Late steps of egg cell differentiation are accelerated by pollination in Zea mays L.

Abstract

Egg cells were analysed cytologically during the female receptivity period in maize (Zea mays L., line A 188). Three classes of egg cell were distinguished: type A--small, non-vacuolated cells with a central nucleus; type B--larger cells with small vacuoles surrounding the perinuclear cytoplasm located in the middle of the cell; type C--big cells with a large apical vacuole and the mid-basal perinuclear cytoplasm. The less-dense cytoplasm of the vacuolated egg cells usually contained numerous cup- or bell-shaped mitochondria. The three egg types appear to correspond to three late stages of egg cell differentiation. The frequencies of each of the three egg types were monitored in developing maize ears before and after pollination. In young ears, with the silks just extending out of the husks, small A-type cells were found in about 86% of ovules. Their frequency decreased to about 58% at the optimum silk length, remained unchanged in non-pollinated ears, and fell to 16% at the end of the female receptivity period. However, after pollination and before fertilisation the frequency of these cells decreased to about 33%, and the larger vacuolated egg cells (types B and C) prevailed. At various stages of the receptivity period, pollination accelerated changes in the egg population, increasing the number of ovules bearing larger, vacuolated egg cells. Experiments with silk removal demonstrated that putative pollination signals act immediately after pollen deposition and are not species-specific.

A comparison of early molecular fertilization mechanisms in animals and flowering plants

“…Mól et al (2000) reported that egg cell maturation is essentially accelerated after pollination in maize, but removal of silk directly after pollination did not repress accelerated egg maturation, suggesting an electric nature of the pollen signal. Even wounding using sea sand is able to promote egg maturation (R. Mól, personal communication), indicating the potential existence of long-range signals unrelated to peptides/ proteins.…”

Experimental Analysis of the Fertilization Process

“…If pollination is withheld, S-phase in the egg cell is completed at 84 h, which is more than a 1-day delay; thus, pollination may accelerate cell cycle progression in the egg cell. In maize, maturation of the female gamete also is accelerated by several hours with the presence of pollen tubes in the style (Mó l et al, 2000).…”

“…Polyspermy is reportedly blocked successfully, and chemical signals, including a calcium influx, have been recorded (see Calcium Waves, Site of Fusion, and Egg Activation below). The model seems to work best on slightly immature egg cells (Mó l et al, 2000) and is a bit technically demanding because receptivity may decline quickly after isolation. However, this model system does not seem to predict all types of gametic behavior.…”

“…For female gametes, the best understood model is maize, in which egg maturation is categorized into three classes (Mó l et al, 2000). Small, densely cytoplasmic egg cells with a centrally positioned nucleus indicating a nonpolarized condition are classified as type A.…”

Haploid embryos of lettuce (Lactuca sativa) induced by alien pollen or chemical factors

“…It is supposed that the pollen-stigma interaction initiates this process. In some species (in Zea mays L.), pollination with pollen of the same species accelerates maturation and changes in the structure of egg cells (Mól et al, 2000). Moreover, during compatible or incompatible pollination, pollen tubes ( and papillae on the stigma sur-face) release a higher concentration of calcium ions Ca 2+ , which allows their penetration through the transmission tissue into the style (Śnieżko and Winiarczyk, 1993;Lenartowska et al, 2001).…”