The dead, hardened floral bracts of dispersal units of wild wheat function as storage for active hydrolases and in enhancing seedling vigor

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.

The Dead Can Nurture: Novel Insights into the Function of Dead Organs Enclosing Embryos

“…Many of the identified proteins possess catalytic activity including hydrolytic activity and oxireductase activity; some of the stored proteins, such as nucleases remain active after long periods (10-50 years) of storage within DOEEs [16,20,21]. Proteins released from DOEEs include several plant defensin-like (DEFL) molecules also known as low molecular weight cysteine-rich (LCR) proteins, which are implicated in defense response to fungus [22][23][24].…”

“…Chitinase and glucanase genes were often over-expressed in plants to confer resistance against fungal pathogens [28][29][30]. The proteome data also revealed S1 type endonucleases, which are released from DOEEs upon hydration [16,20,21]. Endonucleases, in general, have been implicated in diverse cellular processes including DNA synthesis and DNA repair [31] as well as in fragmentation of genomic DNA during PCD [32][33][34].…”

“…RNases were shown to inhibit growth of pathogenic fungi and application of S-like RNase NE into the extracellular space of leaves inhibits the development of Phytophthora parasitica [38,39], an oomycete soil borne pathogen with a wide range of host plants. Also, Notably, the proteome analysis identified Pathogenesis Related 4 (PR4 RNase) homologue ribonucleases released from dead seed coats of white mustard (Sinapis alba) and from dead glumes of wild emmer wheat [16,20]. The Wheatwin1 PR4 RNase was shown to enter inside fungal cells without affecting the integrity of cell walls and possesses antifungal activity, which is dependent on its enzymatic activity [40].…”

“…It is commonly believed that during PCD most macromolecules such as DNA, RNA and proteins are degraded and their constituents remobilized into other plant parts [17][18][19]. Contrary to this view, recent reports have demonstrated the capacity of DOEEs to store and maintain the integrity of hundreds of proteins, some can persist in active forms for decades, and are released to the immediate surrounding of the dispersal unit upon hydration [16,20,21].…”

The Dead Can Nurture: Novel Insights into the Function of Dead Organs Enclosing Embryos

“…Contrary to this view, recent reports have demonstrated the capacity of DOEEs to store and maintain the integrity of hundreds of proteins; some can persist in active forms for decades, and are released to the immediate surrounding of the dispersal unit upon hydration [16,20,21]. …”