Jack Christopher

The role of root architectural traits in adaptation of wheat to water-limited environments

Better understanding of root system structure and function is critical to crop improvement in water-limited environments. The aims of this study were to examine root system characteristics of two wheat genotypes contrasting in tolerance to water limitation and to assess the functional implications on adaptation to water-limited environments of any differences found. The drought tolerant barley variety, Mackay, was also included to allow inter-species comparison. Single plants were grown in large, soil-filled root-observation chambers. Root growth was monitored by digital imaging and water extraction was measured. Root architecture differed markedly among the genotypes. The drought-tolerant wheat (cv. SeriM82) had a compact root system, while roots of barley cv. Mackay occupied the largest soil volume. Relative to the standard wheat variety (Hartog), SeriM82 had a more uniform rooting pattern and greater root length at depth. Despite the more compact root architecture of SeriM82, total water extracted did not differ between wheat genotypes. To quantify the value of these adaptive traits, a simulation analysis was conducted with the cropping system model APSIM, for a wide range of environments in southern Queensland, Australia. The analysis indicated a mean relative yield benefit of 14.5% in water-deficit seasons. Each additional millimetre of water extracted during grain filling generated an extra 55 kg ha-1 of grain yield. The functional implications of root traits on temporal patterns and total amount of water capture, and their importance in crop adaptation to specific water-limited environments, are discussed.

Traits and selection strategies to improve root systems and water uptake in water-limited wheat crops

Wheat yields globally will depend increasingly on good management to conserve rainfall and new varieties that use water efficiently for grain production. Here we propose an approach for developing new varieties to make better use of deep stored water. We focus on water-limited wheat production in the summer-dominant rainfall regions of India and Australia, but the approach is generally applicable to other environments and root-based constraints. Use of stored deep water is valuable because it is more predictable than variable in-season rainfall and can be measured prior to sowing. Further, this moisture is converted into grain with twice the efficiently of in-season rainfall since it is taken up later in crop growth during the grain-filling period when the roots reach deeper layers. We propose that wheat varieties with a deeper root system, a redistribution of branch root density from the surface to depth, and with greater radial hydraulic conductivity at depth would have higher yields in rainfed systems where crops rely on deep water for grain fill. Developing selection systems for mature root system traits is challenging as there are limited high-throughput phenotyping methods for roots in the field, and there is a risk that traits selected in the lab on young plants will not translate into mature root system traits in the field. We give an example of a breeding programme that combines laboratory and field phenotyping with proof of concept evaluation of the trait at the beginning of the selection programme. This would greatly enhance confidence in a high-throughput laboratory or field screen, and avoid investment in screens without yield value. This approach requires careful selection of field sites and years that allow expression of deep roots and increased yield. It also requires careful selection and crossing of germplasm to allow comparison of root expression among genotypes that are similar for other traits, especially flowering time and disease and toxicity resistances. Such a programme with field and laboratory evaluation at the outset will speed up delivery of varieties with improved root systems for higher yield.

Genotypic variation in seedling root architectural traits and implications for drought adaptation in wheat (Triticum aestivum L.)

Root system characteristics are of fundamental importance to soil exploration and below-ground resource acquisition. Root architectural traits determine the in situ space-filling properties of a root system or root architecture. The growth angle of root axes is a principal component of root system architecture that has been strongly associated with acquisition efficiency in many crop species. The aims of this study were to examine the extent of genotypic variability for the growth angle and number of seminal roots in 27 current Australian and 3 CIMMYT wheat (Triticum aestivum L.) genotypes, and to quantify using fractal analysis the root system architecture of a subset of wheat genotypes contrasting in drought tolerance and seminal root characteristics. The growth angle and number of seminal roots showed significant genotypic variation among the wheat genotypes with values ranging from 36 to 56 (degrees) and 3 to 5 (plant−1), respectively. Cluster analysis of wheat genotypes based on similarity in their seminal root characteristics resulted in four groups. The group composition reflected to some extent the genetic background and environmental adaptation of genotypes. Wheat cultivars grown widely in the Mediterranean environments of southern and western Australia generally had wider growth angle and lower number of seminal axes. In contrast, cultivars with superior performance on deep clay soils in the northern cropping region, such as SeriM82, Baxter, Babax, and Dharwar Dry exhibited a narrower angle of seminal axes. The wheat genotypes also showed significant variation in fractal dimension (D). The D values calculated for the individual segments of each root system suggested that, compared to the standard cultivar Hartog, the drought-tolerant genotypes adapted to the northern region tended to distribute relatively more roots in the soil volume directly underneath the plant. These findings suggest that wheat root system architecture is closely linked to the angle of seminal root axes at the seedling stage. The implications of genotypic variation in the seminal root characteristics and fractal dimension for specific adaptation to drought environment types are discussed with emphasis on the possible exploitation of root architectural traits in breeding for improved wheat cultivars for water-limited environments.

Identification and mapping of polymorphic SSR markers from expressed gene sequences of barley and wheat

The growing availability of EST sequences from a range of crop plantsprovides a potentially valuable source of new DNA markers. We have examined theInternational Triticeae EST Cooperative database for the presence ofdinucleotide and trinucleotide simple sequence repeats. Analysis of 24,344 ESTsidentified 388 dinucleotide repeats and 978 trinucleotide repeats in ESTs,representing 1.6% and 4.0% of the total number of ESTs, respectively. To testthe utility and cross-species transferability of EST-derived SSR markers,primers were designed to the flanking regions of 41 barley SSRs and used toscreen 11 barley and 15 wheat varieties. Sixteen of the barley SSR markers werepolymorphic in barley and five were polymorphic in wheat. This represents arelatively high level of transferability of SSR markers between barley andwheat, which has important implications for the development of new markers andcomparative mapping of barley, wheat and other cereals. An additional 56 SSRsfrom wheat ESTs were tested in the same barley and wheat varieties. Four wheatEST SSR markers were polymorphic in wheat and one in barley. Chromosomallocations in barley and wheat were determined for the majority of polymorphicmarkers.

Developmental and physiological traits associated with high yield and stay-green phenotype in wheat

Water availability is a key limiting factor in wheat production in the northern grain belt of Australia. Varieties with improved adaptation to such conditions are actively sought. The CIMMYT wheat line SeriM82 has shown a significant yield advantage in multi-environment screening trials in this region. The objective of this study was to identify the physiological basis of the adaptive traits underpinning this advantage. Six detailed experiments were conducted to compare the growth, development, and yield of SeriM82 with that of the adapted cultivar, Hartog. The experiments were undertaken in field environments that represented the range of moisture availability conditions commonly encountered by winter crops grown on the deep Vertosol soils of this region. The yield of SeriM82 was 6-28% greater than that of Hartog, and SeriM82 exhibited a stay-green phenotype by maintaining green leaf area longer during the grain-filling period in all environments where yield was significantly greater than Hartog. However, where the availability of deep soil moisture was limited, SeriM82 failed to exhibit significantly greater yield or to express the stay-green phenotype. Thus, the stay-green phenotype was closely associated with the yield advantage of SeriM82. SeriM82 also exhibited higher mean grain mass than Hartog in all environments. It is suggested that small differences in water use before anthesis, or greater water extraction from depth after anthesis, could underlie the stay-green phenotype. The inability of SeriM82 to exhibit stay-green and higher yield where deep soil moisture was depleted indicates that extraction of deep soil moisture is important.

Patterns of Carbon Partitioning in Leaves of Crassulacean Acid Metabolism Species during Deacidification

Carbohydrates stored during deacidification in the light were examined in 11 Crassulacean acid metabolism (CAM) species from widely separated taxa grown under uniform conditions. The hypothesis that NAD(P) malic enzyme CAM species store chloroplastic starch and glucans, and phosphoenolpyruvate carboxykinase species store extrachloroplastic sugars or polymers was disproved. Of the six malic enzyme species examined, Kalanchoe tubiflora, Kalanchoe pinnata, Kalanchoe daigremontiana, and Vanilla planifolia stored mainly starch. Sansevieria hahnii stored sucrose and Agave guadalajarana did not store starch, glucose, fructose, or sucrose. Of the five phosphoenolpyruvate carboxykinase species investigated, Ananus comosus stored extrachloroplastic carbohydrate, but Stapelia gigantea, Hoya carnosa, and Portea petropolitana stored starch, whereas Aloe vera stored both starch and glucose. Within families, the major decarboxylase was common for all species examined, whereas storage carbohydrate could differ both between and within genera. In the Bromeliaceae, A. comosus stored mainly fructose, but P. petropolitana stored starch. In the genus Aloe, A. vera stored starch and glucose, but A. arborescens is known to store a galactomannan polymer. We postulate that the observed variation in carbohydrate partitioning between CAM species is the result of two principal components: (a) constraints imposed by the CAM syndrome itself, and (b) diversity in biochemistry resulting from different evolutionary histories.

Frost trends and their estimated impact on yield in the Australian wheatbelt

Highlight Over the last decades, the impact of post-heading frost on yield has increased in major parts of the Australian wheatbelt. Despite global warming, frost remains a high priority for breeding.

High-throughput phenotyping of seminal root traits in wheat.

BackgroundWater availability is a major limiting factor for wheat (Triticum aestivum L.) production in rain-fed agricultural systems worldwide. Root system architecture has important functional implications for the timing and extent of soil water extraction, yet selection for root architectural traits in breeding programs has been limited by a lack of suitable phenotyping methods. The aim of this research was to develop low-cost high-throughput phenotyping methods to facilitate selection for desirable root architectural traits. Here, we report two methods, one using clear pots and the other using growth pouches, to assess the angle and the number of seminal roots in wheat seedlings– two proxy traits associated with the root architecture of mature wheat plants.ResultsBoth methods revealed genetic variation for seminal root angle and number in the panel of 24 wheat cultivars. The clear pot method provided higher heritability and higher genetic correlations across experiments compared to the growth pouch method. In addition, the clear pot method was more efficient – requiring less time, space, and labour compared to the growth pouch method. Therefore the clear pot method was considered the most suitable for large-scale and high-throughput screening of seedling root characteristics in crop improvement programs.ConclusionsThe clear-pot method could be easily integrated in breeding programs targeting drought tolerance to rapidly enrich breeding populations with desirable alleles. For instance, selection for narrow root angle and high number of seminal roots could lead to deeper root systems with higher branching at depth. Such root characteristics are highly desirable in wheat to cope with anticipated future climate conditions, particularly where crops rely heavily on stored soil moisture at depth, including some Australian, Indian, South American, and African cropping regions.

Soil coring at multiple field environments can directly quantify variation in deep root traits to select wheat genotypes for breeding

Summary Variation in deep root traits among wheat genotypes were identified in the field using high-throughput soil coring. Some weakly significant relationships were found between deep root traits and above-ground surrogates.

Single nucleotide polymorphisms in cytochrome P450 genes from barley

Abstract.Plant cytochrome P450s are known to be essential in a number of economically important pathways of plant metabolism but there are also many P450s of unknown function accumulating in expressed sequence tag (EST) and genomic databases. To detect trait associations that could assist in the assignment of gene function and provide markers for breeders selecting for commercially important traits, detection of polymorphisms in identified P450 genes is desirable. Polymorphisms in EST sequences provide so-called perfect markers for the associated genes. The International Triticeae EST Cooperative data base of 24,344 ESTs was searched for sequences exhibiting homology to P450 genes representing the nine known clans of plant P450s. Seventy five P450 ESTs were identified of which 24 had best matches in Genbank to P450 genes of known function and 51 to P450s of unknown function. Sequence information from PCR products amplified from the genomic template DNA of 11 barley varieties was obtained using primers designed from six barley P450 ESTs and one durum wheat P450 EST. Single nucleotide polymorphisms (SNPs) between barley varieties were identified using five of the seven PCR products. A maximum of five SNPs and three haplotypes among the 11 barley lines were detected in products from any one primer pair. SNPs in three PCR products led to changes between barley varieties in at least one restriction site enabling genotyping and mapping without the expense of a specialist SNP detection system. The overall frequency of SNPs across the 11 barley varieties was 1 every 131 bases.