and Jones, 1999;Commuri and Jones, 2001; Duke and Doehlert, 1996; Engelen-Eigles et al., 2000; Heat stress is a major factor limiting crop yield in many agricultural Singletary et al., 1994). The extent of damage caused regions. During the early stage of kernel development, heat stress is particularly detrimental to subsequent dry matter accumulation since it by heat stress depends on the time of exposure in relacauses disruption of cell division, sugar metabolism, and starch bio-tion to the stage of kernel development (Gibson and synthesis in the endosperm. The effects of heat stress on protein accu- Paulsen, 1999). By exposing maize kernels to heat stress mulation, however, is less well understood. Therefore, the objective of during the lag phase (starting at 5 DAP), cell division this study was to determine the mechanisms by which heat stress decreases and ultimately the number of endosperm cells and starch protein accumulation and alters composition of developing maize (Zea granules are severely reduced (Commuri and Jones, 1999; mays L.) kernels. In this study, maize ears were heat stressed for 2 and Engelen-Eigles et al., 2000), which is also associated with 4 d at continuous 35؇C starting at 5 d after pollination (DAP). Endosperms a reduction in cytokinin levels (Cheikh and Jones, 1994). were analyzed for the relative proportion of each Osborne protein solubil-Starch metabolism is particularly repressed, which apity class fraction and for individual zein proteins. The 2-and 4-d heat pears to be due to a reduction of the activity of ADP stress (DHS) treatment caused a 20 and 48% reduction in kernel final dry weight, respectively, and protein content was similarly reduced. glucose pyrophosphorylase and soluble starch synthaseSpecifically, zein content was reduced by an average of 53%, but zein (Commuri, 1997). These findings are in agreement with composition was only mildly affected. The concentrations of glutelin those of Duke and Doehlert (1996), Keeling et al. (1994), and albumin plus globulin tended to increase throughout most of 4- Singletary et al. (1994), andWilhelm et al. (1999), who DHS kernel development. L-[ 35 S]-methionine incorporation in theshowed that, even when maize kernels are exposed to zein fraction was delayed by the 4 DHS when compared to control heat stress at later developmental stages (e.g., the linear and 2-DHS treated kernels. Therefore, we concluded that heat stress fill period), there is a significant repression in starch during early stages of endosperm development repressed zein accumubiosynthesis because of the reduction in the activity of lation at the synthesis level. In contrast, later in development, zein these two enzymes. accumulation appeared to be repressed mainly by protein degradation, Maize protein accumulation seems less susceptible to which appears to be a part of the natural progression of kernel development since there was no apparent significant effect of heat stress heat stress (Bhullar and Jenner, 1985;Wilhelm et al., 1999) treatments on this process. and w...

Maize (Zea mays L.) endosperm transfer cells are essential for kernel growth and development so they have a significant impact on grain yield. Although structural and ultrastructural studies have been published, little is known about the development of these cells, and prior to this study, there was a general consensus that they contain only flange ingrowths. We characterized the development of maize endosperm transfer cells by bright field microscopy, transmission electron microscopy, and confocal laser scanning microscopy. The most basal endosperm transfer cells (MBETC) have flange and reticulate ingrowths, whereas inner transfer cells only have flange ingrowths. Reticulate and flange ingrowths are mostly formed in different locations of the MBETC as early as 5 days after pollination, and they are distinguishable from each other at all stages of development. Ingrowth structure and ultrastructure and cellulose microfibril compaction and orientation patterns are discussed during transfer cell development. This study provides important insights into how both types of ingrowths are formed in maize endosperm transfer cells.

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Maize coenocytic and cellularizing endosperm development were characterized at optical and transmission electron microscopy levels. Samples were collected daily in 3 consecutive years under different temperature regimes; therefore, the developmental stages were expressed on a growing degree basis. Soon after the primary endosperm nucleus is formed, it starts dividing without cytokinesis, leading to the formation of the coenocyte. The nuclei divide freely on the periphery of the coenocyte and spread from the micropylar region toward the chalazal region. The first anticlinal walls are formed as the nuclei come closer to each other, followed by periclinal wall formation also starting in the micropylar region. As cellularization proceeds, new anticlinal and periclinal walls assume a less organized pattern until the central vacuole is fully occupied by newly formed cells. Overall, our findings suggest that the developmental pattern of maize coenocytic and cellularizing endosperm has many similarities to other species of the Poaceae family.

Endosperm transfer cells in maize have extensive cell wall ingrowths that play a key role in kernel development. Although the incorporation of lignin would support this process, its presence in these structures has not been reported in previous studies. We used potassium permanganate staining combined with transmission electron microscopy – energy dispersive X-ray spectrometry as well as acriflavine staining combined with confocal laser scanning microscopy to determine whether the most basal endosperm transfer cells (MBETCs) contain lignified cell walls, using starchy endosperm cells for comparison. We investigated the lignin content of ultrathin sections of MBETCs treated with hydrogen peroxide. The lignin content of transfer and starchy cell walls was also determined by the acetyl bromide method. Finally, the relationship between cell wall lignification and MBETC growth/flange ingrowth orientation was evaluated. MBETC walls and ingrowths contained lignin throughout the period of cell growth we monitored. The same was true of the starchy cells, but those underwent an even more extensive growth period than the transfer cells. Both the reticulate and flange ingrowths were also lignified early in development. The significance of the lignification of maize endosperm cell walls is discussed in terms of its impact on cell growth and flange ingrowth orientation.