Abstract
It is commonly assumed that the dead, hardened floral bracts of the dispersal unit of grasses have been evolved to protect seeds from predation and / or assist in fruit/caryopsis dispersal. While these structures have important agronomical and economical implications, their adaptive value has not been fully explored. We investigated the hypothesis that the maternally derived hardened floral bracts have been evolved not just as a means for caryopsis protection and dispersal, but also as storage for substances that might affect seed germination and seedling vigor. Dead glumes as well as lemmas and paleas of wild emmer wheat (Triticum turgidum var dicoccoides) were found to store and release upon hydration active hydrolases including nucleases and chitinases. High nuclease activity was released upon hydration from glumes derived from wild strains of wheat including Triticum urartu and wild emmer wheat, while very low nuclease activity was detected in glumes derived from domesticated, free-threshing wheat cultivars (e.g., durum wheat). Germination from the intact dispersal unit of wild emmer wheat was delayed, but post germination growth was better than those of separated caryopses. Most notable was a significant increase in lateral root production on seedlings germinated from the intact dispersal unit. Proteome analysis of wild emmer wheat glumes revealed many proteins stored and released upon hydration including S1-type nucleases, peptidases, antifungal hydrolases such as chitinases and β-1,3-glucanase as well as pectin acetylesterase, a protein involved in cell wall degradation and remodeling. Also, reactive oxygen species (ROS)-detoxifying enzymes such as superoxide dismutase and ascorbate peroxidase were overrepresented in dead glumes of wild emmer wheat. Thus our study highlighted previously unknown features of the dispersal unit in wild wheat in which the dead, hardened floral bracts enclosing the caryopsis store active hydrolases and nutritional elements and probably growth promoting substances that facilitate seed longevity and germination and increase seedling vigor.
Highlights
Flowering plants represent the most diverse group of land plants
These results demonstrate that dead, hardened floral bracts enclosing the caryopsis are not just passive entities involved in caryopsis protection and dispersal but rather an active unit that store and release upon hydration active hydrolases that may increase survival rate of germinating seeds
Data presented here highlighted two aspects of the dispersal unit of Poaceae species, namely, Triticum and Avena species, which have not been recognized previously. These include the finding that dead floral bracts of the dispersal unit function as storage for active hydrolases that are released upon hydration to the ‘seedsphere’ and that seedling vigor is enhanced when germinated from the whole dispersal unit compared to the thrashed seed
Summary
Flowering plants (angiosperms) represent the most diverse group of land plants They have evolved a plethora of strategies to increase fitness through extensive modification of their reproductive organs, influorescence and flower parts [1]. Species in the grass family (Poaceae), wheat, rice and maize provide a major staple food for human and are agricultural and economical most important crops. This family has evolved unique, modified influorescences and structures surrounding the flowers, which are commonly arranged in spikelets, each may contain one or multiple flowers [2]. A spikelet is composed of two hardened bracts at the base called glumes, which enclosed the flowers. The fruit is a caryopsis composed of one seed in which the seed coat is fused with the pericarp
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