DAYANANDAN, P.1* and J. PONSAMUEL2. 1Dept. of Botany, Madras Christian College, Tambaram, India 600059; 2Plant Transformation and Gene Expression, Dow AgroSciences, 9330 Zionsville Road, Indianpolis IN 46268-1054. - Electron microscopy of terpenoid secreting cells of neem (Azadirachta india A.Juss.).
The neem plant is known to accumulate about 25 diterpenoids and 100
different triterpenoids, including the biologically most active
azadirachtins. Azadirachtins possess a wide spectrum of therapeutic
and powerful pesticidal properties. The terpenoids are synthesized and
accumulate within idioblasts known as secretory cells. The cotyledons
of neem are the richest source of terpenoids, up to 3% of seed weight.
Cotyledons also store considerable quantity of neem oil. The secretory
cells differentiate about 40 days after pollination in cotyledons that
are 4-6 mm long. Development of a secretory cell commences with cell
enlargement, increase in the size the nucleus and vacuole. The cell
walls of secretory cells are strengthened by the addition of wall
material by the adjacent cells. Small vesicles containing terpenoids
accumulate in the cytoplasm. The terpenoid vesicles appear to
originate by the enlargement of endoplasmic reticulum (ER). A string
of terpenoid vesicles interconnected by narrow regions gives a beaded
appearance. The mature vesicles do not possess a boundary membrane.
Each vesicle consists of many small droplets, in the range of 50-100
nm in diameter. The vesicles grow by the addition of terpenoids
synthesized by ER in the vicinity of the terpenoid vesicles. As the
vesicles develop the vacuole fragments into two or more bodies. The
cytoplasm contains numerous ribosomes and ER. Plastids, mitochondria
and microbodies, and a few dictyosomes also occur. Lipid bodies
accumulate at later stages of development. The plastids are likely to
provide the isoprenoids that may be further elaborated by other
organelles. In addition, the vacuoles, cytoplasm and ER may all
provide the enzymes and compartments necessary for the synthesis of a
large variety of terpenoids. In vitro extraction of terpenoids is
possible only if the cultured neem tissues are induced to
differentiate secretory cells.
Key words: Azadirachta indica, Neem, secretory cells, terpenoids, ultrastructure