The Crease and the Nucellar Projection

The primary function of the tissues of the crease and the nucellar projection is the transport of assimilates to the developing grain. As with many tissues of the grain its character and construction changes through its functional life.

The crease, the mass of tissue at the bottom of the ventral groove, forms, at about the time of fertilization, from the re-directed and fused growth of two maternal tissues, the funiculus (the short stalk that joins the ovule to the carpel wall) and the tissue of the chalazal region, (which is also the source of the integuments).

It is composed of several infolded layers of cells, including: a vascular bundle originating in the maternal pericarp and a pigment strand which differentiates after the 'free nuclear division' phase of endosperm development. The nucellus in the crease region has a different fate to the nucellus of the dorsal and lateral sides of the grain and here it is called the nucellar projection (sometimes the nucellar pillar). At first the tissues of the crease grow by cell extension, but later, an intercalary meristem at the base of the vascular strand is activated. As the aleurone differentiates from the outer cells of the endosperm, it too becomes modified in the region of the crease.

The vascular bundle is the main source of nutrients to the developing grain with assimilates being unloaded into cells of the chalazal region, transported across the nucellar projection and unloaded into the endosperm cavity before active uptake into cells of the modified groove aleurone and on to the starchy endosperm.

The pigment strand develops between the vascular bundle and the nucellar projection, accumulating hydrophobic compounds of a corky and waxy nature. Once the nucellar and integument cuticles have been attached a water resistant layer is formed around the endosperm and embryo.

The tissues of the crease region, unlike those in the rest of the grain, do not become crushed as the endosperm grows and the nucellar cells remain, for some time, as a pillar of tissue running along the inner margin of the crease. But, as the aleurone layer develops, the nucellar projection becomes digested to leave only a narrow strand of occluded cells and a cuticularised epidermis.

TS of the crease region during grain growth

The pictures above are taken from transverse sections of grains harvested at 6 and 11 days after flowering. In the intervening time the endosperm has developed quickly, being well supplied with nutrients transported from the vascular bundle, across the chalaza and the nucellar projection and uploaded by the cells of the modified groove aleurone.

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