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

Thurber CS, Hepler PK, Caicedo AL - BMC Plant Biol. (2011)

Bottom Line: 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.Consistent with previous work, shattering wild rice individuals possess clear, defined abscission layers at flowering, whereas non-shattering cultivated rice individuals do not.

View Article: PubMed Central - HTML - PubMed

Affiliation: Biology Department, University of Massachusetts, Amherst, MA 01003, USA.

ABSTRACT

Background: Seed 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.

Results: Consistent 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.

Conclusions: Seed 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.

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Comparison of abscission layers across weedy Oryza populations. Panels A-F are shattering BHA_1A05, Panels G-L are shattering SH_1A08, Panels M-R are non-shattering MXSH_1B06. Each individual was collected 1 week prior to flowering (Prior), at flowering (Flowering) and 1 week after flowering (After). Arrows point to the region of the abscission zone while white boxes outline the region magnified further. Abscission layers can be seen as darkly stained bands. Images at left were taken at 10× magnification while those at right are 60× magnification. Scale bars on bottom right represent 100 μm for 10× images and 50 μm for 60× images.
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Figure 2: Comparison of abscission layers across weedy Oryza populations. Panels A-F are shattering BHA_1A05, Panels G-L are shattering SH_1A08, Panels M-R are non-shattering MXSH_1B06. Each individual was collected 1 week prior to flowering (Prior), at flowering (Flowering) and 1 week after flowering (After). Arrows point to the region of the abscission zone while white boxes outline the region magnified further. Abscission layers can be seen as darkly stained bands. Images at left were taken at 10× magnification while those at right are 60× magnification. Scale bars on bottom right represent 100 μm for 10× images and 50 μm for 60× images.

Mentions: One week prior to flowering, all five shattering weedy rice accessions, including the two shown in Figure 2 (SH_1A08 and BHA_1A05) possess well-defined abscission layers (Figure 2A, G). Inspection with a higher magnification 60× lens shows that the BHA and SH weedy rice abscission layers prior to flowering (Figure 2B, H) are similar in staining and organization to the wild rice at flowering stage (Figure 1B, D, F); the highly lignified cells are darkly stained and starting to swell slightly, while the cells around the region are parallel to the plane of abscission. In contrast, the non-shattering MX weed shows only unbalanced, diffuse staining in the abscission zone with no clear organization of cells surrounding the zone (Figure 2M, N).


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

Thurber CS, Hepler PK, Caicedo AL - BMC Plant Biol. (2011)

Comparison of abscission layers across weedy Oryza populations. Panels A-F are shattering BHA_1A05, Panels G-L are shattering SH_1A08, Panels M-R are non-shattering MXSH_1B06. Each individual was collected 1 week prior to flowering (Prior), at flowering (Flowering) and 1 week after flowering (After). Arrows point to the region of the abscission zone while white boxes outline the region magnified further. Abscission layers can be seen as darkly stained bands. Images at left were taken at 10× magnification while those at right are 60× magnification. Scale bars on bottom right represent 100 μm for 10× images and 50 μm for 60× images.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3025945&req=5

Figure 2: Comparison of abscission layers across weedy Oryza populations. Panels A-F are shattering BHA_1A05, Panels G-L are shattering SH_1A08, Panels M-R are non-shattering MXSH_1B06. Each individual was collected 1 week prior to flowering (Prior), at flowering (Flowering) and 1 week after flowering (After). Arrows point to the region of the abscission zone while white boxes outline the region magnified further. Abscission layers can be seen as darkly stained bands. Images at left were taken at 10× magnification while those at right are 60× magnification. Scale bars on bottom right represent 100 μm for 10× images and 50 μm for 60× images.
Mentions: One week prior to flowering, all five shattering weedy rice accessions, including the two shown in Figure 2 (SH_1A08 and BHA_1A05) possess well-defined abscission layers (Figure 2A, G). Inspection with a higher magnification 60× lens shows that the BHA and SH weedy rice abscission layers prior to flowering (Figure 2B, H) are similar in staining and organization to the wild rice at flowering stage (Figure 1B, D, F); the highly lignified cells are darkly stained and starting to swell slightly, while the cells around the region are parallel to the plane of abscission. In contrast, the non-shattering MX weed shows only unbalanced, diffuse staining in the abscission zone with no clear organization of cells surrounding the zone (Figure 2M, N).

Bottom Line: 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.Consistent with previous work, shattering wild rice individuals possess clear, defined abscission layers at flowering, whereas non-shattering cultivated rice individuals do not.

View Article: PubMed Central - HTML - PubMed

Affiliation: Biology Department, University of Massachusetts, Amherst, MA 01003, USA.

ABSTRACT

Background: Seed 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.

Results: Consistent 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.

Conclusions: Seed 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.

Show MeSH
Related in: MedlinePlus