Let us make an in-depth study of the polyamines in plants. After reading this article you will learn about 1. Discovery and Chemical Nature of Polyamines 2. Occurrence of Polyamines in Plants 3. Important Physiological Effects of Polyamines in Plants 4. Biosynthesis of Polyamines and 5. Deactivation of Polyamines.
Discovery and Chemical Nature of Polyamines:
Polyamines are a group of polyvalent cationic organic compounds containing two or more amino groups. More than three centuries ago, they were first observed as crystals in human semen (hence, the name spermine of one of the polyamines) by Van Leeuwenhoek under his own crude microscope.
Polyamines were chemically characterised in 1920s and were primarily of interest to Chemists. These are now known to be ubiquitous bioactive compounds occurring in cells of almost all types of organisms including micro-organisms, plants and animals.
It was only in later half of the 20th century especially in early 1970s, that polyamines began to attract the attention of plant physiologists. Many polyamines were found to be physiologically active and implicated in a number of physiological effects in plants. Their role in controlling plant growth and development is being increasingly recognised now. However, polyamines have not yet been given status of plant hormones by plant physiologists unequivocally.
Polyamines differ from plant hormones in two respects:
(i) While plant hormones are present in very small concs. (micro-molar), the polyamines are present in abundance (millimolar concs.) and
(ii) The polyamines are poorly trans located.
Evans and Malmberg (1989) have reviewed role of polyamines in plant development and have concluded that “polyamines are not plant hormones although they might be considered as plant growth regulators or merely one of several kinds of metabolites needed for certain developmental processes.”
Chemical structures of common physiologically active polyamines in plants are given in Fig. 17.35.
At neutral pH, polyamines are polyvalent cations. Their positively charged amino groups can bind to negatively charged phosphate groups of polyvalent anions such as DNA, RNA and phospholipids of plasma-membrane in cells and stabilize these macromolecules. This binding characteristic may also affect membrane permeability and increase transcription of DNA and translation of RNA in plants and animal cells. Spermine and spermidine have been used in DNA packaging.
Occurrence of Polyamines in Plants:
The polyamines occur in plant cells in two different forms:
(i) Free polyamines and
(ii) Bound polyamines.
In bound forms, the polyamines are conjugated with some phenolic compounds such as hydroxycinnamic acid, coumaric acid or caffeic acid. Bound polyamines constitute significant fraction of total polyamines and may also be functionally as important as free polyamines.
Important Physiological Effects of Polyamines in Plants:
i. In control of flowering (as component of floral stimulus).
ii. Regulation of embryogenesis.
iii. Delaying senescence of detached leaves.
iv. Stabilization of membranes and isolated protoplasts.
v. Promotion of development of some fruits.
vi. Minimize water stress of many types of cells.
Biosynthesis of Polyamines:
Polyamines are biosynthesized in organisms including plants from one of the two amino acids, lysine and arginine.
Cadaverine is synthesized from lysine by decarboxylation in presence of lysine decarboxylase.
Putrescine, spermidine and spermine are synthesized from arginine (Fig. 17.36).
Putrescine can be synthesized from arginine by two different routes, (i) by involving ornithine as intermediate or (ii) by involving agmatine as intermediate. However, the second route through agmatine appears to be the principal route of putrescine synthesis in plants.
Spermidine and spermine are synthesized from putrescine by sequential addition of H2N – (CH2)3 – groups from decarboxylated S-Adenosyl methionine (SAM) in the presence of spermidine synthase and spermine synthase respectively (Fig. 17.36). SAM is an intermediate of ethylene biosynthesis in plants.
Deactivation of Polyamines:
Polyamines are deactivated by oxidative deamination of terminal amino groups in the presence of the enzymes polyamine oxidases.