Miniature1-Encoded Cell Wall Invertase Is Essential for Assembly and Function of Wall-in-Growth in the Maize Endosperm Transfer Cell    

Abstract

The miniature1 (mn1) seed phenotype in maize (Zea mays) is due to a loss-of-function mutation at the Mn1 locus that encodes a cell wall invertase (INCW2) that localizes exclusively to the basal endosperm transfer cells (BETCs) of developing seeds. A common feature of all transfer cells is the labyrinth-like wall-in-growth (WIG) that increases the plasma membrane area, thereby enhancing transport capacity in these cells. To better understand WIG formation and roles of INCW2 in the BETC development, we examined wild-type and mn1 mutant developing kernels by cryofixation and electron microscopy. In Mn1 seeds, WIGs developed uniformly in the BETC layer during 7 to 17 d after pollination, and the secretory/endocytic organelles proliferated in the BETCs. Mitochondria accumulated in the vicinity of WIGs, suggesting a functional link between them. In the mn1 BETCs, WIGs were stunted and their endoplasmic reticulum was swollen; Golgi density in the mutant BETCs was 51% of the Mn1 Golgi density. However, the polarized distribution of mitochondria was not affected. INCW2-specific immunogold particles were detected in WIGs, the endoplasmic reticulum, Golgi stacks, and the trans-Golgi network in the Mn1 BETCs, while immunogold particles were extremely rare in the mutant BETCs. Levels of WIG development in the empty pericarp4 mutant was heterogeneous among BETCs, and INCW2 immunogold particles were approximately four times more abundant in the larger WIGs than in the stunted WIGs. These results indicate that polarized secretion is activated during WIG formation and that INCW2 is required for normal development of WIGs to which INCW2 is localized.

MaizeRough Endosperm3Encodes an RNA Splicing Factor Required for Endosperm Cell Differentiation and Has a Nonautonomous Effect on Embryo Development

“…Mitochondria within rgh3 BETL cells are distributed throughout the cytosol ( Figure 5E, white arrowheads) as opposed to normal, polar distribution along on the basal cell wall of differentiating BETL cells (Kang et al, 2009). Moreover, rgh3 cells in the BETL region have a cuboidal cell shape and contain electron-dense bodies similar to protein storage bodies in aleurone cells ( Figure 5E).…”

“…However, multiple dek mutants disrupt BETL differentiation without inhibiting embryo development. Mutants in defective endosperm17, minature1, reduced grain-filling1, and baseless1 (bsl1) produce viable embryos but have morphological BETL defects correlated with reduced grain fill (Lowe and Nelson, 1946;Brink and Cooper, 1947;Maitz et al, 2000;Gutié rrez-Marcos et al, 2006;Kang et al, 2009). Similar to rgh3, bsl1 shows a range of seed phenotypes, including an embryo-lethal phenotype in seeds with severely affected endosperm tissues (Gutié rrez-Marcos et al, 2006).…”

“…Developing ears were harvested from 10 to 20 DAP. Kernels were processed for semithin (500 nm) and thin (80 nm) sections as described by Kang et al (2009). Representative pictures of semithin, toluidine blue (0.05%) stained sections were taken using an Olympus BH2 microscope and Retiga 200R digital camera (QImaging).…”

Kin Conflict in Seed Development: An Interdependent but Fractious Collective

“…Thus, seed maturation is associated with a rapid early increase in fresh weight, accompanied by a slower, but steady, increase in dry weight (Mansfield & Briarty 1992;Brown et al 1999;Baud et al 2002). Seed size in grasses is also determined by early endosperm expansion followed by deposition of food reserves but, unlike Arabidopsis, embryos remain relatively small and reserves are deposited in persistent endosperm (Olsen 2004 (Kang et al 2010). Loss of Mn1 activity leads to endosperms with fewer and smaller cells (Vilhar et al 2002).…”

Understanding the Role of Defective Invertases in Plants: Tobacco Nin88 Fails to Degrade Sucrose

“…cwINVs often play a crucial role in phloem unloading in sink tissues by creating a Suc gradient, enhancing sink strength-mediated long-distance transport of Suc (Sturm, 1999). Their function is most prominent in sinks with no plasmodesmatal connections between cells, such as developing seeds and pollen (Proels et al, 2006;Hirsche et al, 2009;Kang et al, 2009). The expression and activity of cwINVs are regulated by a number of stimuli that are known to affect carbohydrate requirements, such as phytohormones (Balibrea-Lara et al, 2004;Hayes et al, 2010) and pathogen infection (Benhamou et al, 1991;Siemens et al, 2011).…”