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Drought is one of the most important factors affecting plant growth and productivity. The previous results on drought tolerance (DT) genetic system in soybean indicated a complex of genes not only few ones were involved in the trait. This study is featured with a relatively thorough identification of QTL-allele/candidate-gene system using an efficient restricted two-stage multi-locus multi-allele genome-wide association study, on two comprehensive DT indicators, membership index values of relative plant weight (MPW) and height (MPH), instead of a single biological characteristic, in a large sample (564 accessions) of the Chinese cultivated soybean population (CCSP). Based on 24,694 multi-allele markers, 75 and 64 QTL with 261 and 207 alleles (2–12/locus) were detected for MPW and MPH, explaining 54.7% and 47.1% of phenotypic variance, respectively. The detected QTL-alleles were organized into a QTL-allele matrix for each indicator, indicating DT is a super-trait conferred by two (even more) QTL-allele systems of sub-traits. Each CCSP matrix was separated into landrace (LR) and released cultivar (RC) sub-matrices, which showed significant differentiation in QTL-allele constitutions, with 58 LR alleles excluded and 16 new ones emerged in RC. Using the matrices, optimal crosses with great DT transgressive recombinants were predicted. From the detected QTL, 177 candidate genes were annotated and validated with quantitative Real-time PCR, and grouped into nine categories, with ABA and stress responders as the major parts. The key point of the above results is the establishment of relatively full QTL-allele matrices composed of numerous gene functions jointly conferring DT, therefore, demonstrates the complexity of DT genetic system and potential of CCSP in DT breeding. [ABSTRACT FROM AUTHOR]

The DREB transcription factors regulate multiple stress response genes, and are therefore useful for molecular plant breeding. AhDREB, a stress-inducible gene, was isolated from Atriplex hortensis L. and introduced into Populus tomentosa Carrière under the control of the CaMV35S promoter. Under salt stress, the chlorophyll content and net photosynthetic rate were higher in transgenic lines than in the wild type (WT). Moreover, the rate of electrolyte penetration (REC) was lower in the transgenic lines. Additional analyses revealed that the AhDREB transgenic plants generally displayed lower malondialdehyde (MDA) activity but higher superoxide dismutase (SOD) and peroxidase (POD) activities and proline content than the WT under salt stress. RNA sequencing indicated that AhDREB could enhance tolerance to salt by activating various downstream genes in the transgenic plants. Furthermore, no growth inhibition was detected in transgenic plants expressing AhDREB driven by the constitutive CaMV35S promoter. The transcriptome showed 165 and 52 differentially expressed genes in transgenic plants under stress and non-stress conditions, respectively, among which no significant metabolic pathway was enriched and no unintended effects have yet been identified. Together, these results suggest that AhDREB may be a good candidate gene for increasing salt tolerance in transgenic poplar breeding. [ABSTRACT FROM AUTHOR]

Populus tomentosa grow rapidly, but are salt susceptible. To quickly and efficiently gain new poplar breeds with better salt resistance, a DREB transcription factor derived from Atriplex hortensis was transformed into triploid Populus tomentosa by our lab, which significantly improved the salt tolerance of host plants. However, environmental impacts of transgenic plants must be assessed before large-scale cultivation in China. Here, we conducted a field trial of AhDREB1 transgenic and non-transgenic triploid Populus tomentosa to assess the impact of transgenic trees on rhizospheric soil microbial communities and allelopathic activity of leaves. No significant differences in the number of soil microbes present were detected between the transgenic lines and the non-transgenic controls. The allelopathic activity of leaves from both the transgenic and non-transgenic lines varied with sampling time, but did not differ significantly between the transgenic and non-transgenic lines. These results indicate that the impact on the environment of AhDREB1 transgenic P. tomentosa did not differ significantly from that of the non-transformed controls for the variables observed in this field trial. We also investigated the persistence of AhDREB1 genes in decomposing transgenic poplar leaf on the soil under natural conditions for five months, and our data indicated that fragments of the genetically modified DNA were not detectable in the field after more than two months. We used a triphenyl tetrazolium chloride test (TTC) (or pollen germination method) and hybridization to test the pollen viability and fertility, respectively, of the transgenic and non-transgenic trees and the results showed that the pollen viability of both the transgenic and non-transgenic trees was extremely low in 2016; the receptor plant may have been sterile. [ABSTRACT FROM AUTHOR]