Date of Graduation

12-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Plant Science (PhD)

Degree Level

Graduate

Department

Plant Pathology

Advisor/Mentor

Korth, Kenneth L.

Committee Member

Clark, John R.

Second Committee Member

Johnson, Donn T.

Third Committee Member

Rothrock, Craig S.

Fourth Committee Member

Rupe, John C.

Keywords

Microarray; Salt Stress; Soybean; Soybean Aphid; Soybean Mosaic Virus

Abstract

With the increase in saline soils worldwide, understanding the mechanisms for salt tolerance in plants is important to reduce yield loss due to salt stress. Soybean, Glycine max (L.) Merr., genotypes differ in chloride uptake with genotypes that take up chloride into foliar tissues tending to be salt-sensitive whereas those that partially exclude chloride from the leaves are more salt-tolerant. Transcriptional and physiological responses were measured in two soybean cultivars, Clark and Manokin, which differ in chloride uptake in response to salt stress and in combination with Soybean mosaic virus (SMV) and its aphid vector, Aphis glycines . The interaction of cultivar and salt treatment revealed using Gene ChipTM analysis a total of 386 genes were differentially regulated. These results were validated with RT-PCR and RT-qPCR. NaCl stress caused damage in Clark and to a lesser extent in Manokin. Likewise, the photosynthesis rate in Clark was greatly reduced by salt stress and was reduced to a lesser extent in Manokin. Reciprocal grafting demonstrated that soybean roots were responsible for salt tolerance and should be the focus of future studies on the genetic basis of salt tolerance. To study the interaction between SMV infection and salt stress, soybeans were treated with salts either prior to or after inoculation. SMV levels and gene expression levels were measured using RT-PCR, RT-qPCR, and ELISA. The SMV level in both treatment regimens was reduced in Manokin in response to the sub-lethal NaCl and CaCl2 levels and in Clark in response to CaCl2. The rate of photosynthesis was not decreased in CaCl2-treated soybeans that were infected with SMV; however, SMV infection did not alter the salt damage phenotype and salt stress did not impact SMV symptoms. Aphid populations were reduced by salt stress in both cultivars. The production of aphid-induced volatile terpenes were suppressed with salt stress and SMV infection. SMV levels were reduced in salt stressed Manokin with aphid feeding, but not in Clark

or any other treatment combination. The results show a potential synergistic interaction between SMV, salt stress, and the soybean aphid with implications for growers facing biotic and abiotics stresses in the field.

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