The Miniature1 Seed Locus of Maize Encodes a Cell Wall Invertase Required for Normal Development of Endosperm and Maternal Cells in the Pedicel

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 when RNA gel blot analyses were used. 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 total enzyme activity (~90%) was dispensable (i.e., nonlimiting). However, below the threshold level of ~6% of 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 the soluble forms of invertase, which are most likely encoded by two separate genes, presumably through metabolic controls mediated by the sugars.

New Insights into Roles of Cell Wall Invertase in Early Seed Development Revealed by Comprehensive Spatial and Temporal Expression Patterns of GhCWIN1 in Cotton

“…With an exception in tobacco (Nicotiana tabacum; Tomlinson et al, 2004), high and low Glc-to-Suc ratios generally favor cell division or promote cell differentiation, respectively, during seed development (for review, see Weber et al, 2005). Consistently, high CWIN activities correlate with high Glc level and mitotic activity in faba bean embryo (Weber et al, 1996(Weber et al, , 1997 and maize endosperm (Cheng et al, 1996;Vilhar et al, 2002). Loss of CWIN expression reduces seed filling in maize (Cheng et al, 1996) and rice , whereas elevation of CWIN activity by silencing its inhibitor increases seed size in tomato (Jin et al, 2009), demonstrating the critical role of CWIN in seed development.…”

“…Extensive studies have shown essential roles of CWIN in seed filling, probably through regulating Suc unloading and the establishment and maintenance of sink strength (Cheng et al, 1996;Weber et al, 1996;Wang et al, 2008;Jin et al, 2009;Zanor et al, 2009). However, it remains largely unknown how CWIN exerts its regulation early in seed development .…”

“…Importantly, genetic evidence shows that hexoses released from CWIN activity, rather than from exogenous supply, are critical for mitotic division (Cheng and Chourey, 1999). Stimulation of endosperm cell division by hexose has been observed in the monocotyledonous maize (Cheng et al, 1996;Vilhar et al, 2002). However, in maize and barley grains, CWIN is specifically localized in basal endosperm TCs at the prestorage to storage phase (Cheng et al, 1996;Dumas and Rogowsky, 2008), and it remains to be determined if CWIN is expressed in maize nuclear endosperm.…”

“…Therefore, CWINs are central to phloem unloading and carbon partitioning, especially in cellular sites lacking symplasmic (plasmodesmal) connections (Patrick and Offler, 2001;Weschke et al, 2003). A positive correlation between CWIN activity and seed development has been observed in maize (Zea mays; Cheng et al, 1996;Vilhar et al, 2002;Chourey et al, 2006), faba bean (Vicia faba; Weber et al, 1995Weber et al, , 1996, barley (Hordeum vulgare; Weschke et al, 2003;Sreenivasulu et al, 2004), rice (Oryza sativa; Hirose et al, 2002;Wang et al, 2008), and tomato (Solanum lycopersicum; Zanor et al, 2009). With an exception in tobacco (Nicotiana tabacum; Tomlinson et al, 2004), high and low Glc-to-Suc ratios generally favor cell division or promote cell differentiation, respectively, during seed development (for review, see Weber et al, 2005).…”

RNA sequencing reveals the complex regulatory network in the maize kernel

“…Among them, 14 genes have one eQTL and 2 genes have two eQTLs. The mn1 gene, which encodes an endospermspecific cell wall invertase and determines the kernel size 26 , is predicted to be regulated by a gene encoding the UDP-glycosyl transferase ( Supplementary Fig. S16).…”