Background and Aims Seeds of carob, Chinese senna, date and fenugreek are hard due to thickened endosperm cell walls containing mannan polymers. How the radicle is able penetrate these thickened walls to complete seed germination is not clearly understood. The objective of this study was to determine if radicle emergence is related to the production of endo-b-mannanase to weaken the mannan-rich cell walls of the surrounding endosperm region, and/or if the endosperm structure itself is such that it is weaker in the region through which the radicle must penetrate. Methods Activity of endo-b-mannanase in the endosperm and embryo was measured using a gel assay during and following germination, and the structure of the endosperm in juxtaposition to the radicle, and surrounding the cotyledons was determined using fixation, sectioning and light microscopy. Key Results The activity of endo-b-mannanase, the major enzyme responsible for galactomannan cell wall weakening increased in activity only after emergence of the radicle from the seed. Thickened cell walls were present in the lateral endosperm in the hard-seeded species studied, but there was little to no thickening in the micropylar endosperm except in date seeds. In this species, a ring of thin cells was visible in the micropylar endosperm and surrounding an operculum which was pushed open by the expanding radicle to complete germination. Conclusions The micropylar endosperm presents a lower physical constraint to the completion of germination than the lateral endosperm, and hence its structure is predisposed to permit radicle protrusion.

Grains of indica rice (Oryza sativa cv. Peiza 67) exhibit an increase in endo-b-mannanase activity, mostly after the completion of germination. According to tissue blots, the initial increase occurs in association with the embryo, and possibly the scutellum, although the largest sustained increase in activity is in the peripheral regions of the endosperm. The aleurone layer, being the only living region of the endosperm, is presumably the site of synthesis and secretion of the enzyme into the non-living, starch-laden region. b-Mannosidase activity is low throughout germination and subsequent seedling growth, particularly in the endosperm regions. Its activity profile does not mimic that of endo-b-mannanase. In the intact grain, gibberellin (GA) causes a relatively small increase in endo-b-mannanase activity, while abscisic acid (ABA) causes a large decrease; this inhibition is overcome to a considerable extent when GA is supplied along with ABA. b-Mannosidase activity is little affected by either GA or ABA. Embryoless half-grains imbibed in water exhibit only a small increase in endo-b-mannanase activity with time of imbibition, showing the necessity for a stimulus from the embryo for this to occur. Incubating half-grains in the presence of GA results in a large increase in enzyme activity; ABA reduces the amount of activity compared to the water controls. GA is capable of reversing the inhibitory effect of ABA with respect to endo-b-mannanase activity. As in the intact grains, b-mannosidase activity in the half-grains is unaffected by either GA or ABA. It is concluded that the major site for the production of endo-b-mannanase activity is the aleurone layer, and this event is influenced by the presence of the embryo; in the absence of the latter, the increase in enzyme activity is stimulated by GA. b-Mannosidase activity is low throughout germination and post-germination, it is not influenced by GA and ABA, and thus its activity is not regulated in a coordinated manner with that of endo-bmannanase.

文章从铁吸收的生理生化机制出发，对铁吸收中的相关基因进行分析，进而探讨这些基因的潜在利用价值，以此来说明基因工程获得富铁植物的可行性。