Genomic approaches for designing durum wheat ready for climate change with a focus on drought.

Climate change is projected to have a significant impact on temperature and precipitation profiles in the Mediterranean basin. The incidence and severity of drought will become commonplace and this will reduce the productivity of rain-fed crops such as durum wheat. Genetic diversity is the material basis for crop improvement and plant breeding has exploited naturally occurring variation to deliver cultivars with improved resistance to abiotic stresses. The coupling of new genomic tools, technologies, and resources with genetic approaches is essential to underpin wheat breeding through marker-assisted selection and hence mitigate climate change. Improvements in crop performance under abiotic stresses have primarily targeted yield-related traits and it is anticipated that the application of genomic technologies will introduce new target traits for consideration in wheat breeding for resistance to drought. Many traits relating to the plant's response and adaptation to drought are complex and multigenic, and quantitative genetics coupled with genomic technologies have the potential to dissect complex genetic traits and to identify regulatory loci, genes and networks. Full realization of our abilities to manipulate metabolism, transduction pathways, and transcription factors for crop improvement ultimately relies on our basic understanding of the regulation of plant networks at all levels of function.

A high-density genetic map of hexaploid wheat (Triticum aestivum L.) from the cross Chinese Spring x SQ1 and its use to compare QTLs for grain yield across a range of environments.

A population of 96 doubled haploid lines (DHLs) was prepared from F1 plants of the hexaploid wheat cross Chinese Spring x SQ1 (a high abscisic acid-expressing breeding line) and was mapped with 567 RFLP, AFLP, SSR, morphological and biochemical markers covering all 21 chromosomes, with a total map length of 3,522 cM. Although the map lengths for each genome were very similar, the D genome had only half the markers of the other two genomes. The map was used to identify quantitative trait loci (QTLs) for yield and yield components from a combination of 24 site x treatment x year combinations, including nutrient stress, drought stress and salt stress treatments. Although yield QTLs were widely distributed around the genome, 17 clusters of yield QTLs from five or more trials were identified: two on group 1 chromosomes, one each on group 2 and group 3, five on group 4, four on group 5, one on group 6 and three on group 7. The strongest yield QTL effects were on chromosomes 7AL and 7BL, due mainly to variation in grain numbers per ear. Three of the yield QTL clusters were largely site-specific, while four clusters were largely associated with one or other of the stress treatments. Three of the yield QTL clusters were coincident with the dwarfing gene Rht-B1 on 4BS and with the vernalisation genes Vrn-A1 on 5AL and Vrn-D1 on 5DL. Yields of each DHL were calculated for trial mean yields of 6 g plant(-1) and 2 g plant(-1) (equivalent to about 8 t ha(-1) and 2.5 t ha(-1), respectively), representing optimum and moderately stressed conditions. Analyses of these yield estimates using interval mapping confirmed the group-7 effects on yield and, at 2 g plant(-1), identified two additional major yield QTLs on chromosomes 1D and 5A. Many of the yield QTL clusters corresponded with QTLs already reported in wheat and, on the basis of comparative genetics, also in rice. The implications of these results for improving wheat yield stability are discussed.

The regulation of component processes of photosynthesis in transgenic tobacco with decreased phosphoribulokinase activity.

Tobacco plants (Nicotiana tabacum L.) transformed with an inverted cDNA encoding ribulose 5-phosphate kinase (phosphoribulokinase,PRK; EC 2.7.1.19) were employed to study the in vivo relationship between photosynthetic electron transport and the partitioning of electron transport products to major carbon metabolism sinks under conditions of elevated ATP concentrations and limited ribulose 1,5-bisphosphate (RuBP) regeneration. Simultaneous measurements of room temperature chlorophyll fluorescence and CO2 gas exchange were conducted on intact leaves. Under ambient CO2 concentrations and light intensities above those at which the plants were grown, transformants with only 5% of PRK activity showed 'down-regulation" of PS II activity and electron transport in response to a decrease in net carbon assimilation when compared to wild-type. This was manifested as a decline in the efficiency of PS II electron transport (ΦPS II), an increase in dissipation of excess absorbed light in the antennae of PS II and a decline in: total linear electron transport (J1), electron transport dedicated to carbon assimilation (JA) and electron transport allocated to photorespiration (JL). The transformants showed no alteration in the Rubisco specificity factor measured in vitro and calculated in vivo but had a relatively smaller ratio of RuBP oxygenation to carboxylation rates (vo/vc), due to a higher CO2 concentration at the carboxylation site (Cc). The relationship between ΦPS II and ΦCO 2was similar in transformants and wild-type under photorespiratory conditions demonstrating no change in the intrinsic relationship between PS II function and carbon assimilation, however, a novel result of this study is that this similar relationship occurred at different values of quantum flux, J1, JA, JL and vo/vc in the transformant. For both wild-type and transformants, an assessment was made of the possible presence of a third major sink for electron transport products, beside RuBP oxygenation and carboxylation, the data provided no evidence for such a sink.

The consequences of chlorophyll deficiency for photosynthetic light use efficiency in a single nuclear gene mutation of cowpea.

The light harvesting and photosynthetic characteristics of a chlorophyll-deficient mutant of cowpea (Vigna unguilata), resulting from a single nuclear gene mutation, are examined. The 40% reduction in total chlorophyll content per leaf area in the mutant is associated with a 55% reduction in pigment-proteins of the light harvesting complex associated with Photosystem II (LHC II), and to a lesser extent (35%) in the light harvesting complex associated with Photosystem I (LHC I). No significant differences were found in the Photosystem I (PS I) and Photosystem II (PS II) contents per leaf area of the mutant compared to the wildtype parent. The decreases in the PS I and PS II antennae sizes in the mutant were not accompanied by any major changes in quantum efficiencies of PS I and PS II in leaves at non-saturating light levels for CO2 assimilation. Although the chlorophyll deficiency resulted in an 11% decrease in light absorption by mutant leaves, their maximum quantum yield and light saturated rate of CO2 assimilation were similar to those of wildtype leaves. Consequently, the large and different decreases in the antennae of PS II and PS I in the mutant are not associated with any loss of light use efficiency in photosynthesis.