Early carbon mobilization and radicle protrusion in maize germination

Abstract

Considerable amounts of information is available on the complex carbohydrates that are mobilized and utilized by the seed to support early seedling development. These events occur after radicle has protruded from the seed. However, scarce information is available on the role of the endogenous soluble carbohydrates from the embryo in the first hours of germination. The present work analysed how the soluble carbohydrate reserves in isolated maize embryos are mobilized during 6–24 h of water imbibition, an interval that exclusively embraces the first two phases of the germination process. It was found that sucrose constitutes a very significant reserve in the scutellum and that it is efficiently consumed during the time in which the adjacent embryo axis is engaged in an active metabolism. Sucrose transporter was immunolocalized in the scutellum and in vascular elements. In parallel, a cell-wall invertase activity, which hydrolyses sucrose, developed in the embryo axis, which favoured higher glucose uptake. Sucrose and hexose transporters were active in the embryo tissues, together with the plasma membrane H+-ATPase, which was localized in all embryo regions involved in both nutrient transport and active cell elongation to support radicle extension. It is proposed that, during the initial maize germination phases, a net flow of sucrose takes place from the scutellum towards the embryo axis and regions that undergo elongation. During radicle extension, sucrose and hexose transporters, as well as H+-ATPase, become the fundamental proteins that orchestrate the transport of nutrients required for successful germination.

Spatial Mapping and Profiling of Metabolite Distributions during Germination

“…These oligosaccharides are broken down to their constituent Glc units and are transported to and taken up by the scutellum, where they are reassembled into Suc, the disaccharide likely visualized by MSI (Bewley, 2001;Nonogaki, 2008). The localization of the presumable Suc disaccharide in the emerging radicle is consistent with the previously reported mobilization of Suc within the scutellum and the subsequent transport to the embryonic axis, specifically to fuel cell elongation in the radicle (Sánchez-Linares et al, 2012).…”

“…Disaccharide also is detected within other tissues, such as the endosperm and pericarp and/or aleurone, but at lower abundance. These nonhomogenous distributions of the polysaccharides are conserved in both B73 and Mo17 inbreds and are consistent with the mobilization of Suc within the scutellum and transport to the embryonic axis, specifically the radicle (Sánchez-Linares et al, 2012).…”

Metabolite profiling of Ricinus communis germination at different temperatures provides new insights into thermo-mediated requirements for successful seedling establishment

“…Initial imbibition is often accompanied by large metabolic changes which set the course for subsequent radicle protrusion and are essential for successful seedling establishment [1,30]. Within minutes after the start of imbibition, reactivation of enzymes that were stored during maturation can be observed, followed by an increase in respiratory metabolism and gene expression [38,39]. Our results show that temperature affects the metabolite composition of seeds already at the beginning of imbibition.…”

“…In germinating seeds, monosaccharides can be produced from the breakdown of stored starch, sucrose hydrolysis and through gluconeogenesis [5,39,40]. R. communis seeds contain low levels of starch and sucrose levels showed little variation during seed imbibition and germination.…”

Proline and trehalose in maize seeds germinating under low osmotic potentials

“…Also, the negative correlation between the levels of proline in the embryo axis and trehalose in the endosperm (Table 2) reinforces the idea that, under water germination. Because of that, the endosperm normally has a lower metabolism activity compared to that of embryo axis, and it seems that its metabolic activity is even more reduced under drought (Sánchez-Linares et al, 2012). These facts are, probably, what may justify the low proline synthesis and accumulation in the endosperm tissue, in seeds germinating under water limitations.…”