Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/14/2009
Publication Date: 11/1/2009
Citation: Kang, B., Xiong, Y., Williams, D.S., Diego, P., Chourey, P.S. 2009. Miniature1-encoded cell wall invertase is essential for assembly and function of wall-in-growth in the maize endosperm transfer cell. Plant Physiology. 151:1366-1376. Interpretive Summary: Soon after pollination / fertilization, a developing seed is comprised of two distinctive domains, endosperm and embryo, that are inter-dependent not only during development but also after maturation and in seed germination where the former nourishes the latter, which produces a plant. Immature endosperm cells at the basal end form highly specialized cellular structures called wall-in-growths (WIGs) that increase surface area of these cells many fold. Function of the WIGs has been however elusive thus far even though mutational WIG-deficiency or irregularities are always associated with highly shriveled or miniaturized seeds that are lethal. A cooperative investigation between scientists from Chemistry Unit, CMAVE, USDA ARS and the University of Florida, at Gainesville, FL, provide critical insights that show for the first time that the WIGs harbor a critical enzyme which cleaves incoming sucrose from the mother plant to simple sugars essential to fuel the entire machinery of normal seed development. The loss of this enzyme in the miniature1 (mn1) seed mutant leads to many abnormalities including stunted WIGs (shown here) and, ultimately, a loss of ~70% seed weight; i.e., the mn1 seed. Further molecular studies are under way to explore many unknown genes that may be critical to WIG formation and normal seed development.
Technical Abstract: The miniature1 (mn1) seed phenotype in maize is due to a loss-of-function mutation at the Mn1 locus that encodes a cell wall invertase, INCW2, which localizes exclusively to the basal endosperm transfer cells (BETC) of developing seeds. A common feature of all transfer cells is the labyrinth-like wall-in-growth (WIG) that increases the plasma membrane area, which could confer greater solute transport and/or nutrient uptake capacity to these cells. To better understand WIG formation and possible roles of INCW2 in BETC maturation, we carried out electron microscopy analysis of developing kernels of the two genotypes. In Mn1 seeds, WIGs developed uniformly during 7 to 17 days after pollination (DAP) and the secretory/endocytic organelles including Golgi/trans-Golgi network (TGN) complexes and multivesicular bodies (MVBs) proliferated in the BETCs. Mitochondria accumulated to the basal cell wall where WIGs are most elaborated and this accumulation began before any WIG was detected. In the mn1 BETCs, WIGs were stunted, the endoplasmic reticulum (ER) was swollen, and Golgi density was 51% of the Mn1. However, the polar distribution of mitochondria was not affected. Immunogold labeling using INCW2-specifc antibody detected gold particles on WIGs, the ER, Golgi stacks and the TGN in the Mn1; however, the mutant kernels showed extremely low to undetectable levels of immunogold at these sites. INCW2 localization in the empty pericarp4 (emp4) kernels, which is associated with heterogenous WIG sizes, showed that the density of INCW2 Immunogold particles was ~4 times higher in the thicker WIGs than in the undersized WIGs. Overall, these results indicate that polarized secretion in the BETC is activated during WIG formation and that INCW2 is essential for normal WIG development but its deposition is dependent on sustained WIG growth.