Submitted to: The Plant Cell
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 25, 1996
Publication Date: N/A
Interpretive Summary: Sucrose is the sole source of carbon entering a developing corn seed where it is utilized to drive numerous metabolic reactions and is also stored as starch. Obviously, the reactions leading to an appropriate utilization of sucrose are critical to agronomic productivity; yet, very little is known concerning the genetic basis of the specific metabolic reactions needful to sucrose utilization in a developing seed. The research described here identifies one such single gene which encodes an enzyme invertase known to split sucrose to glucose and fructose. A genetic mutation leading to an invertase deficient grain leads to a loss of nearly 70% of the seed weight. This is the first such mutant which is associated with such a drastic effect on seed size and weight. The gene is now molecularly cloned and is characterized in terms of its expression in various parts of the plant. As expected, the highest levels of expression are seen in a developing seed. In fact, it is the first enzyme both in time (during development) and the physical site in a grain to drive the incoming sucrose into the downstream reactions leading to a normal seed development. Further studies are aimed to better understand how this gene and other related genes are regulated leading to greater agricultural productivity.
Technical Abstract: Collective evidence demonstrates that the Miniature1 (Mn1) seed locus in maize encodes an endosperm-specific isozyme of cell wall invertase, CWI-2. The evidence includes, (1)isolation and characterization of ethyl methanesulfonate-induced mn1 mutants with altered enzyme activity and (2)a near linear relationship between gene/dose and invertase activity and the CWI-2 protein. In addition, molecular analyses showed that the cDNA clone incw2 maps to the Mn1 locus and differentiates the six ethyl methanesulfonate-induced mn1 mutants of independent origin into two classes by RNA gel blot analyses. We also report two unexpected observations that provide significant new insight into the physiological role of invertase and its regulation in a developing seed. First, a large proportion of the total enzyme activity (-90%) was dispensable (i.e., nonlimiting). However, below the threshold level of -6% of the wild-type activity, the endosperm enzyme controlled both the sink strength of the developing endosperm as well as the developmental stability of maternal cells in the pedicel in a rate-limiting manner. Our data also suggest an unusually tight coordinate control between the cell wall-bound and soluble forms of invertase, which are most likely encoded by two separate genes, presumably through metabolic controls mediated by the sugars.