Senescence: The Compromised Time of Death That Plants May Call on Themselves.

Matin Miryeganeh

doi:10.3390/genes12020143

Abstract

Plants synchronize their life history events with proper seasonal conditions, and as the fitness consequences of each life stage depend on previous and/or subsequent one, changes in environmental cues create cascading effects throughout their whole life cycle. For monocarpic plants, proper senescence timing is very important as the final production of plants depends on it. Citing available literatures, this review discusses how plants not only may delay senescence until after they reproduce successfully, but they may also bring senescence time forward, in order to reproduce in favored conditions. It demonstrates that even though senescence is part of aging, it does not necessarily mean plants have to reach a certain age to senesce. Experiments using different aged plants have suggested that in interest of their final outcome and fitness, plants carefully weigh out environmental cues and transit to next developmental phase at proper time, even if that means transiting to terminal senescence phase earlier and shortening their lifespan. How much plants have control over senescence timing and how they balance internal and external signals for that is not well understood. Future studies are needed to identify processes that trigger senescence timing in response to environment and investigate genetic/epigenetic mechanisms behind it.

Highlights

Plant Epigenetics Unit, Okinawa Institute of Science and Technology Graduate University, 1919-1 Tancha, Abstract: Plants synchronize their life history events with proper seasonal conditions, and as the fitness consequences of each life stage depend on previous and/or subsequent one, changes in environmental cues create cascading effects throughout their whole life cycle
No evidence for senescence was found as growth and fecundity did not decrease at older ages, and survival and reproductive value increased with age
As all cohorts within each greenhouse synchronized senescence with each other and not with their replicates growing in the other greenhouses, this supported the hypothesis that seasonal conditions have a greater impact on senescence timing than age, and it shows that plants control senescence by adjusting the timing of life-history events based on the environment in which they are growing (Figure 1)

Summary

Whole Plant Senescence

There are two main types of senescence in plants. The sequential or organ senescence, which happens in a continuum pattern at a certain developmental stage after accomplishing certain tasks, when senescing organs recycle their extra nutrients towards developing and growing ones [30]. Investigating senescence in pea plants (Pisum sativum L.) led scientists to reject the simplified source–sink view that says senescence is induced by developed flowers and fruits [30] They suggested that senescence is the “consequence” of reproductive phase; meaning the commitment of plant to redirect the nutrients towards reproductive sinks is “required” but “not enough”, and monocarpic senescence timing is influenced by many factors including environmental condition and the previous developmental life stages. Studying the effect of day length on senescence, as one of the main environmental factors, using different ecotypes of A. thaliana showed that long day only causes earlier senescence in early flowering accessions, and not in late flowering accessions, which shows senescence being influenced by both environment and genetic [55] They found that senescence was linked to other developmental traits such as flowering and fruit number, which was evidenced in other studies as well where correlation between flowering time and seed set/senescence was reported (see, e.g., in [56,57,58,59,60]). In order to do the proper adjustment with the environment, flowering and senescence seem to interact with each other, and studies have shown that whole plant senescence is associated with both flowering-dependent and flowering-independent pathways [16,17,29,34,42,61]

Plants Schedule Their Life Events Based on Environmental Signals

The Art of Senescence Synchrony and Harmonizing with Environmental Cues

Senescence Timing and Productivity in Plants

The Importance of Molecular Analyses of Senescence

Findings

Conclusions and Future Plans