[en] The research presented across this doctoral Thesis explores the roles of mitochondrial respiration and fibrillin proteins in plant metabolism, primarily using Arabidopsis (Arabidopsis thaliana) and tomato (Solanum lycopersicum) plants as model organisms. In particular, previous studies have reported metabolic functions of fibrillin 6 (FBN6) and mitochondrial alternative oxidase (AOX) in plants, however, their roles during carotenoid biosynthesis and stress responses remain poorly understood. The objectives of the present Thesis were focused on (i) investigating the relevance of FBN6 for the production of carotenoids and subsequent responses to high light stress in Arabidopsis; (ii) studying the role of AOX respiration during the biosynthesis of stress-related primary metabolites in Arabidopsis; and (iii) analyze the contribution, in vivo regulation and (iv) different roles of mitochondrial respiration during carotenoid biosynthesis and ripening in tomato fruits. These objectives were addressed along four different chapters. In chapter 1, the Arabidopsis FIBRILLIN6 (AtFBN6) protein is shown to be crucial for carotenoid biosynthesis, particularly under high light conditions. AtFBN6 promotes the activity of phytoene synthase (PSY), a key enzyme in the carotenoid biosynthetic pathway, highlighting its significant role in enhancing carotenoid levels. High light stress induces the expression of AtFBN6, which directly interacts with PSY to support increased carotenoid production. The absence of AtFBN6 results in decreased carotenoid levels, indicating its essential function in this biosynthetic process and its role in protecting plants from photooxidative damage under high light stress. In chapter 2, the critical role of the AOX pathway in maintaining respiratory activity and redox balance during acute water and salt stress is shown by using Arabidopsis mutants lacking AOX1a gene (AtAOX1a). The aox1a mutants exhibit decreased photosynthetic performance and higher levels of oxidative stress-related metabolites, underscoring the importance of AOX in cell redox balancing and preventing excessive reactive oxygen species (ROS) accumulation. Additionally, the AOX pathway is shown to provide metabolic flexibility, enabling the synthesis of stress-related metabolites and supporting cell osmoprotection. In chapter 3, mitochondrial respiration and electron partitioning studies in tomato fruits of wild-type (WT) plants and chromorespiration-defective ghost mutants reveal that mitochondrial respiration is the primary energy and metabolite source during fruit ripening. The in vivo activity of the AOX pathway significantly increases at the breaker stage, becoming the main contributor to climacteric respiration. Such climacteric upregulation of the AOX pathway is mediated by a post-translational activation of AOX protein, likely through interactions with key organic acid activators. Overall, AOX respiration coincided with changes in primary metabolism suggesting its role in providing carbon precursors required for fruit ripening. Finally in chapter 4, the generation of new aox1a mutants in tomato reveal the importance of AOX1a for normal growth and development of vegetative as well as fruit tissues in this species. Fruits from aox1a mutants exhibit altered respiration patterns and delayed ethylene-induced gene expression, which impacted ethylene and carotenoid metabolism during fruit ripening. Despite these early metabolic disruptions, aox1a mutants eventually compensate through the upregulation of AOX1b, partially restoring metabolic functions and carotenoid levels by the end of the ripening process. Overall, these findings highlight the multifaceted roles of AOX and fibrillin proteins in plant metabolism, stress response, and development. They underscore the complexity of metabolic pathways involved in plant growth and adaptation, offering insights into how plants manage and respond to environmental stresses through intricate biochemical networks. These insights have potential applications in agricultural practices, particularly in developing crops with enhanced stress tolerance and optimized metabolic efficiency.

[es] Esta tesis doctoral explora los roles de la respiración mitocondrial y las proteínas fibrilinas en el metabolismo de las plantas, utilizando principalmente Arabidopsis (Arabidopsis thaliana) y tomate (Solanum lycopersicum) como organismos modelo. En particular, estudios anteriores han señalado funciones metabólicas de la fibrilina 6 (FBN6) y la oxidasa alternativa mitocondrial (AOX) en plantas, pero sus roles durante la biosíntesis de carotenoides y las respuestas al estrés no se comprenden completamente. Los objetivos de esta tesis se centraron en (i) investigar la relevancia de FBN6 para la producción de carotenoides y las respuestas al estrés de luz intensa en Arabidopsis; (ii) estudiar el papel de la respiración AOX durante la biosíntesis de metabolitos primarios relacionados con el estrés en Arabidopsis; y (iii) analizar la contribución, regulación in vivo y (iv) los diferentes roles de la respiración mitocondrial durante la biosíntesis de carotenoides y la maduración en los frutos de tomate. Estos objetivos se abordaron en cuatro capítulos. En el capítulo 1, se demuestra que la proteína FIBRILLIN6 (AtFBN6) de Arabidopsis es crucial para la biosíntesis de carotenoides, particularmente bajo condiciones de luz intensa. AtFBN6 promueve la actividad de la fitoeno sintasa (PSY), una enzima clave en la vía biosintética de carotenoides, destacando así su importante función en el aumento de los niveles de carotenoides. El estrés por alta luz induce la expresión de AtFBN6, que interactúa directamente con PSY para aumentar la producción de carotenoides. La ausencia de AtFBN6 resulta en niveles disminuidos de carotenoides, indicando su función esencial en este proceso y su papel protector frente al daño fotooxidativo. En el capítulo 2, se subraya el papel crítico de la vía AOX en el mantenimiento de la actividad respiratoria y el equilibrio redox durante el estrés hídrico y salino, utilizando mutantes de Arabidopsis que carecen del gen AOX1a (AtAOX1a). Los mutantes aox1a muestran un rendimiento fotosintético reducido y mayores niveles de metabolitos relacionados con el estrés oxidativo, resaltando la importancia de AOX en el equilibrio redox celular y la prevención de la acumulación excesiva de especies reactivas de oxígeno (ROS). Además, la vía AOX ofrece flexibilidad metabólica, permitiendo la síntesis de metabolitos relacionados con el estrés y apoyando la osmo-protección celular. En el capítulo 3, estudios sobre la respiración mitocondrial y el reparto de electrones en frutos de tomate de plantas silvestres y mutantes con defectos en la cromorespiración revelan que la respiración mitocondrial es la principal fuente de energía durante la maduración. La actividad in vivo de la vía AOX aumenta significativamente en la etapa de "breaker", convirtiéndose en el principal contribuyente a la respiración climatérica. Esta regulación de la vía AOX está mediada por la activación postraduccional de la proteína AOX. En general, la respiración AOX coincide con cambios en el metabolismo primario. Finalmente, en el capítulo 4, la generación de nuevos mutantes aox1a en tomate revela la importancia de AOX1a para el crecimiento y desarrollo normal de los tejidos vegetativos y de fruto.