Xuekun Zhang

Recent progress in drought and salt tolerance studies in Brassica crops

Water deficit imposed by either drought or salinity brings about severe growth retardation and yield loss of crops. Since Brassica crops are important contributors to total oilseed production, it is urgently needed to develop tolerant cultivars to ensure yields under such adverse conditions. There are various physiochemical mechanisms for dealing with drought and salinity in plants at different developmental stages. Accordingly, different indicators of tolerance to drought or salinity at the germination, seedling, flowering and mature stages have been developed and used for germplasm screening and selection in breeding practices. Classical genetic and modern genomic approaches coupled with precise phenotyping have boosted the unravelling of genes and metabolic pathways conferring drought or salt tolerance in crops. QTL mapping of drought and salt tolerance has provided several dozen target QTLs in Brassica and the closely related Arabidopsis. Many drought- or salt-tolerant genes have also been isolated, some of which have been confirmed to have great potential for genetic improvement of plant tolerance. It has been suggested that molecular breeding approaches, such as marker-assisted selection and gene transformation, that will enhance oil product security under a changing climate be integrated in the development of drought- and salt-tolerant Brassica crops.

The Transcriptome of Brassica napus L. Roots under Waterlogging at the Seedling Stage

Although rapeseed (Brassica napus L.) is known to be affected by waterlogging, the genetic basis of waterlogging tolerance by rapeseed is largely unknown. In this study, the transcriptome under 0 h and 12 h of waterlogging was assayed in the roots of ZS9, a tolerant variety, using digital gene expression (DGE). A total of 4432 differentially expressed genes were identified, indicating that the response to waterlogging in rapeseed is complicated. The assignments of the annotated genes based on GO (Gene Ontology) revealed there were more genes induced under waterlogging in “oxidation reduction”, “secondary metabolism”, “transcription regulation”, and “translation regulation”; suggesting these four pathways are enhanced under waterlogging. Analysis of the 200 most highly expressed genes illustrated that 144 under normal conditions were down-regulated by waterlogging, while up to 191 under waterlogging were those induced in response to stress. The expression of genes involved under waterlogging is mediated by multiple levels of transcriptional, post-transcriptional, translational and post-translational regulation, including phosphorylation and protein degradation; in particular, protein degradation might be involved in the negative regulation in response to this stress. Our results provide new insight into the response to waterlogging and will help to identify important candidate genes.

Transcriptome Analysis of Canola (Brassica napus) under Salt Stress at the Germination Stage

Canola (Brassica napus) is one of the most important oil crops in the world. However, its yield has been constrained by salt stress. In this study, transcriptome profiles were explored using Digital Gene Expression (DGE) at 0, 3, 12 and 24 hours after H2O (control) and NaCl treatments on B. napus roots at the germination stage. Comparisons of gene-expression between the control and the treatment were conducted after tag-mapping to the sequenced Brassica rapa genome. The differentially expressed genes during the time course of salt stress were focused on, and 163 genes were identified to be differentially expressed at all the time points. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses revealed that some of the genes were involved in proline metabolism, inositol metabolism, carbohydrate metabolic processes and oxidation-reduction processes and may play vital roles in the salt-stress response at the germination stage. Thus, this study provides new candidate salt stress responding genes, which may function in novel putative nodes in the molecular pathways of salt stress resistance.

Imbibition behavior and flooding tolerance of rapeseed seed (Brassica napus L.) with different testa color

In this study, the correlations among these characters were investigated in 18 rapeseed (Brassica napus L.) accessions with different seed coat color. The results indicated that seed water uptake and flooding tolerance were significantly correlated with seed color and melanin pigment content of testa. The red or black-seeded accessions had higher melanin content in testa, showed slower water uptake and lower leakage and higher flooding tolerance. The majority of yellow-seeded rapeseed accessions which had low melanin pigments content in testa showed a rapid water uptake and higher leakage than the red or black-seeded, which led to imbibition damage and lower flooding tolerance. The results suggest that the yellow-seeded cultivars experienced poor field emergence and more serious pre-harvest sprouting in raining weather than the dark-seeded cultivars. Some yellow-seeded accessions showed a slow imbibition behavior and relative high flooding tolerance, indicated these accessions can be used as a genetic resource to improve the flooding tolerance and reduce imbibition damage for the yellow-seeded B. napus L.

Relationship Among Yield Components and Selection Criteria for Yield Improvement in Early Rapeseed （Brassica napus L.）

Abstract The objective of this study was to determine the relationship between seed yield and other important agronomic traits of early-maturing rapeseed as a rotation crop in a double-cropping rice area using Pearsons correlation coefficient as well as to estimate direct and indirect effects of specific yield component traits on seed yield via path analysis. Nineteen rapeseed genotypes were grown at ten environments in South China during 2008-2009 and 12 characters were evaluated. Analysis of variance showed that environment had a significant impact on all characters. For most characters the genotype by environment interaction was weak and not statistically significant. Simple correlation analysis indicated that the number of primary branches (PB), number of pods on the main raceme (PR), and number of seeds per pod made significant contributions to seed yield per plant (SYP), while 1 000-seed weight was negatively correlated with SYP (r=−0.485, P

A Comparison of Screening Methods to Identify Waterlogging Tolerance in the Field in Brassica napus L. during Plant Ontogeny

Waterlogging tolerance is typically evaluated at a specific development stage, with an implicit assumption that differences in waterlogging tolerance expressed in these systems will result in improved yield performance in fields. It is necessary to examine these criteria in fields. In the present study, three experiments were conducted to screen waterlogging tolerance in 25 rapeseed (Brassica napus L.) varieties at different developmental stages, such as seedling establishment stage and seedling stage at controlled environment, and maturity stage in the fields. The assessments for physiological parameters at three growth stages suggest that there were difference of waterlogging tolerance at all the development stages, providing an important basis for further development of breeding more tolerant materials. The results indicated that flash waterlogging restricts plant growth and growth is still restored after removal of the stress. Correlation analysis between waterlogging tolerance coefficient (WTC) of yield and other traits revealed that there was consistency in waterlogging tolerance of the genotypes until maturity, and good tolerance at seedling establishment stage and seedling stage can guarantee tolerance in later stages. The waterlogging-tolerant plants could be selected using some specific traits at any stage, and selections would be more effective at the seedling establishment stage. Thus, our study provides a method for screening waterlogging tolerance, which would enable the suitable basis for initial selection of a large number of germplasm or breeding populations for waterlogging tolerance and help for verifying their potential utility in crop-improvement.

Combining Ability and Genetic Effects of Germination Traits of Brassica napus L.Under Waterlogging Stress Condition

Abstract Cross combinations from six rapeseed cultivars and lines were evaluated under waterlogging stress condition in order to understand the genetic mechanism of waterlogging tolerance of Brassica napus L. and provide reasonable improvement programs. There were six germination traits investigated on combining ability and heritability using complete diallel crossing method designed for 30 combinations from those six cultivars and lines. The traits included relative root length, stem length, fresh weight per plant, survival rate, electrical conductivity, and vigor index. After flooding treatment, the six traits of parents and F 1 were analyzed. The general combining ability (GCA) and special combining ability (SCA) of germination traits were analyzed using Griffing I method. Among 30 cross combinations, the GCA was significantly different among six waterlogging resistance traits. The SCA of these traits was significantly different except the SCA of electrical conductivity. As a representative trait of waterlogging tolerance in rapeseed, relative vigor index had the highest narrow heritability and relatively low broad heritability. The cultivars Zhongshuang 9 and P79 had higher tolerance potential to waterlogging stress. It can be concluded that combining ability and genetic effects of relative vigor index during germination stage could be used to identify the waterlogging tolerance of rapeseed in breeding program.

Screening and Identification of Waterlogging Tolerant Rapeseed (Brassica Napus L) During Germination Stage

To develop a suitable screening criterion of water logging tolerant varieties, germination rate, radical length, hypocotyl length and seedling fresh weight were investigated for 50 rapeseed varieties under water logging. The results revealed radical length, hypocotyl length and seedling fresh weight were significantly reduced under water logging. Germination rate was relatively insensitive compared to the other three traits and was not a continuous distribution. Water logging tolerance index combining seed germination rate and the other 3 index was significantly related with all the traits evaluated. Therefore, the water logging tolerance index could be used as a screening tool for selecting the tolerant varieties.

Comparison of transcriptomes undergoing waterlogging at the seedling stage between tolerant and sensitive varieties of Brassica napus L.

Abstract RNA sequencing of the sensitive GH01 variety of Brassica napus L. seedling roots under 12 h of waterlogging was compared with previously published data of the ZS9 tolerant variety to unravel genetic mechanisms of waterlogging tolerance beyond natural variation. A total of 2 977 genes with similar expression patterns and 17 genes with opposite expression patterns were identified in the transcription profiles of ZS9 and GH01. An additional 1 438 genes in ZS9 and 1 861 genes in GH01 showed strain specific regulation. Analysis of the overlapped genes between ZS9 and GH01 revealed that waterlogging tolerance is determined by ability to regulate genes with similar expression patterns. Moreover, differences in both gene expression profiles and abscisic acid (ABA) contents between the two varieties suggest that ABA may play some role in waterlogging tolerance. This study identifies a subset of candidate genes for further functional analysis.

Identification of QTLs for branch number in oilseed rape (Brassica napus L.).

Oilseed rape(Brassica napus L.)is one of the most important oil crops worldwide and provides about 50 percent of the vegetable oil supply in China(Yin et al.,2009).The development of rapeseed varieties with higher yield is an effective measure to optimize balance between the supply and demand