Document Type

Thesis - Open Access

Award Date

2020

Degree Name

Master of Science (MS)

Department / School

Agronomy, Horticulture, and Plant Science

First Advisor

Xing-You Gu

Keywords

Ehd1, Flowering time, QTL, rice (O.sativa L.), Seed dormancy

Abstract

This research aimed to understand the evolutionary mechanism of seed dormancy and flowering time and to provide basic knowledge for the manipulation of germination features in crop breeding. Flowering time and seed dormancy are two adaptive traits for flowering plants. Seed dormancy (SD) helps regulate the timing of germination to promote the survival of seed-bearing plants in adverse environments. Cereal crops and weedy/wild relatives, such as Asian cultivated rice and weedy rice (Oryza sativa L.), have weak and strong seed dormancy, respectively. Thus, weedy and cultivated rice has been used as a model system to investigate mechanisms of seed dormancy in the previous research. Flowering time (FT) determines the timing of the seed set. Both seed dormancy and flowering time may associate with each other to complete the life cycle in natural ecosystems. A cluster of quantitative trait loci (QTL) for SD/FT (SD10/FT10) was identified on chromosome 10, which was responsible for an SD-FT association in rice (O. sativa L.). Fine-mapping delimited SD10/FT10 to a 200-kb region, which contains Early heading1 (Ehd1). Ehd1 encodes a Myb transcriptional factor (TF) and promotes flowering. Thus, Ehd1 could be an underlying gene of FT10. However, Ehd1 was not reported for an effect on SD. Thus, the research was conducted to determine if Ehd1 is an underlying gene for the QTL FT10 only or both FT10 and SD10. The first objective of this research was to evaluate RNA interference (RNAi)-mediated effects of Ehd1 on FT and SD in generations of transgenic lines. An Ehd1-RNAi construct was used to transform the cultivar Nippponbare (O. sativa, ssp. Japonica), and then introduced into the genetic background of EM93-1 (O. sativa, ssp. indica), the donor of the flowering-promoting allele at FT10/SD10 or Ehd1, by marker-assisted backcrossing (BC). Single plant-derived BC2F3 (selected from #14 and #47), BC2F4 and BC2F5 (selected from #81 and #84) lines were evaluated for the presence or absence of the Ehd1-RNAi transgene using Hygromycin-B testing, for time (days) from germination to flowering (DTF), and the degree of SD by standard germination testing. These generations of data demonstrated that there was one copy of the transgene segregating in the lines, silencing Ehd1 delayed flowering and also enhanced seed dormancy, and there was a negative correlation between DTF and germination percentage. These results indicate that Ehd1 has pleiotropic effects on both FT and SD. The second objective of this research was to identify genes related by Ehd1 to influences FT and SD. Quantitative trait analysis (QTL) was used to identify the possible genes in a mapping population of about 300 F2 plants from a cross between an Ehd1- RNAi transgenic line and EM93-1. The mapping population was evaluated for DTF, SD, and the resist (R) or susceptible (S) response to Hygromicyn B, and genotyped for about 50 DNA markers distributed along the 12 chromosomes. A single copy of the RNAi construct (T-DNA) was mapped to chromosome 7, and the position was flanked by markers RM295 and RM3325. Large differences in both FT and SD between the R and S groups were observed in the mapping population. Five QTL for FT and two QTL for SD were detected in the population. Both of the FT and SD QTL includes one mapped at the transfer DNA (T-DNA) insertion position of the Ehd1-RNAi transgene. All these QTLs were located on chromosomes 7 or 8, but not on chromosome 10 or near Ehd1. The QTL differed in effect on FT or SD between the R and S groups. These results suggested that Ehd1 influences FT and SD at least partly by regulating expressions of genes underlying the QTL on chromosomes 7 and 8. This research provided evidence that Ehd1 has phenotypic effects on flowering time and seed dormancy, and Ehd1 also regulates expressions of some other genes to influence the SD-FT association.

Library of Congress Subject Headings

Rice -- Genetics.
Rice -- Flowering time.
Rice -- Seeds -- Dormancy.

Format

application/pdf

Number of Pages

121

Publisher

South Dakota State University

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Rights Statement

In Copyright