Mechanisms underpinning the onset of seed coat impermeability and dormancy-break in Astragalus adsurgens

Abstract

Impermeable seed coats, i.e. physical dormancy (PY) influence the germination ecology of plants from 18 angiosperm families. Astragalus adsurgens (Fabaceae; Papilinoidaae) is a perennial plant widespread in temperate regions that is thought to produce both permeable and impermeable seeds. Why seeds vary in the permeability of their coat, in addition to the mechanisms by which impermeable seeds break dormancy, are not completely understood. However, seeds are often consumed by herbivores; a phenomenon that might facilitate the germination of impermeable seeds. Here, we tested whether: (1) moisture content plays a significant role in the onset of seed coat impermeability (and therefore PY) at similar ranges reported for species from tropical ecosystems; and (2) the presence of impermeable coats offer any benefits for seed survival when consumed by animals. We tested these hypotheses using A . adsurgens seeds collected from Inner Mongolia, China. Freshly collected seeds with a moisture content of 9.7% were permeable to water and therefore not physically dormant. However, seeds became impermeable when dried below a threshold of 6.5% moisture content. Treating impermeable seeds with hydrochloric acid effectively broke dormancy. Scanning electron microscope (SEM) revealed that HCl treated seeds had a narrow opening in the hilum and extra-hilar regions, through which water entered. Seeds with impermeable coats survived significantly better than permeable seeds when consumed by cows. Irrespective of coat permeability, most seeds were egested between 12 and 24 h. In seeds that maintained dormancy after gut passage, this was broken by additional acid scarification. Overall results suggest that: (1) seed coat impermeability is induced by reduced moisture content; (2) imbibition primarily occurs at the hilum and extra-hilar region; and (3) impermeable seeds may benefit from endozoochory.

Review: Plant Eco-Evolutionary Responses to Climate Change: Emerging Directions

“…Additionally, temperature and water availability affect the timing and success of seed germination [17][18][19]. Increased temperatures could reduce seed viability [20] and decreased moisture availability could prevent seeds from breaking dormancy [21]. Despite the considerable effort to understand thermal and moisture requirements for germination [22][23][24][25] (Table 1), relatively little is known about how climate change will affect the genetic, molecular, and physiological mechanisms associated with germination [26].…”

Natural quantitative genetic variation in seed size, dormancy, and seedling vigor in upland and lowland ecotypes of a C4 perennial grass

“…We tested the water permeability of seed from both parental lines across six time intervals (0hr, 1hr, 3hrs, 6hrs, 12hrs and 24hrs) following the protocol described in Jaganathan et al (2019). We replicated this test three times for both ecotypes.…”

“…These dormancy types are common in wild species and considered the main drivers for seed dormancy across plant species (Baskin and Baskin 1998;Baskin et al 2000;Baskin 2003b;Baskin and Baskin 2004). One candidate,TT7 maintains seed dormancy by reducing the permeability of the seed coat to water and chemical compounds (Jaganathan et al 2019). The other gene, BAN, increases seed dormancy by promoting seed ABA levels by an indirect mechanism involving proanthocyanidin accumulation in seeds which increases the ABA level (Jia et al 2012).…”

Characterization of the Common Genetic Basis Underlying Seed Hilum Size, Yield, and Quality Traits in Soybean

“…Plus, plant seeds with intact hilums exhibit relatively high seed vigor ( Kumar et al, 2019 ), whereas plants with injured hilums produce poor quality seeds that are susceptible to significant yield losses, possibly due to bacterial infections and reduced nutrient flows ( Hsieh et al, 2005 ). In addition, the seed hilum is also the channel for water uptake and efflux during germination ( Pietrzak et al, 2002 ; Zhang et al, 2004 ; Muramatsu et al, 2008 ; Jaganathan et al, 2019 ) and as the seed matures ( Hyde, 1954 ). Furthermore, hilum attributes might have been selected during domestication, since this tissue serves as a hygroscopically activated valve in the impermeable epidermis of the testa, which is critical for seed dormancy ( Hyde, 1954 ).…”