Submitted to: Molecular and General Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 26, 1998
Publication Date: N/A
Interpretive Summary: Photosynthesized sucrose is transported to developing seeds where it serves as the sole source of carbon for both metabolic as well as structural and storage functions which, ultimately, contribute to crop yields. Despite such an important role of sucrose, basic knowledge on how sucrose is unloaded in a regulated manner and what the initial reactions inside a developing seed are, remain unknown. ARS scientists in the Crop Genetics & Environmental Research Unit have shown previously that the incoming sucrose is first cleaved to simple sugars by invertase prior to its utilization in a seed. This reaction is of importance because a maize mutation, miniature seed, lacking invertase is associated with a loss of nearly 70 to 80% of seed weight at maturity. Experiments reported here were designed to test if the genetic defect of the miniature seed mutation can be corrected by directly feeding the simple sugars to a developing seed. Results were negative. The external source of sugars is not utilized in the mutant. Our interpretation of the data is that the cleavage reaction itself, inside plant cells, may be an essential function to allow the normal utilization of incoming sucrose. Further studies are needed to better understand as yet unknown metabolic events (genes and proteins) which regulate the flow of sugars for the normal development of seeds in maize and all cereals.
Cell wall-bound invertase (CWI) is spatially and temporally the first enzyme which metabolizes the incoming sucrose in a developing seed of maize (Zea mays). Our previous studies have shown that CWI-2 isozyme encoded by the wild-type gene of the Miniature1 (Mn1) seed locus plays a critical role in seed development (Cheng et al., 1996). Null mutations of the gene, such as the mn1 seed mutant which lack invertase activity, are associated with a loss of ~70 to 80% of the normal seed weight. We show here that under in vitro kernel culture conditions the hexose-based medium was similar to the sucrose-based medium in promoting normal development of kernels of the Mn1, but not the mutant mn1 genotype. Anatomical, biochemical, and immunohistological data showed that, unlike the Mn1 kernels, the mn1 kernels retain their mutant phenotype regardless of sucrose or hexoses in the culture medium. Based on these data, we suggest that the metabolic release of hexoses plays a critical role in the normal assimilation of photoassimilated sucrose in a developing kernel.