subset contrasting for RCA were grown under well-watered and water stressed conditions at two research stations and under natural drought on farms across two agroecological zones in Malawi. We found substantial variation for RCA in local germplasm ranging from 0% to 37% of root cross sectional area. Suboptimal water availability increased RCA from 54% to 77% across environments. Under water stress high RCA genotypes had 23% to 29% greater leaf relative water content, 67% to 96% greater shoot biomass 70 days after planting, and 78% to 143% greater grain yield than low RCA genotypes. Our results are consistent with the hypothesis that RCA improves plant growth under drought by reducing the metabolic costs of soil exploration and improving water acquisition. RCA has value as a selection criterion to improve the performance of maize and possibly other cereal crops in water limited environments.

Root cortical aerenchyma enhances nitrogen acquisition from low-nitrogen soils in maize.

Suboptimal nitrogen (N) availability is a primary constraint for crop production in developing nations, while in rich nations, intensive N fertilization carries substantial environmental and economic costs. Therefore, understanding root phenes that enhance N acquisition is of considerable importance. Structural-functional modeling predicts that root cortical aerenchyma (RCA) could improve N acquisition in maize (Zea mays). We evaluated the utility of RCA for N acquisition by physiological comparison of maize recombinant inbred lines contrasting in RCA grown under suboptimal and adequate N availability in greenhouse mesocosms and in the field in the United States and South Africa. N stress increased RCA formation by 200% in mesocosms and by 90% to 100% in the field. RCA formation substantially reduced root respiration and root N content. Under low-N conditions, RCA formation increased rooting depth by 15% to 31%, increased leaf N content by 28% to 81%, increased leaf chlorophyll content by 22%, increased leaf CO2 assimilation by 22%, increased vegetative biomass by 31% to 66%, and increased grain yield by 58%. Our results are consistent with the hypothesis that RCA improves plant growth under N-limiting conditions by decreasing root metabolic costs, thereby enhancing soil exploration and N acquisition in deep soil strata. Although potential fitness tradeoffs of RCA formation are poorly understood, increased RCA formation appears be a promising breeding target for enhancing crop N acquisition.

Root cortical aerenchyma inhibits radial nutrient transport in maize (Zea mays).

Formation of root cortical aerenchyma (RCA) can be induced by nutrient deficiency. In species adapted to aerobic soil conditions, this response is adaptive by reducing root maintenance requirements, thereby permitting greater soil exploration. One trade-off of RCA formation may be reduced radial transport of nutrients due to reduction in living cortical tissue. To test this hypothesis, radial nutrient transport in intact roots of maize (Zea mays) was investigated in two radiolabelling experiments employing genotypes with contrasting RCA. In the first experiment, time-course dynamics of phosphate loading into the xylem were measured from excised nodal roots that varied in RCA formation. In the second experiment, uptake of phosphate, calcium and sulphate was measured in seminal roots of intact young plants in which variation in RCA was induced by treatments altering ethylene action or genetic differences. In each of three paired genotype comparisons, the rate of phosphate exudation of high-RCA genotypes was significantly less than that of low-RCA genotypes. In the second experiment, radial nutrient transport of phosphate and calcium was negatively correlated with the extent of RCA for some genotypes. The results support the hypothesis that RCA can reduce radial transport of some nutrients in some genotypes, which could be an important trade-off of this trait.

Root cortical burden influences drought tolerance in maize.

Root cortical aerenchyma (RCA) increases water and nutrient acquisition by reducing the metabolic costs of soil exploration. In this study the hypothesis was tested that living cortical area (LCA; transversal root cortical area minus aerenchyma area and intercellular air space) is a better predictor of root respiration, soil exploration and, therefore, drought tolerance than RCA formation or root diameter. RCA, LCA, root respiration, root length and biomass loss in response to drought were evaluated in maize (Zea mays) recombinant inbred lines grown with adequate and suboptimal irrigation in soil mesocosms. Root respiration was highly correlated with LCA. LCA was a better predictor of root respiration than either RCA or root diameter. RCA reduced respiration of large-diameter roots. Since RCA and LCA varied in different parts of the root system, the effects of RCA and LCA on root length were complex. Greater crown-root LCA was associated with reduced crown-root length relative to total root length. Reduced LCA was associated with improved drought tolerance. The results are consistent with the hypothesis that LCA is a driver of root metabolic costs and may therefore have adaptive significance for water acquisition in drying soil.

Root Cortical Aerenchyma Enhances the Growth of Maize on Soils with Suboptimal Availability of Nitrogen, Phosphorus, and Potassium

Root cortical aerenchyma (RCA) is induced by hypoxia, drought, and several nutrient deficiencies. Previous research showed that RCA formation reduces the respiration and nutrient content of root tissue. We used SimRoot, a functional-structural model, to provide quantitative support for the hypothesis that RCA formation is a useful adaptation to suboptimal availability of phosphorus, nitrogen, and potassium by reducing the metabolic costs of soil exploration in maize (Zea mays). RCA increased the growth of simulated 40-d-old maize plants up to 55%, 54%, or 72% on low nitrogen, phosphorus, or potassium soil, respectively, and reduced critical fertility levels by 13%, 12%, or 7%, respectively. The greater utility of RCA on low-potassium soils is associated with the fact that root growth in potassium-deficient plants was more carbon limited than in phosphorus- and nitrogen-deficient plants. In contrast to potassium-deficient plants, phosphorus- and nitrogen-deficient plants allocate more carbon to the root system as the deficiency develops. The utility of RCA also depended on other root phenes and environmental factors. On low-phosphorus soils (7.5 μM), the utility of RCA was 2.9 times greater in plants with increased lateral branching density than in plants with normal branching. On low-nitrate soils, the utility of RCA formation was 56% greater in coarser soils with high nitrate leaching. Large genetic variation in RCA formation and the utility of RCA for a range of stresses position RCA as an interesting crop-breeding target for enhanced soil resource acquisition.

Roots of the Second Green Revolution

The Green Revolution boosted crop yields in developing nations by introducing dwarf genotypes of wheat and rice capable of responding to fertilisation without lodging. We now need a second Green Revolution, to improve the yield of crops grown in infertile soils by farmers with little access to fertiliser, who represent the majority of third-world farmers. Just as the Green Revolution was based on crops responsive to high soil fertility, the second Green Revolution will be based on crops tolerant of low soil fertility. Substantial genetic variation in the productivity of crops in infertile soil has been known for over a century. In recent years we have developed a better understanding of the traits responsible for this variation. Root architecture is critically important by determining soil exploration and therefore nutrient acquisition. Architectural traits under genetic control include basal-root gravitropism, adventitious-root formation and lateral branching. Architectural traits that enhance topsoil foraging are important for acquisition of phosphorus from infertile soils. Genetic variation in the length and density of root hairs is important for the acquisition of immobile nutrients such as phosphorus and potassium. Genetic variation in root cortical aerenchyma formation and secondary development (‘root etiolation’) are important in reducing the metabolic costs of root growth and soil exploration. Genetic variation in rhizosphere modification through the efflux of protons, organic acids and enzymes is important for the mobilisation of nutrients such as phosphorus and transition metals, and the avoidance of aluminum toxicity. Manipulation of ion transporters may be useful for improving the acquisition of nitrate and for enhancing salt tolerance. With the noteworthy exceptions of rhizosphere modification and ion transporters, most of these traits are under complex genetic control. Genetic variation in these traits is associated with substantial yield gains in low-fertility soils, as illustrated by the case of phosphorus efficiency in bean and soybean. In breeding crops for low-fertility soils, selection for specific root traits through direct phenotypic evaluation or molecular markers is likely to be more productive than conventional field screening. Crop genotypes with greater yield in infertile soils will substantially improve the productivity and sustainability of low-input agroecosystems, and in high-input agroecosystems will reduce the environmental impacts of intensive fertilisation. Although the development of crops with reduced fertiliser requirements has been successful in the few cases it has been attempted, the global scientific effort devoted to this enterprise is small, especially considering the magnitude of the humanitarian, environmental and economic benefits being forgone. Population growth, ongoing soil degradation and increasing costs of chemical fertiliser will make the second Green Revolution a priority for plant biology in the 21st century.