Year of Publication

2019

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Agriculture, Food and Environment

Department

Plant and Soil Sciences

First Advisor

Dr. Douglas D. Archbold

Abstract

Abiotic stresses are responsible for limiting crop production worldwide. Among diverse abiotic stresses, drought and salinity are the most challenging. Plants under these conditions have diverse strategies for tolerating stress. Osmotic adjustment and osmoprotection occur in plants during salinity and drought stress through accumulation of compatible solutes to a high level without interfering with cellular metabolism. Polyols (sugar alcohols) including sorbitol and ribitol are one such class of compatible solutes. Using plants of wild-type (WT) and three genetically-modified lines of tomato (Solanum lycopersicum cv. ‘Ailsa Craig’), an empty vector line ‘TR22’, and 2 sdh anti-sense lines ‘TR45’, and ‘TR49’ designed to severely limit sorbitol metabolism, the objective of this work was to characterize the sorbitol cycle in tomato in response to abiotic stresses. Sorbitol and ribitol content, as well as the enzymatic activities, protein accumulation, and gene expression patterns of the key sorbitol cycle enzymes ALDOSE-6-PHOSPHATE REDUCTASE (A6PR), ALDOSE REDUCTASE (AR), and SORBITOL DEHYDROGENASE (SDH), were measured in mature leaves in response to drought stress by withholding water and by using polyethylene glycol as a root incubation solution to mimic drought stress, to salt stress by incubating roots in NaCl solution, and to incubation of roots in 100 mM sorbitol and ribitol.

A6PR, not previously reported for tomato, and AR both exhibited increased activity correlated to sorbitol accumulation during the drought osmotic, and salt stresses, with SDH also increasing in WT and TR22 to metabolize sorbitol. The level of sorbitol accumulation was considerably lower than that of the common sugars glucose and fructose so was not enough to have a significant impact on tissue osmotic potential but could provide other important osmoprotective effects. Use of the sdh antisense lines indicated that SDH has the key role in sorbitol metabolism in tomato as well as a likely role in ribitol metabolism. Like sorbitol, ribitol also accumulated significantly more in the antisense lines during the stresses. Expression and/or activity of A6PR, AR, and SDH were also induced by the polyols, although it is not clear if the induction was due to a polyol signal, the osmotic effect of the incubation solution, or both. In addition, a unique post-abiotic stress phenotype was observed in the sdh anti-sense lines. After both drought and salt stresses and during a recovery phase after re-watering, the antisense lines failed to recover. This may have been due to their accumulation of ribitol. The sdh anti-sense lines were uniquely sensitive to ribitol but not sorbitol, with an apparent foliar and seed germination toxicity to ribitol. The determination that sorbitol, and perhaps ribitol as well, plays a role in abiotic responses in tomato provides a cornerstone for future studies examining how they impact tomato tolerance to abiotic stresses, and if their alteration could improve stress tolerance.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2019.311

Available for download on Sunday, January 26, 2020

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