Document Type

Dissertation

Degree

Doctor of Philosophy

Major

Biology

Date of Defense

8-10-2016

Graduate Advisor

Wendy M Olivas, PhD

Co-Advisor

Taylor, Nigel

Committee

Wang, Xuemin

Zolman, Bethany

Kutchan, Toni

Abstract

The tropical root crop cassava is cultivated for its large starchy storage roots. Understanding critical processes in root tuberization is essential if improvement programs are to secure future yields for farmers. Studies were undertaken to identify critical components of storage root development at the anatomical and gene expression levels. Two types of roots were identified from greenhouse-grown stem cuttings: basal roots, which develop from the stem cut end and are prolific in nature, and nodal roots, which originate from the region of the buried axillary bud. Only nodal roots develop to produce storage organs. Anatomical sectioning was performed to determine the origin of both root types. Basal roots were seen to develop from the cambium of the semi-woody stems, while nodal roots originated from deep within the secondary xylem or pith regions. This data contradicts accepted knowledge that storage roots develop from a subset of the fibrous roots. As a result, it is proposed here that storage and basal/fibrous roots are fundamentally different organs, originate through different rhizogenic processes, and are committed to their different developmental fates from initiation onwards. cDNA microarray analysis was performed on roots at different stages of storage root development. Gene Set Enrichment Analysis revealed up-regulation of the jasmonic acid biosynthesis pathway during the initiation stage of tuberization. K-means clustering identified three clusters of up-regulated genes at storage root initiation and later developmental stages, while Heatmap analysis revealed major latex allergen Hev b 4 proteins to be highly up-regulated at the initiation stage. Three candidate genes seen to be highly up-regulated at the later starch filling stages were identified as possible homologues of Mec1, cassava ATDI21 and ENOD40-like genes. RT-PCR analysis revealed their enhanced expression in storage roots compared to fibrous roots and leaves. Mec1 has previously been associated with cassava storage roots, but no reports exist for the involvement of ATDI21 or ENOD40. The homologues of the latter two genes require further characterization to determine their functional role in storage root development. Integration of anatomical studies with functional genomics tools has provided new knowledge of root tuberization in cassava and identified new avenues of research.

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