Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)

Department / School

Plant Science

First Advisor

Senthil Subramanian


Auxin, Auxin biosynthesis, Auxin signaling, Rhizobia, Soybean


Nitrogen(N) is one of the most important plant nutrients for plant growth and yield, however, its abundance in the soil is not sufficient for profitable crop production. The use of chemical fertilizers helps address soil N deficiency in agriculture. However, due to the environmental pollution resulting from excessive use of fertilizers, alternative forms of N for agriculture are a necessity. Leguminous plants such as soybean (Glycine max) form a symbiotic association with N-fixing rhizobia to meet their N demands. Legume-rhizobia symbiosis results in the formation of unique structures called nodules where rhizobia convert atmospheric nitrogen into plant usable forms, thus reducing the need for chemical fertilizer application. The plant hormone auxin plays a crucial role in determining the number of nodules and their rate of maturity in soybean. A better understanding of what roles auxin plays in regulating nodule number and nitrogen fixation would help devise strategies to enhance or optimize N supply from nodules of soybeans and other legumes. Indole-3-acetic acid (IAA) is the most abundant natural form of auxin produced by plants. The conversion of tryptophan to IAA through the two-step IPA pathway is the main source of auxin in Arabidopsis. Auxin action is mediated by a group of F-box proteins belonging to the TIR/AFB family that act as receptors. In this study, we evaluated the role of auxin biosynthesis and signaling in soybean root, nodule development, and nitrogen fixation using yucasin and PEO-IAA, chemicals that inhibit auxin biosynthesis and signaling respectively. The suppression of auxin biosynthesis with the use of yucasin in soybean roots led to a significant increase in root length and to a significant reduction in the expression of the IAA9, an auxin-responsive marker gene. Moreover, the suppression of auxin biosynthesis in soybean roots led to a significant increase in total nodule and mature nodule number at 21 days post rhizobia inoculation (dpi). The expression levels of FWL1, ENOD2, ENOD40, and NSP1 showed an increasing trend but the expression level of these markers was not significantly increased. The application of yucasin did not significantly affect nodule nitrogenase activity per plant and nodule. Pod number, seed number, seed weights, and seed protein concentration were used as a grain yield measurement in soybean and results showed that yucasin treatment did not affect grain yield. The suppression of auxin signaling with the use of PEO-IAA in soybean root led to increase in root length and a reduction in lateral root density. The expression level of auxinresponsive genes showed variable expression levels. Moreover, the application of PEOIAA in soybean root increased the total number of nodules and the mature number of nodules at 10 and 20 μM at 21 dpi. The expression of ENOD2, ENOD40, NIN, FWL1, and NSP1 had a consistent trend of increased marker gene expression at 10μM and 20μM PEOIAA. There was a significant increase in the expression of FWL1, ENOD2, and NIN at 10μM PEO-IAA treatment. Moreover, the application of PEO-IAA did not affect any of the grain yield-related traits evaluated. Overall, our results confirmed that auxin biosynthesis and signaling play an important role in soybean root and nodule development, and that manipulation of auxin biosynthesis and signaling could be used to optimize nodule numbers and potentially nitrogen fixation and grain yield.

Library of Congress Subject Headings

Auxin -- Synthesis.
Soybean -- Roots.
Soybean -- Development.
Growth (Plants)
Plant hormones.

Number of Pages



South Dakota State University



Rights Statement

In Copyright