ABSTRACT
PREMISE OF THE STUDY: Many flowers are pollinated by potentially hungry insects, yet flowers also contain gametes and embryos which must be protected from predation. Microscopic calcium oxalate crystals in plant tissues have been proposed to protect against herbivory. Aroids, which have an unusual diversity of such crystals, also exhibit diverse pollination strategies. Many species have pollinators that do not feed while visiting the flowers, while other species, especially those pollinated by beetles, offer sterile staminodia as food rewards. We examined flowers of 21 aroid species with various pollination strategies to test the hypothesis that crystals protect vital gametes and embryos while allowing consumption of food bribes. METHODS: Aroid inflorescences collected from the field or from greenhouse material were sectioned, cleared, and examined by bright field and polarization microscopy. KEY RESULTS: All species examined, regardless of pollination strategy, arrayed crystals around unshed pollen and ovules. Less vital tissues, such as odoriferous appendages, had few crystals. Staminodia offered as food to beetle pollinators, however, differed greatly between species in their crystal contents. Some had minimal crystals; some had crystals in patterns suggesting they limit beetle feeding; still others had abundant crystals in no obvious pattern. CONCLUSIONS: The results are consistent with crystals protecting against insect predation of gametes and embryos. However, the role of crystals in food-bribe staminodia is unclear. They may limit and direct feeding by beetles in some species, while in others they might have no protective role.
Subject(s)
Araceae/metabolism , Calcium Oxalate/metabolism , Herbivory , Inflorescence/metabolism , Pollination , Animals , InsectaABSTRACT
Although cells that synthesize crystals are known throughout the plant kingdom, their functional significance is still unknown. Mechanical support, mineral balance, waste sequestration, and protection against herbivores have all been proposed as crystal functions. To seek clues to their role(s), I systematically examined all organs except fruit of Dieffenbachia seguine (Araceae) for crystals. Crystals were found in nearly every organ. Raphides (long, slim, pointed crystals) were most common, but druses (crystal aggregates) and prisms were also found. Raphides varied in size by a factor of 10 and also in organization from tightly bundled to loosely organized. Biforines, a type of cell capable of expelling raphides, or biforine-like cells, were found in nearly all organs, but especially in leaves, spathes, and anthers. Different organs had different crystal complements, and characteristic crystals were found at specific locations, such as among pollen, along the undersides of leaf veins, and at root branch points. All crystals appeared to be composed of calcium oxalate, based on acid solubility. Possible roles of the crystals are discussed in light of these findings.
ABSTRACT
Mesophyll cells of Zinnia elegans L., cultured in the presence of phytohormones, will transdifferentiate and undergo programmed cell death to become tracheary elements, thick-walled cells of the xylem. This system is a model system for study of plant cell development and differentiation. We report that a high concentration of extracellular Ca(2+) is necessary during the first 6 h of culturing for tracheary elements to form. Extracellular Ca(2+) is still required at later times, but at a much lower concentration. When cells transdifferentiate in adequate Ca(2+), microsomal phospholipase C activity increases and levels of inositol 1,4,5-trisphosphate rise at about hour 4 of culturing. The production of inositol 1,4,5-trisphosphate appears to be important for tracheary element formation, since inhibitors of phospholipase C inhibit both inositol 1,4,5-trisphosphate production and tracheary element formation. Pertussis toxin, an inhibitor of GTP-binding proteins, inhibits transdifferentiation and eliminates inositol 1,4,5-trisphosphate production. Tracheary element formation was not completely abolished by inhibitors that eliminated inositol 1,4,5-trisphosphate production, suggesting the involvement of other pathways in regulating transdifferentiation.
