Browning and Gunning demonstrated that placental transfer tissues increase plant reproductive succes

In the 1970s, plant cell biologists Adrian Browning and Brian Gunning explored placental transfer tissue function. By using a simple moss experimental system, they determined that placental transfer tissues increase the rate at which radioactively labelled carbon moves through placental transfer tissues from green gametophytes into young sporophytes. This process is expected to increase the ability of mature sporophytes to produce progeny spores, thereby increasing reproductive success. Recall that embryos are very young, few-celled sporophytes and that in mosses and other bryophytes all stages of sporophyte development are nutritionally dependent on gametophyte tissues. Browning and Gunning investigated nutrient flow into young sporophytes because these slightly older and larger developmental stages were easier to manipulate in the laboratory than were tiny embryos.

In a first step, the investigators grew many gametophytes of the moss Funaria hygrometrica in a greenhouse until young sporophytes developed as the result of sexual reproduction. In a second step, they placed black glass sleeves over young sporophytes as a shade to prevent photosynthesis, enclosed moss gametophytes and their attached sporophytes within transparent jars, and supplied the plants with radioactively labelled carbon dioxide for measured times known as pulses. Because the moss gametophytes were not shaded, their photosynthetic cells were able to convert the radioactively labelled carbon dioxide into labelled organic compounds, such as sugars and amino acids. Shading prevented the young sporophytes, which possess some photosynthetic tissue, from using labelled CO2 to produce organic compounds.

In a third step, the researchers added an excess amount of nonradioactive CO2 to prevent the further uptake of the radioactive CO2 from their experimental system, a process known as a chase. This process stopped the radiolabelling of photosynthetic products. (Such experiments as these are known as pulse-chase experiments.) In a final step, Browning and Gunning plucked young sporophytes of different sizes (ages) from gametophytes and measured the amount of radioactive organic carbon present in the separated gametophyte and sporophyte tissues at various times following the chase.

From these data, they were able to calculate the relative amount of organic carbon that had moved from the photosynthetic moss gametophytes to sporophytes. Browning and Gunning discovered that about 22% of the organic carbon produced by gametophyte photosynthesis was transferred to the young sporophytes during an eight-hour chase period. They also calculated the rate of nutrient transfer between generations and compared this rate with the rate at which organic carbon moves in several other plant tissues that lack specialized transfer cells (determined in other studies). By so doing, Browning and Gunning discovered that organic carbon moved from moss gametophytes to young sporophytes nine times as fast as organic carbon moves within other plant tissues. These investigators inferred that the increased rate of nutrient movement could be attributed to placental transfer cell structure, namely, the fact that cell-wall ingrowths enhanced plasma membrane surface area. By comparing the amount of radioactive carbon accumulated by young sporophytes of differing ages, they also learned that larger sporophytes absorbed labelled carbon about three times as fast as smaller ones.

These data are consistent with the hypothesis that placental transfer tissues increase plant reproductive success by providing embryos and older sporophytes with more nutrients than they would otherwise receive. Supplied with these greater amounts of nutrients, sporophytes are better able to grow larger than they otherwise would, and eventually they produce more progeny spores.