The evolution of life history strategies in plants and its relationship to the evolution of protective cellular mechanisms against reactive oxygen species

Abstract

Purpose. The Oxidative Stress Theory of Aging (OSTA) states that accumulation of oxidative damage is a major contributor to aging; however, until now, no studies have examined whether perennial plants exhibit cellular mechanisms to better protect themselves against oxidative damage than annual plants, nor how these mechanisms may have evolved. Methods. We undertook three approaches to evaluate the capacity for annual and perennial plants to resist oxidative damage. The first approach involved using an electrolyte leakage assay to assess the rate of cellular damage in leaves exposed to exogenous H 2 O 2 . The second approach involved determining the concentration of exogenous H 2 O 2 required to maximize germination rates, which provides insight about the antioxidant levels in seeds. The third approach involved assessing the susceptibility of chlorophyll a and chlorophyll b to exogenous H 2 O 2 and determining chlorophyll a/b ratios. We also conducted an ancestral state reconstruction of life history strategies in order to interpret our results in an evolutionary context. Results. Leaves from deciduous and evergreen perennials showed a lower rate of cellular damage than leaves from annuals when exposed to exogenous H 2 O 2 . Seeds from deciduous perennials—but not biennials or evergreen perennials—required a higher H 2 O 2 concentration to maximize germination rate compared to seeds from annuals, suggesting that seeds from deciduous perennials have higher antioxidant levels. Although chlorophyll b was found to be more susceptible to damage from exogenous H 2 O 2 , chlorophyll a/b ratios did not differ among life history strategies. Ancestral state reconstruction revealed that the ancestral plant was most likely an evergreen perennial. Conclusion. Our results showcase that resistance to oxidative stress is necessary for perennial plants to survive over multiple years. The mechanisms responsible for the increased tolerance of perennial species to oxidative stress has not been fully elucidated by this study, but it does not involve changes to chlorophyll a/b ratios, as such changes could disrupt photosynthesis. The developmental onset of these protective mechanisms was delayed in evergreen perennials compared to deciduous perennials, perhaps because the ancestral evergreens were primarily focused on rapid colonization of the terrestrial environment, which requires faster germination rates induced by higher H 2 O 2 levels.