The Evolution of Plant Hormones: From Metabolic Byproducts to Regulatory Hubs

Abstract

As sessile organisms, plants adapt to environmental challenges through flexible developmental and physiological programs. Hormones play a central role in this adaptability, integrating environmental signals into coordinated responses that regulate growth and stress tolerance. Comparative studies across photosynthetic lineages reveal that several core hormone functions are remarkably conserved, despite major evolutionary changes in hormone perception, biosynthesis, metabolism, and transport. This conservation suggests that plant hormones have played a pivotal evolutionary role-not only preserving essential biological functions but also enabling increased complexity in plant form and function. A similar dual role is observed in evolutionary endocrinology in animals, where hormones contribute to the emergence and regulation of complex traits. We propose that hormones such as cytokinins, auxins, brassinosteroids, strigolactones, and abscisic acid originated as metabolic derivatives closely tied to core physiological functions essential for survival and reproduction, including reproductive success, nutrient sensing, and dehydration tolerance. Over time, these compounds were progressively integrated into increasingly sophisticated regulatory networks, where they now serve as central coordinators and key targets of evolutionary selection. This model advances our understanding of hormone evolution by providing a structured framework to interpret the persistence, specialization, and integration of plant hormones across evolutionary timescales.

Keywords: evolution; modularity; plant hormones; receptor signaling; stress physiology.

Figure 1

Cytokinins (CKs) are reproduction maximization hormones. CK promotes reproductive growth when conditions are favorable by enhancing shoot development and supporting resistance to shoot-specific stress (e.g., oxidative damage, pathogens). In contrast, survival-focused hormones such as auxins (AUX), abscisic acid (ABA), and gibberellic acids (GA) downregulate reproductive allocation under adverse conditions. CK functions as a permissive signal for maximal reproductive output and is repressed when environmental cues signal stress.

Figure 2

Model of hormone emergence and evolution. The model proposes three phases of hormone evolution: (1) Association Phase, where pre-hormone metabolites correlate with key adaptive traits such as reproduction or stress tolerance; (2) Causation Phase, in which these compounds acquire regulatory roles and begin actively promoting these traits; and (3) Integration Phase, marked by the evolution of hormone-specific biosynthesis, transport, metabolism, and high-sensitivity receptor-mediated signaling. The red arc indicates the strengthening regulatory role of the hormone across evolutionary time.

Figure 3

Modular hormone responses to progressive drought stress. This schematic illustrates the sequential activation of hormone modules as drought severity intensifies. Each module—CK, brassinosteroids (BR), AUX, strigolactones (SL), and ABA—initiates distinct physiological responses, enabling plants to progressively shift from maximal reproduction toward survival strategies such as root expansion, growth inhibition, and dormancy.

Figure 4

Divergent evolutionary trajectories of ABA and SL as chloroplast-derived hormone modules for dehydration stress adaptation. A conceptual model depicting the shared chloroplast-derived origins of ABA and SLs and their distinct evolutionary trajectories. ABA evolved as a module enhancing stress tolerance and dormancy, while SL diversified to promote architectural adjustments and escape responses under severe drought.

Figure 5

Emergence and diversification of salicylic acid (SA) as a hormone module. The figure maps the emergence of SA via the phenylpropanoid pathway, from trans-cinnamic acid (t-CA) through benzoic acid (BA), highlighting its ancestral link to stress defense and its subsequent regulatory specialization. The evolution of multiple SA receptors and biosynthetic routes reflects its dual role in biotic and abiotic stress adaptation.