Semester

Summer

Date of Graduation

2022

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Biology

Committee Chair

Jennifer S. Hawkins

Committee Co-Chair

Vagner Benedito

Committee Member

Vagner Benedito

Committee Member

Edward Brzostek

Committee Member

Jonathan Cumming

Committee Member

Timothy Driscoll

Abstract

Drought, one of the most common abiotic stressors, is a result of the precipitation and temperature fluctuations influenced by climate change. As consistent weather patterns are crucial for the maintenance of crop yield, drought threatens food security through its impact on plant growth and development. It is essential to ensure the quality, availability, and affordability of grain-based products in the face of climate change due to expectations of population growth. Therefore, shedding light on the mechanisms associated with drought tolerance is integral to maintaining agricultural production under water-limited conditions. My dissertation work aimed to uncover the morphological, physiological, and genetic controls of drought resistance in Sorghum, a C4 grain crop grown for food, feed, and biofuel. In Chapter 3, two Sorghum bicolor accessions that differ in their pre-flowering responses to drought were evaluated following long-term drought exposure across juvenile and adult vegetative stages. Findings from this work emphasized accession-specific responses to drought, indicating that morphological/histological and physiological strategies both play roles in promoting hydraulic safety in response to drought, and these mechanisms may be mutually exclusive. Chapter 4 expanded upon the findings of Chapter 3 by uncovering the evolutionary origins of the morphological and physiological responses associated with drought exposure. Using quantitative trait loci (QTL) mapping in a Sorghum recombinant inbred line (RIL) population, eight QTL unique for drought exposure were detected. S. bicolor alleles controlled reductions in height and enhanced aboveground biomass, emphasizing the impact of grain Sorghum varieties (i.e. TX7000) on drought-responsive phenotypes. These biological impacts may be influenced by the candidate genes with these QTL, specifically those involved in reproductive processes. These gene products facilitate grain production and may promote early flowering, a common drought escape mechanism that influences the transition into reproduction before stress becomes too severe. Physiologically, S. bicolor alleles increased leaf temperature while Sorghum propinquum alleles increased relative water content; these species-specific strategies reflect their variable belowground growth and impact of domestication on drought-responsive phenotypes. The QTL detected for relative water content and leaf temperature contained genes involved in auxin and abscisic acid (ABA) synthesis and signaling. In addition to playing roles in root development and water uptake, phytohormones can also affect aboveground responses, such as growth and stomatal closure. Therefore, our findings highlight the contribution of plant hormones to root-to-shoot communication and water uptake and loss through both above- and belowground strategies. The relationship between above- and belowground responses and hormone signaling was explored further in Chapter 5. Using the same Sorghum RIL population, five QTL for belowground responses to drought exposure were identified. Three of these QTL co-localized on chromosome four and with a root biomass QTL detected in this same population evaluated under salinity stress, suggesting shared genetic control of belowground traits under osmotic stress. Further, these traits were all controlled by S. bicolor alleles. This control demonstrated that root system architecture is reorganized under osmotic stress by the domesticated parent to favor vertical growth while also increasing root biomass, suggesting a main goal of enhanced water uptake in the osmotic stress response. Candidate genes within these QTL were associated with root development and hormone synthesis/recognition, contributing additional support to the allelic effects described in this work, as well as to the role of water acquisition described in Chapter 4. Genes within the two remaining QTL detected in the drought population were also involved in plant hormone responses, specifically abscisic acid (ABA). Genes encoding pentatricopeptide repeat (PPR)-containing proteins and Late Embryogenesis Abundant- like (LEA) proteins were identified in these regions. PPR’s have established roles in ABA signaling in Arabidopsis and were also shown to be up-regulated in response to heat and drought stress in Sorghum. Further, LEA proteins are induced upon ABA and osmotic stress exposure, and function as molecular chaperones. Altogether, these findings further highlight the contribution of phytohormones in drought resistance, particularly through intricate signal cascades that influence plant functioning under drought, at the morphological, physiological, and molecular levels.

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