Timing is everything: early degradation of abscission layer is associated with increased seed shattering in U.S. weedy rice

Carrie S Thurber,Peter K Hepler,Ana L Caicedo

doi:10.1186/1471-2229-11-14

Abstract

BackgroundSeed shattering, or shedding, is an important fitness trait for wild and weedy grasses. U.S. weedy rice (Oryza sativa) is a highly shattering weed, thought to have evolved from non-shattering cultivated ancestors. All U.S. weedy rice individuals examined to date contain a mutation in the sh4 locus associated with loss of shattering during rice domestication. Weedy individuals also share the shattering trait with wild rice, but not the ancestral shattering mutation at sh4; thus, how weedy rice reacquired the shattering phenotype is unknown. To establish the morphological basis of the parallel evolution of seed shattering in weedy rice and wild, we examined the abscission layer at the flower-pedicel junction in weedy individuals in comparison with wild and cultivated relatives.ResultsConsistent with previous work, shattering wild rice individuals possess clear, defined abscission layers at flowering, whereas non-shattering cultivated rice individuals do not. Shattering weedy rice from two separately evolved populations in the U.S. (SH and BHA) show patterns of abscission layer formation and degradation distinct from wild rice. Prior to flowering, the abscission layer has formed in all weedy individuals and by flowering it is already degrading. In contrast, wild O. rufipogon abscission layers have been shown not to degrade until after flowering has occurred.ConclusionsSeed shattering in weedy rice involves the formation and degradation of an abscission layer in the flower-pedicel junction, as in wild Oryza, but is a developmentally different process from shattering in wild rice. Weedy rice abscission layers appear to break down earlier than wild abscission layers. The timing of weedy abscission layer degradation suggests that unidentified regulatory genes may play a critical role in the reacquisition of shattering in weedy rice, and sheds light on the morphological basis of parallel evolution for shattering in weedy and wild rice.

Highlights

Seed shattering, or shedding, is an important fitness trait for wild and weedy grasses
We have found that most U.S weedy rice readily shatters its seeds to a similar degree as wild rice [18]
In an effort to understand how weedy rice may have re-evolved the shattering trait after its loss in domesticated ancestors, we investigate here the morphological basis of shattering in U.S weedy rice groups

Summary

Introduction

Shedding, is an important fitness trait for wild and weedy grasses. To establish the morphological basis of the parallel evolution of seed shattering in weedy rice and wild, we examined the abscission layer at the flower-pedicel junction in weedy individuals in comparison with wild and cultivated relatives. During domestication of grass species (e.g. wheat, rye, barley, and rice), a critical shift occurred towards reductions in seed-shedding ability, facilitating the harvesting of grains [2,3,4,5]. In agricultural weeds – plants that invade cultivated fields – increased seed dispersal is believed to be favored, much as it is in wild species [2]. Seed shattering is a commonly observed trait in agricultural weedy plants that are related to domesticated species [2]. Seed shattering is under opposing selection in crops and weeds inhabiting agricultural complexes

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