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Characterizing the role of plant hormones during plant development and plant immunity in Arabidopsis thaliana and Solanum lycopersicum cv. Micro-Tom




Berry, Hannah Marie, author
Argueso, Cristiana, advisor
Bush, Daniel, committee member
Bedinger, Patricia, committee member
Stasevich, Timothy, committee member

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Plant hormones are major regulators of plant growth, development, and responses to biotic and abiotic stressors. Constitutive activation of immunity is commonly associated with stunted plant growth. This phenomenon, called the growth defense tradeoff, was previously thought to result from limitations in metabolic processes, where resources were redirected from plant growth toward energetically costly defense responses. However, recent studies have shown that metabolic limitations are not solely responsible for the growth defense tradeoff, and that growth and defense can be uncoupled, resulting in plants with increased immunity without compromising plant yield. While the effects on plant growth have been widely characterized in the context of constitutive immunity, developmental impacts such as changes to plant architecture, reproductive development, and leaf morphology, have been studied to a lesser extent. Cytokinin is one of nine major plant hormone families and its role in plant growth, meristematic maintenance, cell division, and senescence are widely known. In conjunction with salicylic acid (SA), a primarily defense related hormone, cytokinin acts to promote SA-dependent defense responses, thus demonstrating a role for cytokinin in plant immunity. Perception of cytokinin initiates a two-component signaling phosphorelay leading to the activation of downstream transcription factors to induce the transcription of cytokinin-responsive genes. One group of these transcription factors is the CYTOKININ RESPONSE FACTORS (CRFs), which is found in all land plant species. In Chapter 2, I show that CRFs are negative regulators of plant growth and positively regulate plant defense responses. To further elucidate the role of CRFs in growth and immunity, I quantified growth and development in crosses between constitutive immunity mutants with elevated SA and CRF overexpressing (hereafter CRFox) lines. Here, these data show that CRFox enhances the growth restriction phenotypes previously characterized in the constitutive immunity mutants. I propose a model with CRF5 at the intersection of CK and SA crosstalk, acting as a regulator of the growth defense tradeoff. How constitutive immunity alters plant development as well as plant growth is not well understood. In Chapter 3, I characterize changes in plant architecture in constitutive immunity mutants SUPPRESSOR OF NPR1-1 (NONEXPRESSER OF PATHOGENESIS RELATED GENES 1), CONSTITUTIVE 1 (snc1) and CONSTITUTIVE EXPRESSION OF PR GENES 5 (cpr5). Both snc1 and cpr5 have elevated levels of endogenous SA and elevated disease resistance. While the reduced growth phenotypes (measured as biomass) of these mutants have been characterized, phyllotaxy has not previously been quantified. Phyllotaxy describes the consistent arrangement or pattern of consecutive organs around a central point. Arabidopsis has a spiral phyllotactic pattern where each consecutive organ is separated by approximately 137.5°. Phyllotactic analysis of shoot apical meristems (SAMs) using scanning electron microscopy and the arrangement of siliques on inflorescence stems in snc1 mutants showed a change in phyllotactic divergence angle originating from reduced shoot apical meristem size and increased plastochron ratio. The plastochron describes the amount of time between organ initiations but can be shown as a ratio when the edge of two consecutive inflorescence primordia to the SAM center is quantified using imaging software. To mimic the phenotypes of constitutive immunity, I inoculated wild type Arabidopsis Col-0 plants with high concentrations of Pseudomonas syringae pv. tomato (Pst) strains: (1) Pst hrcC-, which is lacks the type-III secretion system necessary to introduce bacterial effectors into the plant cell but initiates plant basal immune responses, thus inducing pattern triggered immunity (PTI); (2) Pst DC3000, which causes plant disease via introduction of bacterial effector proteins into the plant cell to change plant metabolism and dampen plant immune responses causing effector triggered susceptibility (ETS); and (3) Pst ArvRpm1 where the bacterial effector protein Rpm1 is recognized by the plant, initiating a high level of defense responses called effector triggered immunity (ETI). Only multiple, concentrated inoculations of Pst DC3000 were able to induce changes to silique phyllotactic patterns. Notably, the SAM was unaltered after Pst DC3000 inoculations, demonstrating, that phyllotactic patterns originating at the meristem are very robust upon infection, and this change in silique patterning was determined to be a result of post-meristematic stem torsion. I conclude that elevated SA throughout the plant life in the constitutive immunity mutants shows a role for SA in regulating meristematic maintenance and/or patterning, but elevated SA after pathogen attack is not sufficient to overcome the tight regulation of the meristem. Arabidopsis has been a primary source of knowledge for elucidating hormone crosstalk during plant development and immunity. However, Arabidopsis cannot inform us about the roles of hormones during fleshy fruit development. Tomatoes are an important agricultural crop species, have a fully sequenced genome, and many genetic resources are available, making tomatoes a model crop species. In my last research chapter, I quantified plant hormones in Solanum lycopersicum cv. Micro-Tom in above- and below-ground plant tissues at four stages of plant development. I selected plant developmental stages based on easily definable traits: the seedling stage was defined as the presence of cotyledons before the emergence of true leaves; the young developmental stage was characterized by the presence of four true leaves before the transition to flowering; the adult or flowering stage was determined by the opening of the first flower; and the fruiting stage was identified by fruit set and fully expanded, breaker, and ripe fruit development stages. While the data collected in this chapter is primarily descriptive, we showed that a single extraction protocol could be used to extract and quantify 18 plant hormones representing 5 of the 9 major hormone families in multiple tissue types including roots, leaves, and fruits. Plant hormone data were integrated into botanical illustrations to create the Plant Hormone Atlas, which was presented at the Art Lab Fort Collins, in Fort Collins, CO.


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cytokinin response factor
salicylic acid


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