Rice Root Morphology Research Articles

Effect and Mechanism of Root Characteristics of Different Rice Varieties on Methane Emissions

Methane (CH4), which is an important component of the greenhouse gases from paddy ecosystems, is a major contributor to climate change. CH4 emissions from paddy ecosystems are closely related to the rice root system; however, how the rice root system affects CH4 emissions remains unclear. We conducted a field experiment in 2023 at the Heping Irrigation District Rice Irrigation Experiment Station in Qing’an County, Heilongjiang Province. The field experiment used five local rice varieties with similar fertility periods to observe rice root morphology and physiology indexes, CH4 emission fluxes, and cumulative CH4 emissions. A structural equation model (SEM) was established to investigate the effects of root characteristics on the CH4 emissions from rice and understand the potential mechanisms of these effects. The results showed that the seasonal patterns of CH4 emission fluxes were similar in different rice varieties, and that, during the tillering to heading–flowering stages, the cumulative CH4 emissions accounted for 89.8–92.6% of the total cumulative CH4 emissions of rice. Significant negative correlations were observed between CH4 emission fluxes and root volume, root dry weight, root oxidation activity (ROA), and root radial oxygen loss (ROL) (r = −0.839, −0.885, −0.401 and −0.934, p < 0.05), while there were significant positive correlations between root diameter; malic acid, citric acid, and succinic acid contents; and CH4 emission fluxes (r = 0.407, 0.753, 0.797, and 0.685, p < 0.05). The SEM showed that CH4 emission fluxes were directly influenced by ROL and organic acid contents, while the other root indicators had indirect effects by modulating ROL and organic acid contents. ROL and root volume had the largest total effect, indicating that ROL and root volume were the most significant root physiological and morphological indicators affecting CH4 emission fluxes. This study provides theoretical support and reference data for achieving sustainable agricultural development in the black soil region of Northeast China.

Variations in Root Morphology and Yield among Rice Varieties in Response to Potassium under Subtropical Conditions

The relationship between rice root morphology and Potassium (K) is a major concern for its growth and development, and it has a substantial impact on yield as well. In light of this, the current pot research was run in the net house of the Department of Agronomy at the Bangladesh Agricultural University, Mymensingh, Bangladesh, throughout the boro (dry season irrigated) season of 2020–2021. Binadhan-10, Hira-2 and BRRI dhan29 were grown with five K fertilizers: 0 kg K ha−1 (K0), 32 kg K ha−1 (K32), 65 kg K ha−1 (K65), 98 kg K ha−1 (K98) and 130 kg K ha−1 (K130). Three replications of the investigation were conducted using a completely randomized design (CRD). The root number (RN), root length (RL), root volume (RV), root porosity (RP) along with leaf area index (LAI), total dry matter (TDM) and yield were assessed. Binadhan-10 with the K65 treatment significantly increased the RN, RL, RV, LAI, TDM and yield. With the further increase in the dosage of K that was under K98 and K130, the value of the root traits and yield did not increase. A positive connection was documented between the grain yield (GY) and all the root traits, excluding the RP. Binadhan-10 can be grown with 65 kg K ha−1 at field level for a satisfactory yield.

Reductive dissolution of iron phosphate modifies rice root morphology in phosphorus-deficient paddy soils

Root morphology reflects plant adaptations to phosphorus (P) deficiency. We hypothesized that changes in rice root morphology reflect P deficiency decrease after ferric iron (Fe(III))-bound phosphate (Fe–P) dissolution in low-redox paddy soils. We developed a novel in-situ32P phosphor-imaging approach under flooding to estimate P uptake by rice roots released from Fe–P dissolution. 32P-labeled ferrihydrite (31 mg P kg−1) was supplied either (1) in polyamide mesh bags (30 μm mesh size) to prevent roots but not microorganisms from direct Fe–P mobilization, or (2) directly mixed with soil to enable roots and microorganisms unrestricted access to the Fe–P. The establishment of low redox conditions (Eh values between −176 and −224 mV) drove the reductive dissolution of Fe–P. Rice root-derived organic acids alone were unable to control Fe–P dissolution, and Fe(III) reduction is predominately a microbially-mediated process. Direct root access to Fe–P raised both the number and mean diameter of crown roots and root tips, and increased P uptake by 149–231%. Crown root elongation rate, 32P activities along roots and root tips were 5–133% higher when roots directly accessed Fe–P compared to Fe–P excluded from roots in mesh bags. Iron accumulation on roots depended on the rice growth stage, but not on their access to Fe–P. Roots’ access to Fe–P increased rice crown roots elongation and branching and increased P accessibility under P deficiency.

Straw and nitrogen amendments improve soil, rice yield, and roots in a saline sodic soil

Straw return can improve the physical properties and nutrient status of the soil. However, there are few studies on straw return coupled with nitrogen (N) fertilizer for rice planting in saline sodic areas. Therefore, we conducted a two-year field experiment to evaluate the effects of straw management (retention or removal) with N fertilizer (0, 90, 180, 270 and 360 kg N ha−1) on soil properties, root characteristics, rice yield and N use efficiency in a saline sodic rice growing area. The results showed that straw return significantly reduced soil bulk density, pH, soil electrical conductivity of saturated paste extraction (ECe), exchangeable sodium (ENa+), exchangeable sodium percentage (ESP), and also significantly increased soil organic matter and cation exchange capacity (CEC). Compared with straw removal, straw return reduced soil pH, ECe, ENa+, and ESP by 2.97%, 10.90%, 8.95%, and 13.78%, respectively. At the same N level, straw retention reduced rice root morphology and root vigor at mid-tillering and panicle initiation, but N fertilizer was able to alleviate root inhibition by straw return. Straw return improved root morphology and root vigor in mid and late rice growth and promoted nutrient uptake. Compared with straw removal, the agronomic efficiency nitrogen and partial factor productivity were significantly increased by 13.97% and 4.59%, respectively, in the straw return treatment. With or without straw retention, rice yield increased and then decreased with increasing N fertilizer application rate, and was maximum at 270 kg N ha−1. Compared to straw removal, rice yield was significantly increased by 4.37% (2020) and 7.17% (2021) for straw return treatments, respectively. Meanwhile, straw return can reduce mineral N fertilizer inputs by about 36–43% without affecting rice yield. Under straw return conditions, soil characteristics, root characteristics, and rice yield were optimal at 270 kg N ha−1. Our results suggest that straw return combined with nitrogen fertilization may be a promising management practice to reduce mineral N application without loss of rice yield in saline sodic soils. Among them, straw return combined with 270 kg N ha−1 was most effective in the early stages of straw return in saline sodic rice growing areas.

Dual Roles of OsGH3.2 in Modulating Rice Root Morphology and Affecting Arbuscular Mycorrhizal Symbiosis.

Several angiosperm GRETCHEN HAGEN 3 (GH3) genes, including tomato SlGH3.4 and rice OsGH3.2 are induced during arbuscular mycorrhizal (AM) symbiosis, but their functions remain largely unclear. Recently, tomato SlGH3.4 was suggested to negatively regulate arbuscule incidence via decreasing auxin levels in colonized cells. In this study, by acquiring rice OsGH3.2pro:β-glucuronidase (GUS) transgenic plants and generating Osgh3.2 mutants via CRISPR/Cas9 technique, the roles of OsGH3.2 in modulating rice root morphology and affecting AM symbiosis were investigated through time course experiments. Unlike SlGH3.4, OsGH3.2 showed asymbiotic expression in rice young lateral roots, and its mutation resulted in a “shallow” root architecture. Such root morphological change was also observed under symbiotic condition and it likely promoted AM fungal colonization, as the mutants exhibited higher colonization levels and arbuscule incidence than wild-type at early stages. Similar to SlGH3.4, OsGH3.2 showed symbiotic expression in cortical cells that have formed mature arbuscules. At late stages of symbiosis, Osgh3.2 mutants showed elongated cortical cells and larger arbuscules than wild-type, indicating elevated auxin level in the colonized cells. Together, these results revealed both asymbiotic and symbiotic roles of OsGH3.2 in modulating rice root architecture and controlling auxin levels in arbusculated cells, which further affected colonization rate and arbuscule phenotype.

Effects of rhizosphere oxygen concentration on root physiological characteristics and anatomical structure at the tillering stage of rice

AbstractOxygen is essential for all aerobic organisms. Higher plants need oxygen to sustain metabolism and growth. After experiencing anaerobic stress for a period of time, most plant tissues will be damaged. This study examined the physiological characteristics and anatomical structures in the root tips of rice seedlings (cultivars Xiushui09 and Chunyou84) in response to different rhizosphere oxygen environments. The results showed that moderate oxygen (MO: 2.5–3.5 mg L−1) increased the dry weight accumulation and formation of rice roots (including length, surface area, number of tips) in both genotypes. Compared to under normal conditions (NC), the magnitudes of increase in these four variables were 4.67, 66.40, 35.44 and 49.50% in Xiushui09 and 12.25, 15.59, 13.57 and 13.48% in Chunyou84, respectively. Low oxygen (LO: 0–1.0 mg L−1) stress decreased the root surface area but increased root volume and average diameter. LO and high oxygen (HO: >6.8 ± 0.21 mg L−1) stress each damaged the apical cells' ultrastructure, reduced the number of organelles, and increased electrical conductivity. Meanwhile, the root activity and respiration of rice seedlings decreased; the abscisic acid content increased compared to levels under NC. Under MO treatment, the cell membrane was not damaged, the root tip organelles were rich, the soluble protein content, root activity, respiration rate and gibberellic acid content increased compared to levels under NC; the magnitudes of these increases were 24.76, 36.00, 8.00 and 283.00% in Xiushui09 and 4.78, 40.00, 15.45 and 49.35% in Chunyou84, respectively. In conclusion, MO optimised rice root morphology and enhanced root physiological activity.

Available Nitrogen in Paddy Soils Depth: Influence on Rice Root Morphology and Plant Nutrition

Nitrogen (N) is a key nutrient for rice (Oryza sativa L.) and its availability in soil profile can influence the plant root morphology and total nutrient uptake. This study evaluated if N from deeper soil layers would increase rice root growth, N uptake and influence plant nutritional status. Flooded rice was grown in undisturbed soil samples from three locations with 15N-labeled ammonium sulfate application at different depths (0, 10, 20, 30, and 45 cm). At rice heading, chlorophyll content and photosynthesis rate were measured at flag leaves and rice biomass production, root morphology, and total N and 15N uptake were evaluated. Higher biomass and N uptake were observed with N application at near soil surface layers. The majority of rice N uptake was derived from soil, which was more evident when N fertilizer was injected at 45 cm deep (on average 74%). The highest root growth was observed at the near surface layers with constant decrease in depth, up to 60 cm. Roots from deeper layers exhibited smaller diameter, as they were mainly highly branched roots. The N application at 30 and 45 cm depths provided N later in the season, resulting in plants with lower total biomass but with higher N content in aboveground biomass, with higher chlorophyll content, photosynthesis rate, and carboxylation efficiency by Rubisco. The N available below 20 cm contributes to plant nutrition and root growth, hence soils with available N in depth likely cause plants to function more optimally in photosynthates supply.

Effects of wheat straw mulch application and nitrogen management on rice root growth, dry matter accumulation and rice quality in soils of different fertility

The root is the most sensitive part of the plant. This study examined the effects of wheat straw mulch, nitrogen (N) management and soil fertility on rice root morphology, dry matter accumulation, grain yield and rice quality and explored the correlation between root morphology and rice quality indexes. Wheat straw mulch had an obvious inhibitory effect on rice root morphology, especially on the total root length and total root surface area at 20 days after transplanting in the high- and low-fertility soil. The low-fertility soil treatments had higher levels of inhibition. However, straw mulch significantly promoted root growth, increased dry matter accumulation in the subsequent growth periods and improved the grain yield and rice quality, which mainly reduced the percentage of chalky kernels and the chalkiness degree. Increasing the nitrogen fertilizer percentage applied as a basal or tillering fertilizer reduced the inhibition in the early growth period; for balanced rice root growth, dry matter accumulation, grain yield and rice quality, 30% as basal fertilizer, 30% as tillering fertilizer and 40% as panicle fertilizer were the best N management strategy. Correlation analysis showed that the rice quality was extremely significantly positively correlated (r = 0.30–0.59, P < 0.05) with the root morphology indexes of the brown rice and protein contents and was significantly or extremely negatively correlated (r = − 0.73 to − 0.28, P < 0.05) with the length–width ratio, percentage of chalky kernels, chalkiness degree and amylose content.

Effect of microcystins on root growth, oxidative response, and exudation of rice (Oryza sativa)

A 30 days indoor hydroponic experiment was carried out to evaluate the effect of microcystins (MCs) on rice root morphology and exudation, as well as bioaccumulation of MCs in rice. MCs were bioaccumulated in rice with the greatest concentrations being observed in the leaves (113.68μgg−1 Fresh weight (FW)) when exposed to 500μgL−1 MCs. Root activity at 500μgL−1 decreased 37%, compared to the control. MCs also induced disruption of the antioxidant system and lipid peroxidation in rice roots. Root growth was significantly inhibited by MCs. Root weight, length; surface area and volume were significantly decreased, as well as crown root number and lateral root number. After 30 days exposure to MCs, an increase was found in tartaric acid and malic acid while the other organic acids were not affected. Glycine, tyrosine, and glutamate were the only amino acids stimulated at MCs concentrations of 500μgL−1. Similarly, dissolved organic carbon (DOC) and carbohydrate at 50 and 500μgL−1 treatments were significantly increased. The increase of DOC and carbohydrate in root exudates was due to rice root membrane permeability changes induced by MCs. Overall, this study indicated that MCs significantly inhibited rice root growth and affected root exudation.

Interactions between selenite and different forms of antimony and their effects on root morphology of paddy rice

Background and Aims The effects of Sb(V), alone or combined with Se, on the growth and root development of plants are unknown. The aim of this study is to investigate the interaction between selenite and different forms of Sb and the effects on their uptake in rice and on rice root morphology.

Mapping Quantitative Trait Loci Associated with Toot Traits Using Sequencing-Based Genotyping Chromosome Segment Substitution Lines Derived from 9311 and Nipponbare in Rice (Oryza sativa L.).

Identification of quantitative trait loci (QTLs) associated with rice root morphology provides useful information for avoiding drought stress and maintaining yield production under the irrigation condition. In this study, a set of chromosome segment substitution lines derived from 9311 as the recipient and Nipponbare as donor, were used to analysis root morphology. By combining the resequencing-based bin-map with a multiple linear regression analysis, QTL identification was conducted on root number (RN), total root length (TRL), root dry weight (RDW), maximum root length (MRL), root thickness (RTH), total absorption area (TAA) and root vitality (RV), using the CSSL population grown under hydroponic conditions. A total of thirty-eight QTLs were identified: six for TRL, six for RDW, eight for the MRL, four for RTH, seven for RN, two for TAA, and five for RV. Phenotypic effect variance explained by these QTLs ranged from 2.23% to 37.08%, and four single QTLs had more than 10% phenotypic explanations on three root traits. We also detected the correlations between grain yield (GY) and root traits, and found that TRL, RTH and MRL had significantly positive correlations with GY. However, TRL, RDW and MRL had significantly positive correlations with biomass yield (BY). Several QTLs identified in our population were co-localized with some loci for grain yield or biomass. This information may be immediately exploited for improving rice water and fertilizer use efficiency for molecular breeding of root system architectures.

Impact of IAA, GA 3 , and ABA on Negative Root Phototropism and Root Growth of Rice

The objective of this research was to investigate the relationship between rice roots negative phototropism and endogenous plant hormone levels, as well as the impact of exogenous hormones on rice root growth. The adventitious roots and primary roots of conventional cultivars Yangdao 6(indica), Nipponbare(japonica), Zhonghua 11(japonica) and its OsPIN1 a over expressed transgenic seedlings were used to observe the process of root negative phototropism, measure contents of endogenous auxin(IAA), gibberellin(GA3) and abscisic acid(ABA) in the bending part of root tips, and investigate the impact of exogenous hormones to rice root morphology and anatomy. Results indicated that seedling roots of both Yangdao 6 and Nipponbare had negative phototropism, and the lateral roots and adventitious roots of Yangdao 6 showed stronger negative phototropism. Plant hormones and negative phototropism of rice root and root development are closely linked. Under unilateral illumination, endogenous IAA, GA3 and ABA levels declined. The content of these phytohormones in the irradiated side was lower than that in the shaded side. The treatment of unilateral light and exogenous IAA induced a large number of root hairs in Yangdao 6. Compared with conventional rice, the angle of negative phototropism was lager in OsPIN1 a transgenic plant. There was no direct relationship between exogenous GA3 and ABA treatments and root negative phototropism of rice. Exogenous ABA inhibited root growth obviously. The total absorption surface area, root number and root length per plant were significantly reduced by ABA treatment. The treatment of 10 μmol L–1ABA not only caused a dysplasia of both meristematic and elongation zone but also promoted the development of the central vascular tissue in root tips.

Root Physiological and Morphological Characteristics of Two Rice Cultivars with Different Nitrogen-Use Efficiency

The variation in nitrogen (N) uptake by rice has been widely studied but differences in rice root morphology that may contribute to this variation are not completely understood. Field and greenhouse experiments were carried out to study N accumulation, root dry weights, total root lengths, root surface areas, and root bleeding rates of two rice cultivars, Elio with low N-use efficiency and Nanguang with high N-use efficiency. Low (1 mmol N L −1) and high (5 mmol N L −1)N applications were established in the greenhouse experiment, and the N rates were 0, 120, and 240 kg ha −1 in the field experiments at Jiangning and Jiangpu farms, Nanjing, China. The results showed that the N accumulation, root dry weight, total root length, and root surface area increased with an increase in N application. At the heading stage, N accumulation in the shoots and roots of Nanguang was greater than that of Elio in the field experiments and that of Elio at 5 mmol N L −1 in the greenhouse experiment. After the heading stage, N accumulation was higher for Nanguang at both 1 and 5 mmol N L −1 in the greenhouse experiment. The total root length and root surface area were significantly different between the two cultivars. Over the range of the fertilizer application rates, the root lengths of Nanguang at Jiangning Farm were 49%–61% greater at booting and 26%–39% greater at heading than those of Elio, and at Jiangpu Farm they were 22%–42% and 26%–38% greater, respectively. Nanguang had a greater root bleeding rate than Elio. It was concluded that the N-use efficiency of the two rice cultivars studied depended to a great extent on the root morphological parameters and root physiological characteristics at different growth stages.

Genotypic variation of rice in phosphorus acquisition from iron phosphate: Contributions of root morphology and phosphorus uptake kinetics

To elucidate the contributions of rice root morphology and phosphorus uptake kinetics to P uptake by rice from iron phosphate, a sand culture experiment with either sufficient P supply (control treatment, 10 mg P/l as NaH2PO4) or Fe-P as the only source of P (40 mg P/pot as FePO4 × 4H2O) and a solution culture experiment supplied with either sufficient P (10 mg P/l) or deficient P (0.5 mg P/l) were conducted. Eight rice cultivars, which differed in P uptake from Fe-P, were investigated. Plant P uptake, root morphology, and P uptake kinetics were determined. There were significant (P < 0.05) genotypic variations in both plant dry weight and P uptake per plant among eight rice (Oryza sativa L.) cultivars when supplied with Fe-P as the P source. The Fe-P treatment significantly (P < 0.05) decreased plant dry weight, P uptake per plant, and P concentration in plant dry matter of all cultivars in comparison with the control plants. In Fe-P treated plants, significant (P < 0.05) genotypic variation was shown in root morphology, including root length, surface area, volume, and number of lateral roots. The P uptake per plant from Fe-P by rice was significantly (P < 0.05) correlated with root surface area and root volume as well as with the number of lateral roots, suggesting that the ability of rice to absorb P from Fe-P was closely related to root morphology. Low P supply in solution significantly increased the Imax (P < 0.05), but significantly decreased the KM (P < 0.05) for P absorption by all rice cultivars. We supposed that kinetic characteristics of root P uptake could not account for the ability of rice to absorb P from Fe-P.

Room‐Induced Changes of Potassium in the Rhizosphere of Lowland Rice

Potassium (K) deficiency in paddy soils has become the most limiting factor for increasing rice (Oriza Sativa L.) yield. It is important to increase K use efficiency in irrigated rice production systems. In this study, the changes of different K fractions in the rhizopshere of lowland rice were examined at low and adequate K supply using a rhizobox technique. The results obtained indicated that the roots of lowland rice caused changes of soil K fractions in the rhizosphere. The depletion of readily available K and exchangeable K in the rhizosphere could extend over 10 mm from the root surface at both the low and adequate K supply levels. The readily released K and slowly released K in the rhizosphere of lowland rice depleted 6–8 mm from the root surface at low K level, whereas up to 2–4 mm from root surface were depleted at the adequate K supply. The rice roots also induced the depletion of mineral‐K in the rhizosphere, within 0–2 mm and 0–3 mm from the root surface for the low K and the adequate K treatments, respectively. The depletion of these five fractions of K and decrease of pH in the rhizosphere of lowland rice was greater at low K supply than at the adequate K supply. The fertilizer K applied was mainly in the forms of readily available and exchangeable fractions in the soil. These results imply that rice roots could increase the solubility and availability of potentially available K and even mineral‐K in the rhizosphere under submerged condition, partially due to the dramatical decrease of pH by rice plant roots in the rhizsophere. The decrease in pH may partially result from the oxidation of Fe (II) in the rhizophere. High K dramatically enhanced Fe (II) oxidation within 1–2 mm from the root surface. It is suggested that improving rice root morphology and physiology for efficiently uptake and use of potentially available K in the rhizosphere would be important for increasing K use efficiency in lowland rice.

Rice root morphology under contrasting moisture regimes and contribution of molecular marker heterozygosity

Evaluation for root morphology and related characters at three different stages and different moisture regimes was made using 9backcross populations obtained by crossing selected doubled haploid lines derived fromIR64 × Azucena with IR64. Marked genotypic differences were observed for all the characters across three samplings. In samplings at 80 days after sowing, significant reductions in mean root and shoot dry weights were noticed from well-watered to severely-stressed conditions, but higher root to shoot dry weight ratios were observed under severe stress condition. In the samplings at maturity, significant increases in mean values of all characters except shoot dry weight and root number, were observed from both well watered to stress conditions. However, individual backcross populations varied with respect to root and shoot dry weights across the moisture regimes. Estimates of heritability were reduced from the 1st to 3rd samplings as well as from WW to SS conditions. In the third sampling, root volume, root thickness and maximum root length were found to be interrelated. The experimental materials were classified into two distinct groups by dendrogram based on 175 markers. This classification was fairly similar to phenotypic classification based on root morphology with IR64 and Azucena falling into the respective main groups. Correlations of marker heterozygosity with hybrid performance and heterosis differed markedly between the 2nd and 3rdsamplings as well as between well watered and severe stress conditions.

Effects of Phenotyping Environment on Identification of Quantitative Trait Loci for Rice Root Morphology under Anaerobic Conditions.

In the rainfed lowlands, rice (Oryza sativa L.) develops roots under anaerobic soil conditions with ponded water, prior to exposure to aerobic soil conditions and water stress. Constitutive root system development in anaerobic soil conditions has been reported to have a positive effect on subsequent expression of adaptive root traits and water extraction during water stress. We examined effects of phenotyping environment on identification of quantitative trait loci (QTLs) for constitutive root morphology traits using 220 doubled-haploid lines (DHLs) from the cross of 'CT9993-5-10-1-M' (CT9993; japonica, upland adapted) x 'IR62266-42-6-2' (IR62266; indica, lowland adapted) in four greenhouse experiments. Broad sense heritability (h(2)) was 75, 60, and 64% on average for shoot biomass, deep root morphology, and root thickness traits, respectively. Quantitative trait loci analysis identified 18 genomic regions associated with deep root morphology traits, but only three were identified consistently across experiments. Three out of a total of eight QTLs for root thickness traits were found in more than one experiment. The maximum genetic effects caused by a single QTL were increments of 0.05 g of deep root mass below a 30-cm soil depth, 0.9% of deep root ratio, 1.6 cm of rooting depth, and 0.09 cm of root thickness, with phenotypic variation explained by a single QTL ranging from 6.8 to 51.8%. The results demonstrate the importance of phenotyping environment and suggest prospects for selection of QTLs for deep root morphology, root thickness, and vigorous seedling growth under anaerobic conditions to improve the constitutive root system of rainfed lowland rice. There was some consistency in QTL regions identified, despite the presence of QTL x environment interactions.

Effects of Phenotyping Environment on Identification of Quantitative Trait Loci for Rice Root Morphology under Anaerobic Conditions

In the rainfed lowlands, rice (Oryza sativa L.) develops roots under anaerobic soil conditions with ponded water, prior to exposure to aerobic soil conditions and water stress. Constitutive root system development in anaerobic soil conditions has been reported to have a positive effect on subsequent expression of adaptive root traits and water extraction during water stress. We examined effects of phenotyping environment on identification of quantitative trait loci (QTLs) for constitutive root morphology traits using 220 doubled-haploid lines (DHLs) from the cross of ‘CT9993-5-10-1-M’ (CT9993; japonica, upland adapted) × ‘IR62266-42-6-2’ (IR62266; indica, lowland adapted) in four greenhouse experiments. Broad sense heritability (h 2) was 75, 60, and 64% on average for shoot biomass, deep root morphology, and root thickness traits, respectively. Quantitative trait loci analysis identified 18 genomic regions associated with deep root morphology traits, but only three were identified consistently across experiments. Three out of a total of eight QTLs for root thickness traits were found in more than one experiment. The maximum genetic effects caused by a single QTL were increments of 0.05 g of deep root mass below a 30-cm soil depth, 0.9% of deep root ratio, 1.6 cm of rooting depth, and 0.09 cm of root thickness, with phenotypic variation explained by a single QTL ranging from 6.8 to 51.8%. The results demonstrate the importance of phenotyping environment and suggest prospects for selection of QTLs for deep root morphology, root thickness, and vigorous seedling growth under anaerobic conditions to improve the constitutive root system of rainfed lowland rice. There was some consistency in QTL regions identified, despite the presence of QTL × environment interactions.

Rhizobium plasmids are involved in the inhibition or stimulation of rice growth and development

This paper originates from an address at the 8th International Symposium on Nitrogen Fixation with Non-Legumes, Sydney, NSW, December 2000 We examined growth responses of rice seedlings (Oryza sativaL. cv. Pelde) to specific Rhizobium strains and their mutants, to investigate the molecular basis of colonization and the stimulation or inhibition of rice growth and development by rhizobia. Inoculation experiments with rice seedlings showed that specific Rhizobium isolates of these rice-associated and legume-associated rhizobia could either promote, inhibit, or have no influence on rice plant growth. There are genes on certain plasmids of Rhizobium leguminosarum bv. trifolii and R. leguminosarum bv. viciae that affect the growth and development of rice root morphology. Additionally, we found that bacteria can intimately associate with, and enter into, rice seedling roots by alternative mechanisms to those encoded by the symbiotic (pSym) and the tumour-inducing (Ti) plasmids. Investigations suggest an involvement of the phytohormone auxin, and possibly nitrate, in this complex rice–Rhizobium interaction.

Locating genes associated with root morphology and drought avoidance in rice via linkage to molecular markers

This research was undertaken to identify and map quantitative trait loci (QTLs) associated with five parameters of rice root morphology and to determine if these QTLs are located in the same chromosomal regions as QTLs associated with drought avoidance/tolerance. Root thickness, root:shoot ratio, root dry weight per tiller, deep root dry weight per tiller, and maximum root length were measured in three replicated experiments (runs) of 203 recombinant inbred lines grown in a greenhouse. The lines were from a cross between indica cultivar Co39 andjaponica cultivar Moroberekan. The 203 RI lines were also grown in three replicated field experiments where they were drought-stressed at the seedling, early vegetative, and late-vegetative growth stage and assigned a visual rating based on leaf rolling as to their degree of drought avoidance/tolerance. The QTL analysis of greenhouse and field data was done using single-marker analysis (ANOVA) and interval analysis (Mapmaker QTL). Most QTLs that were identified were associated with root thickness, root/shoot ratio, and root dry weight per tiller, and only a few with deep root weight. None were reliably associated with maximum root depth due to genotype-by-experiment interaction. Root thickness and root dry weight per tiller were the characters found to be the least influenced by environmental differences between greenhouse runs. Correlations of root parameters measured in greenhouse experiments with field drought avoidance/tolerance were significant but not highly predictive. Twelve of the fourteen chromosomal regions containing putative QTLs associated with field drought avoidance/tolerance also contained QTLs associated with root morphology. Thus, selecting for Moroberekan alleles at marker loci associated with the putative root QTLs identified in this study may be an effective strategy for altering the root phenotype of rice towards that commonly associated with drought-resistant cultivars.