The time to develop new cultivars and introduce them into cultivation is an issue of major importance in plant breeding. This is because plant breeders have an urgent need to help provide solutions to feed a growing world population, while in parallel, time savings are linked to profitability. Plant breeding processes may in general be broken down into the following five key elements: (1) germplasm variation; (2) crossing; (3) generation of new genetic combinations; (4) screening and selection (identification and subsequent fixation of desired allelic combinations); and (5) line/cultivar development. Each of these has implications in relation to the time taken to breed a new cultivar; a brief introduction is given for each to highlight the obstacles that may be targeted in accelerating the breeding process. Specific techniques that provide a time advantage for these elements are then discussed. Some targets for enhancing the efficiency of plant breeding, e.g., the manipulation of meiotic recombination, have proven to be recalcitrant. However, other methods that create new genetic variation along with improvements in selection efficiency compensate to a large extent for this limitation. Progress in accelerating the plant breeding process continues by exploiting new emerging ideas in science and technology.
Stay-green trait enhances sorghum adaptation to post-flowering drought. Six stay-green backcross introgression lines (BILs) carrying one or more stay-green QTLs (Stg1-4) and their parents were characterized under non-stress (W100: 100% of soil field capacity (FC)) and two levels of post-flowering drought (W75: 75% FC; W50: 50% FC) in a controlled condition. We aimed to study the response and identify the drought threshold of these QTLs under different levels of post-flowering drought and find traits closely contributing to grain yield (GY) under different drought severity. W50 caused the highest reduction in BILs performance. From W100 to W50, the GY of the recurrent parent reduced by 70%, whereas that of the BILs reduced by only 36%. W75 and W50 induce different behavior/response compared to W100. Harvest index contributed to the GY under the three water regimes. For high GY under drought transpiration rate at the beginning of drought and mid-grain filling was important at W75, whereas it was important at mid-grain filling and late-grain filling at W50. Stay-green trait can be scored simply with the relative number of green leaves/plants under both irrigated and stress environments. QTL pyramiding might not always be necessary to stabilize or increase the GY under post-flowering drought. The stay-green QTLs increase GY under drought by manipulating water utilization depending on drought severity.
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A simple protocol is presented that tests salt tolerance in rice seedlings. The method is based on a glasshouse hydroponics test in which salt is added to the nutrient hydroponic solution in which the seedlings are grown. A list of equipment is provided including hydroponic hardware and stock solutions. Advice is given on seed storage prior to use and pregermination treatments that promote even germination of test samples. Salt treatments commence after seedling establishment in hydroponics, at the 2-3 leaf stage. Information on responses of standard genotypes (tolerant, intermediate and sensitive) is given to which test seedlings are compared. Visual symptoms of salinity stress include reduced leaf area, whitish appearance of lower leaves, leaf tip death, leaf rolling and seedling death. Scoring is carried out according to the standard evaluation system developed by the International Rice Research Institute (IRRI). Recommended test salt concentrations are given along with a method to recover selected seedlings and examples of use. 4.1 Introduction Rice is one of the most important crops and is consumed by more than half of the world's population. Soil salinity is a major and increasing problem limiting rice growth and leads to huge yield losses every year. The search for new cultivars with improved tolerance to salt stress is a major goal in relieving this problem. This protocol gives an easy-to-follow procedure to select salt-tolerant rice lines for subsequent field testing. 4.2 Equipment All equipment (tanks, trays, containers, drums and platforms) is dark coloured to minimise light penetration into the culture solution, thus reducing algal growth.
Micro-tubers are important propagules in potato breeding and potato production, and they are also dormant and easily transported and therefore good targets for mutation induction in potato mutation breeding. A prerequisite for mutation breeding is to determine optimal mutation treatments. Therefore, radio-sensitivity tests of a tetraploid and a diploid potato to gamma irradiation were undertaken. Effects of different gamma sources on radioactivity were also studied. In vitro potato cuttings were gamma irradiated using a wide dose range (0, 3, 6, 9, 12, 15 and 20 Gy). The irradiated cuttings were then cultured to induce micro-tubers directly in vitro. Microtuber morphotypes were assessed after irradiation of cuttings using three gamma sources with emission activities of 1.8, 7.07 and 139 Gy/min. The diploid species (Solanum verrucosum) was more radio-sensitive than the tetraploid cultivar Desirée (Solanum tuberosum). Gamma dose rates had significant influences on subsequent micro-tuber production at various mutant generations. Effects included reductions in the number, size and weight of micro-tubers produced. Gamma dose was more lethal for the diploid potato genotype and micro-tubers produced were small compared to those produced by the tetraploid genotype after irradiation. Different treatments are recommended for diploid and tetraploid potato irradiation in producing large mutant micro-tuber populations.
Several marker-assisted selection (MAS) or backcrossing (MAB) approaches exist for polygenic trait improvement. However, the implementation of MAB remains a challenge in many breeding programs, especially in the public sector. In MAB introgression programs, which usually do not include phenotypic selection, undesired donor traits may unexpectedly turn up regardless of how expensive and theoretically powerful a backcross scheme may be. Therefore, combining genotyping and phenotyping during selection will improve understanding of QTL interactions with the environment, especially for minor alleles that maximize the phenotypic expression of the traits. Here, we describe the introgression of stay-green QTL (Stg1–Stg4) from B35 into two sorghum backgrounds through an MAB that combines genotypic and phenotypic (C-MAB) selection during early backcross cycles. The background selection step is excluded. Since it is necessary to decrease further the cost associated with molecular marker assays, the costs of C-MAB were estimated. Lines with stay-green trait and good performance were identified at an early backcross generation, backcross two (BC2). Developed BC2F4 lines were evaluated under irrigated and drought as well as three rainfed environments varied in drought timing and severity. Under drought conditions, the mean grain yield of the most C-MAB-introgression lines was consistently higher than that of the recurrent parents. This study is one of the real applications of the successful use of C-MAB for the development of drought-tolerant sorghum lines for drought-prone areas.
Potato (Solanum tuberosum L.) is an important vegetable and staple crop worldwide and mainly propagated vegetatively. Breeding of potato is problematic and therefore induced mutation is an attractive means of improving the crop. In vitro culture systems, and especially the production of microtubers, are ideal for such purposes in potato improvement. Radio-sensitivity testing (growth reduction, GR and lethal dose, LD) allows the determination of irradiation treatments (Gy) for mutation induction. Three schemes incorporating in vitro techniques were tested for mutation induction in potato namely: 1) irradiation of cuttings without leaves and subsequent dissociation of chimeras to produce plantlets or micro-tubers on M 1 V 2 (or further generation) plantlets, 2) irradiation of cuttings with leaves and direct induction of mutant micro-tubers, and 3) induction and irradiation of micro-tubers. Variability among the potato genotypes to gamma irradiation was recorded. Optimized irradiation treatments for mutation induction were established for the various tissues/propagules: cutting growth (GR 50 , 9-6 to 20.6 Gy), cutting tuberization ability (LD 50 , 7.3 to 13 Gy) and micro-tuber sprouting ability (LD 50 , 20.6 to 54.8 Gy). Micro-tubers were found to be more resistant for in vitro mutation induction than in vitro cuttings. This study shows the susceptibility of different plant tissue/propagule and potato genotypes to gamma irradiation. Radio-sensitivity analyses showed that lower gamma doses are required when mutation induction is applied in combination with micro-tuberization.
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