Domestication and Crop Physiology: Roots of Green-Revolution Wheat

Background and aimsMost plant scientists, in contrast to animal scientists, study only half the organism, namely above-ground stems, leaves, flowers and fruits, and neglect below-ground roots. Yet all acknowledge roots are important for anchorage, water and nutrient uptake, and presumably components of yield. This paper investigates the relationship between domestication, and the root systems of landraces, and the parents of early, mid- and late green-revolution bread wheat cultivars. It compares the root system of bread wheat and 'Veery'-type wheat containing the 1RS translocation from rye.MethodsWheat germplasm was grown in large pots in sand culture in replicated experiments. This allowed roots to be washed free to study root characters.Key resultsThe three bread wheat parents of early green-revolution wheats have root biomass less than two-thirds the mean of some landrace wheats. Crossing early green-revolution wheat to an F(2) of 'Norin 10' and 'Brevor', further reduced root biomass in mid-generation semi-dwarf and dwarf wheats. Later-generation semi-dwarf wheats show genetic variation for root biomass, but some exhibit further reduction in root size. This is so for some California and UK wheats. The wheat-rye translocation in 'Kavkaz' for the short arm of chromosome 1 (1RS) increased root biomass and branching in cultivars that contained it.ConclusionsRoot size of modern cultivars is small compared with that of landraces. Their root system may be too small for optimum uptake of water and nutrients and maximum grain yield. Optimum root size for grain yield has not been investigated in wheat or most crop plants. Use of 1RS and similar alien translocations may increase root biomass and grain yield significantly in irrigated and rain-fed conditions. Root characters may be integrated into components of yield analysis in wheat. Plant breeders may need to select directly for root characters

Dosage effect of the short arm of chromosome 1 of rye on root morphology and anatomy in bread wheat

The spontaneous translocation of the short arm of chromosome 1 of rye (1RS) in bread wheat is associated with higher root biomass and grain yield. Recent studies have confirmed the presence of QTL for different root morphological traits on the 1RS arm in bread wheat. This study was conducted to address two questions in wheat root genetics. First, does the presence of the 1RS arm in bread wheat affect its root anatomy? Second, how does root morphology and anatomy of bread wheat respond to different dosages of 1RS? Near-isogenic plants with a different number (0 to 4 dosages) of 1RS translocations were studied for root morphology and anatomy. The F1 hybrid, with single doses of the 1RS and 1AS arms, showed heterosis for root and shoot biomass. In other genotypes, with 0, 2, or 4 doses of 1RS, root biomass was incremental with the increase in the dosage of 1RS in bread wheat. This study also provided evidence of the presence of gene(s) influencing root xylem vessel number, size, and distribution in bread wheat. It was found that root vasculature follows a specific developmental pattern along the length of the tap root and 1RS dosage tends to affect the transitions differentially in different positions. This study indicated that the inherent differences in root morphology and anatomy of different 1RS lines may be advantageous compared to normal bread wheat to survive under stress conditions

Dissection of QTL effects for root traits using a chromosome arm-specific mapping population in bread wheat

A high-resolution chromosome arm-specific mapping population was used in an attempt to locate/detect gene(s)/QTL for different root traits on the short arm of rye chromosome 1 (1RS) in bread wheat. This population consisted of induced homoeologous recombinants of 1RS with 1BS, each originating from a different crossover event and distinct from all other recombinants in the proportions of rye and wheat chromatin present. It provides a simple and powerful approach to detect even small QTL effects using fewer progeny. A promising empirical Bayes method was applied to estimate additive and epistatic effects for all possible marker pairs simultaneously in a single model. This method has an advantage for QTL analysis in minimizing the error variance and detecting interaction effects between loci with no main effect. A total of 15 QTL effects, 6 additive and 9 epistatic, were detected for different traits of root length and root weight in 1RS wheat. Epistatic interactions were further partitioned into inter-genomic (wheat and rye alleles) and intra-genomic (rye–rye or wheat–wheat alleles) interactions affecting various root traits. Four common regions were identified involving all the QTL for root traits. Two regions carried QTL for almost all the root traits and were responsible for all the epistatic interactions. Evidence for inter-genomic interactions is provided. Comparison of mean values supported the QTL detection

Integrated genetic map and genetic analysis of a region associated with root traits on the short arm of rye chromosome 1 in bread wheat

A rye–wheat centric chromosome translocation 1RS.1BL has been widely used in wheat breeding programs around the world. Increased yield of translocation lines was probably a consequence of increased root biomass. In an effort to map loci-controlling root characteristics, homoeologous recombinants of 1RS with 1BS were used to generate a consensus genetic map comprised of 20 phenotypic and molecular markers, with an average spacing of 2.5 cM. Physically, all recombination events were located in the distal 40% of the arms. A total of 68 recombinants was used and recombination breakpoints were aligned and ordered over map intervals with all the markers, integrated together in a genetic map. This approach enabled dissection of genetic components of quantitative traits, such as root traits, present on 1S. To validate our hypothesis, phenotyping of 45-day-old wheat roots was performed in five lines including three recombinants representative of the entire short arm along with bread wheat parents ‘Pavon 76’ and Pavon 1RS.1BL. Individual root characteristics were ranked and the genotypic rank sums were subjected to Quade analysis to compare the overall rooting ability of the genotypes. It appears that the terminal 15% of the rye 1RS arm carries gene(s) for greater rooting ability in wheat

Water use of tall and dwarf crop plants

The recurrent California drought necessitates investigation of the relationship between water application, crop yields, and management practices. The majority of cultivars in many crops are genetically dwarfed which allows the application of larger amounts of water and fertilizer in return for higher yields and ease of harvesting. This project used bread wheat as a model system to investigate the water use and water-use efficiency of taIl and dwarf cultivars. Four near-isogenic lines, rhtrht (tall), RhtlRhtl (semidwarf), Rht2Rht2 (semidwarf), and Rht3Rht3 (dwarf), in 'Maringa' bread wheat background and four of their near-isogenic Fl hybrids derived from crossing the original lines were used to determine the effects of dwarfing genes on plant height, water use, grain yield, total dry matter, and wateruse efficiency in well-watered and droughted pot experiments in the glasshouse. The nearisogenic lines and their six F 1 hybrids were also grown in well-watered and droughted field conditions. The glasshouse season lasted 158 days, whereas the field season took 149 days between planting and harvesting. Carbon isotope discrimination was determined as a measure of transpiration efficiency. The near-isogenic lines used similar amounts of water in well-watered (12 kg per 158 days) and droughted (9 kg per 158 days) pot experiments. The Rht3Rht3 dwarf line actually used 3% less water than the tall line in a well-watered situation, and 5% less water than tall line in a droughted situation, but these differences were not significant in this experiment. Plant height ranged from 60 to 124 em and from 53 to 121 ern in well-watered and droughted pot experiments, and it varied from 50 to 94 cm and from 49 to 90 cm in well-watered and droughted field experiments, respectively. Total dry matter, grain yield, transpiration efficiency (total dry matter/water used), and water-use efficiency (grain yield/water used) declined with plant height in well-watered glasshouse conditions. No significant relationships were found between plant height and these traits in droughted glasshouse conditions. Carbon isotope discrimination was negatively correlated with transpiration efficiency, but significantly so only in well-watered pot experiments. Plant height was negatively associated with carbon isotope discrimination in both well-watered and droughted pot and field experiments. Grain yield and aboveground dry matter also declined with plant height in field conditions. In most cases, the dwarfing genes reduced shoot dry matter more than grain yield, therefore, harvest index of the semidwarf and dwarf lines was higher than that of the tall standard line. The dwarfism caused by Rhtl, Rht2, and Rht3 genes had, in general, depressing effects on transpiration efficiency, water-use efficiency, total dry matter, and grain yield. An optimum range for plant height was determined (90- 100 ern) using these near-isogenic lines, below which shoot dry matter, grain yield, and water-use efficiency were significantly reduced

Path analysis of genotype x environment interactions of wheats to nitrogen

To determine the effect of the application of different levels of nitrogen on grain yield and its components, nine genotypes of bread wheat (Triticum aestivum L.), five of durum wheat (T. turgidum L. var. durum Desf.) and a triticale (x Triticosecale Wittmack) were subjected to three levels of N (0, 65 and 130 kg N ha-1) at three dates of planting: late planting, early planting and mid-season planting, over two growing seasons. A mathematical model based on the method of path analysis was used to study the relationship between grain yield and its components under different N levels. The main effect of N was highly significant for grain yield, aboveground dry matter at anthesis, number of kernels per spike and kernel weight. Genotype x N interaction was significant for these characters, except number of kernels per spike. The greatest variability among the genotypes in response to N occurred during vegetative growth, followed by the seed setting and grain filling periods. Seven genotypes showed sensitivity to N during vegetative growth and seed setting, two during seed setting and grain filling, one during vegetative growth and grain filling and three during both vegetative and reproductive phases. Only 'Express' and to some extent 'Yecora Rojo' were adapted to limited N during the reproductive phase. Development and cultivation of cultivars such as Express that are adapted to limited N could reduce the amount of N required to grow an economic crop and thus reduce the cost of inputs and the possibility of polluting natural resources with the application of excess N. (© Inra/Elsevier, Paris.)Analyse de piste des intéractions génotype x environnement chez des blés traités à l'azote. Pour définir l'effet de différents niveaux d'apport d'azote sur le rendement en grains et ses composantes, neuf génotypes de blé tendre ( Triticum aestivum L.), cinq de blé dur (T. turgidum var. durum Desf.) et un triticale (x Triticosecale Wittmack) ont été soumis à trois traitements d'azote (de 0,65 à 130 kg/ha-1) à trois dates de semis sur deux saisons de culture. Un modèle mathématique basé sur l'analyse de piste a été utilisé pour étudier la relation entre le rendement en grains et ses composantes dans ces différentes conditions. L'effet principal de l'azote a été hautement significatif pour le rendement, la matière sèche aérienne, le nombre de grains par épi et le poids de grain. L'intéraction G x N a été significative pour ces caractères, sauf pour le nombre de grains par épi. La plus grande variabilité génotypique pour la réponse à l'azote a été constatée pendant la croissance végétative, suivie par les périodes de nouaison et de remplissage. Sept génotypes ont montré une sensibilité à l'azote pendant la croissance végétative et la nouaison, deux pendant les phases de nouaison et de remplissage, un pendant la croissance végétative et le remplissage et trois pendant les deux périodes végétative et reproductrice. Seuls « Express » et, dans une moindre mesure, « Yecora Rojo » sont apparus adaptés à des faibles doses d'azote pendant la période reproductrice. Le développement et la culture de cultivars comme « Express », adaptés à de faibles niveaux d'azote, pourraient réduire la quantité d'azote requise pour exploiter une plante d'intérêt agronomique et par conséquent réduire le coût des intrants et les risques de pollution des ressources naturelles par des apports excessifs d'azote. (© Inra/Elsevier, Paris.

Phenotyping and Genetic Analysis of Root and Shoot Traits of Recombinant Inbred Lines of Bread Wheat Under Well- Watered Conditions

Phenotyping root traits and understanding their inheritance are critical for crop improvement, as the root system plays an important role in crop performance under well-watered and drought conditions. A set of 118 F8 recombinant inbred lines (RILs) of bread wheat (Triticum aestivum L.) derived from the cross Iran #49 (a landrace) × Yecora Rojo (a standard variety) plus the two parents was used to phenotype root traits at mid-tillering stage and at physiological maturity in sand-tube experiments under well-watered conditions. Iran #49 and Yecora Rojo were different for grain yield per plant (20.4 vs.13.8), shallow-root weight per plant (5.3 vs. 1.3 g, roots developed between 0 and 30 cm), deep-root weight per plant (4.5 vs. 0.5 g, roots developed below 30 cm), and root biomass per plant (9.8 vs. 1.8 g). Although there were significant differences among the RILs for number of roots longer than 30 cm, total length of roots longer than 30 cm, longest root, shallow-root weight, deep-root weight, and root biomass, the estimate of narrow-sense heritability was relatively low for shallow-root weight (26%), deep- -root weight (14%), and root biomass (22%) at mid-tillering stage. At maturity, the estimate of heritability for these root traits was 81%, 79%, and 83%, respectively. Additive × additive epistasis was detected for deep-root weight at maturity. Genotypic differences in root traits among the RILs were highest at maturity. The root traits measured at mid-tillering and at maturity showed significant, but weak correlation coefficients ranging from 0.20 to 0.40. Grain yield per plant showed significant genotypic correlation with root traits at maturity. Harvest index showed strong negative correlation with root traits ranging from -0.69 to -0.78. Our studies indicated that the appropriate time for phenotyping root traits in wheat is at maturity.Research supported by the California Wheat Commission, the California Agricultural Experiment Station, and the University of California, Riverside, Botanic Gardens

QTLs for root traits at mid-tillering and for root and shoot traits at maturity in a RIL population of spring bread wheat grown under well-watered conditions

Root system traits have positive effects on wheat grain yield, particularly in drought environments. Root traits are difficult to manipulate using conventional selection procedures. Marker-assistedselection (MAS) could be helpful for the improvement of root morphological traits. A recombinant inbred line (RIL) population of 168 lines derived from the cross Iran #49 9 Yecora Rojo was used to map quantitative trait loci (QTLs) for root traits at midtillering stage for one season and for root and shoot traits at plant maturity for two seasons using two different subsets. The RILs were grown in sand-tube experiments in a glasshouse under well-watered conditions. Longest root (LR), total root length longer 30 cm (TRL30), shallow root weight (roots between 0 and 30 cm, SRW), deep root weight (roots bellow 30 cm, DRW), total root biomass (RBio), ratio of root to shoot (RTS) and to plant (RTP) biomass were measured at mid-tillering. At maturity, number of days to booting (DTB), to heading (DTH), to anthesis (DTA), and to maturity (DTM), plant height (PH), flag leaf area (FLA), number of tillers (NTP) and spikes (NSP) per plant, number of grains (NGP), grain weight (GW), grain yield (GY) per plant, LR, SRW, DRW, RBio, PBio, and RTP were measured. At mid-tillering, a total of 18 putative QTLs were detected with individual QTL accounted for between 6.5 and 26.5 % of the variation in the traits. The QTLs were distributed on chromosomes 1B, 2A, 2D, 4B, 6B, 7A, and 7D. A major and two minor QTLs were identified for LR, with the major QTL (qLR-2D) explaining 26.5 % of variation. Two QTLs were detected for DRW on chromosome 4B between markers Gwm6 and Sukkula.1220 that together explained 23.1 % of variation. One region between marker Wmc198 and Cfa2263 on chromosome 2A contained four QTLs affecting PH, SRW, RTS, and RTP. At maturity, 70 putative QTLs were detected across the two seasons with a single QTL accounted for between 7.7 and 40.6 % of variation in the traits. Three major colocalized QTLs for SRW, DRW, and RBio were identified on chromosome 2D between markers Wms515 andWms102 that accounted for 19.8, 20.5, and 22.4 %0f variation, respectively. Two major colocalized QTLs for SRW and RBio were detected on chromosome 3A that explained 17.8 and 13.4 % of variation, respectively. One major QTL for DRW was identified on chromosome 1B that accounted for 20.3 % of variation. Chromosome 2B harbored major QTLs for GY, NGS, and NGP. A major QTL cluster was detected on chromosome 2D and on chromosome 4A relating 11 and eight QTLs for phenological periods, root traits, RTS, and RTP, indicating pleiotropic effects on these traits. Of the four common root traits studied at mid-tillering and at maturity, only SRW had linked QTLs on chromosome 2A at both stages of plant growth, indicating selection for root traits at seedling stage alone may not be effective in changing root morphological characteristics at later stages of plant growth. It appeared that chromosome 2A, 2D, and 4B harbored genes regulating growth of root traits at early and later stages of plant growth. The molecular markers closely linked to QTLs for root and shoot traits may be used in wheat breeding program using MAS procedures.The research was financed by The California Wheat Commission, The University of California- Riverside Botanic Gardens, and the California Agricultural Experiment Station

Dosage effect of the short arm of chromosome 1 of rye on root morphology and anatomy in bread wheat.

The spontaneous translocation of the short arm of chromosome 1 of rye (1RS) in bread wheat is associated with higher root biomass and grain yield. Recent studies have confirmed the presence of QTL for different root morphological traits on the 1RS arm in bread wheat. This study was conducted to address two questions in wheat root genetics. First, does the presence of the 1RS arm in bread wheat affect its root anatomy? Second, how does root morphology and anatomy of bread wheat respond to different dosages of 1RS? Near-isogenic plants with a different number (0 to 4 dosages) of 1RS translocations were studied for root morphology and anatomy. The F(1) hybrid, with single doses of the 1RS and 1AS arms, showed heterosis for root and shoot biomass. In other genotypes, with 0, 2, or 4 doses of 1RS, root biomass was incremental with the increase in the dosage of 1RS in bread wheat. This study also provided evidence of the presence of gene(s) influencing root xylem vessel number, size, and distribution in bread wheat. It was found that root vasculature follows a specific developmental pattern along the length of the tap root and 1RS dosage tends to affect the transitions differentially in different positions. This study indicated that the inherent differences in root morphology and anatomy of different 1RS lines may be advantageous compared to normal bread wheat to survive under stress conditions