Endogenous auxin and abscisic acid in regulation of Equisetum arvense L. sporophyte growth and development

Background. Phytohormones are natural regulators of plant growth and development, with their content and distribution varying across organs and tissues throughout the plant’s life cycle. Indole-3-acetic acid (IAA) regulates organogenesis, delays aging, and is involved in responses to environmental stresses. Abscisic acid (ABA), a stress hormone, controls transpiration, root growth, and plant aging. While extensive research exists on the role of IAA and ABA in the growth and morphogenesis of higher flowering plants, their roles in vascular spore-bearing plants remain poorly understood. Materials and Methods. This study examined the dynamics and distribution of endogenous IAA and ABA in the organs of reproductive and sterile plants of the sporophyte generation of Equisetum arvense L. across nine ontogenetic phases, using HPLC-MS analysis. Results. The study found that during the growth of shoots, rhizomes, and reproductive structures, the active form of IAA accumulates. As growth slows down, organs age, and spores mature, the content of endogenous ABA increases. Across all development phases, hormone levels were higher in the organs of sterile summer plants than in reproductive spring plants, except during the germination phase for IAA and the semi-open and open strobile phases for ABA. The accumulation of free ABA in strobiles during the massive spore shedding indicated its role in regulating spore maturation and strobile aging. Hormone levels in sterile shoots of varying heights increased following the formation and growth of second-order lateral branches. In spring rhizomes, IAA and ABA accumulation occurred during the open strobile phase, while in the rhizomes of summer plants, IAA (due to the bound form) and ABA (due to the free form) accumulated in 40- and 50-cm tall plants. Upon cessation of growth, IAA levels in the rhizomes of 70-cm tall plants decreased, while ABA levels remained unchanged. Conclusions. Active growth processes in both above-ground and underground organs as well as the development of reproductive structures were associated with the accumulation of the active form of IAA. In contrast, the slowing of growth, aging of organs and maturation of spores were accompanied by increased ABA content. The study also revealed similarities in the patterns of IAA and ABA accumulation in the ontogeny of higher spore-bearing and flowering plants, contributing to the fundamental understanding of phytohormonal regulation of plant growth and development.