Screening for drought tolerance in Brassica rapa L.: from genetic variation to gene expression

[Truncated] One of the consequences of climate change is a change in rainfall distribution with more frequent and unpredictable droughts in some regions. Drought threatens supplies of oil produced from oilseed rape/canola (Brassica napus), while at the same time, international demand for oilseeds is growing at an unprecedented rate. B. rapa, one of the diploid progenitor species of the genetically-narrow and relatively recent allopolyploid B. napus, exhibits greater genetic variation and wider natural distribution than B. napus, and therefore is a promising source of drought tolerance alleles for canola breeders.

This thesis aims to develop screening methods for drought tolerance in B. rapa during the reproductive phase, and to identify expressed drought tolerance genes that may be transferred to B. napus. An integrated approach was developed to meet this goal, from assessment of genetic diversity, physiological evaluation of drought tolerance to transcriptome analysis of drought-tolerant and drought-sensitive accessions.

First, genetic diversity in a global collection of 173 accessions of B. rapa from eight regions was assessed by 51 simple sequence repeat (SSR) markers which revealed 715 polymorphic alleles. Three groups of B. rapa were identified by STRUCTURE analysis of SSR data. Group 1 consisted of 42 accessions from Europe and west Asia-north Africa, which had the highest richness of SSR allelic diversity, and included all the seven wild type B. rapa ssp. sylvestris accessions. Group 2 consisted of 73 accessions from east Asia while group 3 was a geographically-mixed group consisting of 58 accessions across all eight regions. Pairwise population probability values for allelic variation revealed the genetic differences among eight geographic regions, which provided the genetic background for selecting representative accessions for drought tolerance screening.