Stress-response relationships related to aging and death of orthodox seeds: A study comparing viability and RNA integrity in soybean (Glycine max) cv. Williams 82

Authors: Walters, Christina; FLEMING, MARGARET; Hill, Lisa; DORR, EMMA - COLORADO STATE UNIVERSITY; Richards, Christopher

Submitted to: Seed Science Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/3/2019
Publication Date: 6/1/2020
Citation: Walters, C.T., Fleming, M.B., Hill, L.M., Dorr, E.J., Richards, C.M. 2020. Stress-response relationships related to aging and death of orthodox seeds: A study comparing viability and RNA integrity in soybean (Glycine max) cv. Williams 82. Seed Science Research. 30(2):161-172. https://doi.org/10.1017/S0960258520000197.
DOI: https://doi.org/10.1017/S0960258520000197

Interpretive Summary: Seeds die during dry storage, but it is impossible to tell when this happens because seeds don't appear to be alive until they receive water. This enigma forms the basis of exploring 'solid state' biology, in which cells are preserved by solidifying the cytoplasm. Aging reactions are slowed when cells become solid matrices and this is why seed banks, like NLGRP, can maintain the viability of germplasm for decades. Monitoring viability, so we know when the seed has eventually succumbed, is very costly and only tells us after-the-fact that the germplasm cannot be used. We are developing methods to reliably predict seed 'expiration dates' by assessing the physics and biochemistry of their solidified cytoplasm. In this body of work, we explore the relationship between accumulated damage to molecules in the cell and eventual mortality. Here, we use RNA degradation as a measure of damage and we show that it correlates well with the rate of viability loss under stressful conditions that accelerate aging. Changes in RNA integrity can be detected before seeds die and can, therefore, serve as an early warning. The work contributes to our understanding of the causes of aging as well as helps curators identify the longevity of seeds in their collections.

Technical Abstract: Unscheduled losses in seed viability is a major problem for seed industries and genetic resource collections (i.e. genebanks). Characterizing nonlethal damage within dry seeds may allow us to detect early signs of aging and accurately predict longevity. We compared RNA degradation and viability loss in seeds exposed to stressful conditions to quantify relationships between degradation rates and stress intensity or duration. We subjected recently-harvested (“fresh”) ‘Williams 82’ soybean seeds to moisture, temperature and oxidative stresses, and measured time to 50% viability (P50) and rate of RNA degradation, the former using standard germination assays and the latter using RNA Integrity Number (RIN). RIN values from fresh seeds were also compared with those from accessions of the same cultivar harvested in the 1980s and 1990s and stored in the refrigerator (5oC), freezer (-18oC) or in vapor above liquid nitrogen (-176oC). Rates of viability loss (P50-1) and RNA degradation (RIN·d-1) were highly correlated in soybean seeds that were exposed to a broad range of temperatures [holding relative humidity (RH) constant at about 30%]. However, the correlation weakened when fresh seeds were maintained at high RH (holding temperature constant at 35oC) or exposed to oxidizing agents. Both P50-1 and RIN·d-1 parameters exhibited breaks in Arrhenius behavior near 50oC, suggesting that constrained molecular mobility regulates degradation kinetics of dry systems. We conclude that the kinetics of aging reactions at RH near 30% can be simulated by temperatures up to 50oC and that RNA degradation can indicate aging prior to and independent of seed death.