Genetic dissection of drought tolerance in chickpea (Cicer arietinum L.).

KEY MESSAGE: Analysis of phenotypic data for 20 drought tolerance traits in 1-7 seasons at 1-5 locations together with genetic mapping data for two mapping populations provided 9 QTL clusters of which one present on CaLG04 has a high potential to enhance drought tolerance in chickpea improvement. Chickpea (Cicer arietinum L.) is the second most important grain legume cultivated by resource poor farmers in the arid and semi-arid regions of the world. Drought is one of the major constraints leading up to 50% production losses in chickpea. In order to dissect the complex nature of drought tolerance and to use genomics tools for enhancing yield of chickpea under drought conditions, two mapping populations-ICCRIL03 (ICC 4958 × ICC 1882) and ICCRIL04 (ICC 283 × ICC 8261) segregating for drought tolerance-related root traits were phenotyped for a total of 20 drought component traits in 1-7 seasons at 1-5 locations in India. Individual genetic maps comprising 241 loci and 168 loci for ICCRIL03 and ICCRIL04, respectively, and a consensus genetic map comprising 352 loci were constructed ( http://cmap.icrisat.ac.in/cmap/sm/cp/varshney/). Analysis of extensive genotypic and precise phenotypic data revealed 45 robust main-effect QTLs (M-QTLs) explaining up to 58.20% phenotypic variation and 973 epistatic QTLs (E-QTLs) explaining up to 92.19% phenotypic variation for several target traits. Nine QTL clusters containing QTLs for several drought tolerance traits have been identified that can be targeted for molecular breeding. Among these clusters, one cluster harboring 48% robust M-QTLs for 12 traits and explaining about 58.20% phenotypic variation present on CaLG04 has been referred as "QTL-hotspot". This genomic region contains seven SSR markers (ICCM0249, NCPGR127, TAA170, NCPGR21, TR11, GA24 and STMS11). Introgression of this region into elite cultivars is expected to enhance drought tolerance in chickpea.

A high-grain protein content locus on barley (Hordeum vulgare) chromosome 6 is associated with increased flag leaf proteolysis and nitrogen remobilization.

Leaf senescence and nitrogen remobilization from senescing tissues are two important factors determining grain protein content (GPC) in cereals. We compared near-isogenic barley (Hordeum vulgare L.) germplasm varying in the allelic state of a major GPC quantitative trait locus on chromosome 6, delineated by molecular markers HVM74 and ABG458 and explaining approximately 46% of the variability in this trait. High GPC was consistently associated with earlier whole-plant senescence. SDS-PAGE and immunoblot analysis of flag leaf proteins indicated earlier leaf protein [including ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)] degradation in high-GPC germplasm. This was accompanied by enhanced availability of ammonium and glutamine in developing kernels, suggesting increased phloem retranslocation of nitrogen. Based on previous microarray analysis, we performed a detailed expression study of six leaf genes, tentatively involved in plastidial proteolysis, vacuolar proteolysis, intermediary N metabolism and N transport. All of these were upregulated in high-GPC barley, mostly around 21 to 28 days past anthesis, prior to or around the time demonstrating maximal differences in leaf protein (including Rubisco) levels. Therefore, these genes represent potential targets to manipulate grain protein accumulation. It appears likely that their functional analysis will enhance our understanding of whole-plant N recycling. Additionally, earlier leaf (photosynthetic) protein degradation may lead to reduced N carbon assimilation in high-GPC germplasm, explaining past studies demonstrating a negative correlation between GPC and yield.

Effects of a barley (Hordeum vulgare) chromosome 6 grain protein content locus on whole-plant nitrogen reallocation under two different fertilisation regimes.

A large fraction of protein N harvested with crop seeds is derived from N remobilisation from senescing vegetative plant parts, while a smaller fraction stems from de novo N assimilation occurring after anthesis. This study contrasts near-isogenic barley (Hordeum vulgare L.) germplasm, varying in the allelic state of a major grain protein content (GPC) locus on chromosome 6. Plant material was grown under both low- and high-N fertilisation levels. The analyses indicated that leaf N remobilisation occurred earlier in high-GPC germplasm under both fertilisation regimes, as indicated by an earlier decrease of total leaf N, chlorophylls, soluble- and membrane-proteins. At the same time, kernel free amino acid levels were enhanced, while leaf free amino acid levels were lower in high-GPC barleys, suggesting enhanced retranslocation of organic N to the developing sinks. Enhanced or longer availability of leaf nitrates was detected in high-GPC varieties and lines, at least under high N fertilisation, indicating that the GPC locus profoundly influences whole-plant N allocation and management. Results presented here, together with data from a recent transcriptomic analysis, make a substantial contribution to our understanding of whole-plant N storage, remobilisation and retranslocation to developing sinks.

Comparative transcriptome profiling of near-isogenic barley (Hordeum vulgare) lines differing in the allelic state of a major grain protein content locus identifies genes with possible roles in leaf senescence and nitrogen reallocation.

To identify genes involved in the regulation and execution of leaf senescence and whole-plant nitrogen reallocation, near-isogenic barley germplasm divergent in senescence timing and protein concentration of mature grains was contrasted. Barley lines differing in allelic state at a major locus on chromosome six, controlling grain protein concentration, were obtained after four generations of backcrossing. Based on physiological data indicating major differences between low- and high-grain protein germplasm at 14-21 d past anthesis, the flag leaf and kernel transcriptomes of the low-protein parent and one high-protein near-isogenic line were compared at these time points, using the 22-k Barley1 Affymetrix microarray. Our data associate several genes with both known (based on sequence comparisons) and unknown functions with the senescence process. These include leucine-rich repeat transmembrane protein kinases, a glycine-rich RNA-binding protein with homology to AtGRP7 and a 'mother of FT/TF1' gene. Our data also indicate upregulation of genes coding for both plastidial and extraplastidial proteases in germplasm with accelerated leaf senescence. Functional characterization of candidate genes identified by this research may contribute to our understanding of the molecular network underlying leaf senescence and nitrogen reallocation.

Molecular and biochemical characterisation of polyphenol oxidases in developing kernels and senescing leaves of wheat (Triticum aestivum).

Polyphenol oxidases (PPOs) have been implicated in plant defence reactions. From an applied point of view, high PPO activity is associated with browning / darkening of fresh and processed food. Owing to its complex genome and economic importance, wheat (Triticum aestivum L.) represents an interesting system to advance our understanding of plant PPO function. We have previously shown that wheat PPOs are organised in a multigene family, consisting of two distinct phylogenetic clusters with three members each. In this study, we demonstrate that members of one cluster are not expressed in developing kernels or senescing flag leaves. Transcriptional regulation of one major gene in the other cluster largely controls PPO levels in these tissues, at least in the wheat varieties used for this study. Our data further indicate that the product of this gene is present as a latent enzyme during early kernel development, and that the latent enzyme is activated during later developmental phases. Enzyme activation can be achieved in vitro by limited tryptic digestion, but our data do not indicate activation by a proteolytic mechanism in vivo. Together, results presented in this study provide important insights into the regulation of wheat PPO function.