COS 151-6 - Lifespan extension via caloric restriction in the aquatic plant Lemna minor

Abstract: Caloric restriction (CR), in which the caloric intake of organisms is reduced below ad libitum levels, is known to extend longevity (both mean and maximum) across a range of animal species. CR-induced lifespan extension is hypothesized to occur as an adaptive trade-off response to conditions of food scarcity. Immediate reproduction is reduced in order to prioritize survival through suboptimal conditions, and to allow future offspring to be born in a more favourable environment. CR-induced lifespan extension has been observed across a broad range of heterotrophic organisms, but little investigation has been done into photoautotrophic organisms, such as plants. We aimed to determine whether CR-induced lifespan extension occurs in the short-lived aquatic angiosperm Lemna minor, and investigate how caloric restriction changed survivorship with age. As plants are typically photoautotrophic and do not ingest their diet, we effected CR via reduced light intensity. This limits photosynthesis and the formation of sugars, and is analogous to restricting food intake in animals. Light treatments ranged from 339 μmol m⁻² s⁻¹ to 4 μmol m⁻² s⁻¹. We found that light restriction significantly extended the lifespan of L. minor, with greater restriction resulting in greater lifespan extension. Surprisingly, lifetime reproduction was not found to decrease with increasing longevity, contrary to results from animal studies. Examining the data with a pace-shape framework showed that while the pace of senescence (i.e., mean lifespan) differed between treatments, the shape of senescence (i.e., age-independent survival trajectory) was similar. A follow-up experiment was performed with larger sample sizes to model survivorship with age, and found logistic models best fit the data. Comparing model parameters between treatments indicated that the rate parameters were the primary difference, further supporting the finding that the rate of senescence was responsible for differences in longevity. In conclusion, our results show that the lifespan-extending effects of caloric restriction can occur in plants as well as animal species. Thus, CR-induced lifespan extension may be a more general phenomenon than previously thought, and further investigation into plant senescence could provide insight on how and why aging occurs. In particular, the lifespan extension observed in L. minor is consistent with a change in the rate of senescence, and does not appear to rely different mechanisms of senescence in high- and low-light environments.