Study Notes on Cyclic AMP (CAMP) | Plant Physiology

The below mentioned article provides study notes on Cyclic AMP (CAMP).

Cyclic AMP was discovered by E.W. Sutherlandt in 1956 in animal liver cells. Since then it has been demonstrated in several animal tissues where its regulatory role has been well demonstrated. There are several evidences regarding the presence of cAMP in plants.

Most of these evidences are based on either analysis of enzymes of cAMP metabolism or their general distribution or their exogenous application evoking defined responses.

cAMP has been reported from Chlamydomonas reinhardii, Euglena gracilis, Dictyostellium, Sacchromyces, Neurospora, Funaria hygrometrica, Avena, coleoptiles, soybean cells, Pinus pollen, germinating lettuce seeds, etc.

The concentration of cAMP varies with plant age. For instance in six day old seedlings of Phaseolus vulgaris it was 2.6 to 9.2 moles/g tissue while in the 15 day old plant it was reduced to one tenth of its original value.

Perennial species had more of cAMP than the annuals. Within a plant apical leaves had large levels whereas root tissue had low levels of cAMP. The membrane bound adenyl cyclase enzyme has been reported from maize coleoptiles, Pinus pollen grains, Neurospora crassa, etc.

The other cAMP metabolizing enzyme is cyclic phosphodiesterase which has been reported from barley seeds, pea seedlings, aleurone layers of some cereals and several lower plants. Kinetic studies have shown that this enzyme differs from that of animals in several respects.

In the following a brief account of positive physiological responses are tabulated (Table 20-4) and some of these responses simulate those of GA, IAA, cytokinin and phytochrome:

Cyclic AMP has also been shown to stimulate the activity of several enzymes like α-amylase, protease, acid phosphatase, RNase, some dehydrogenases, isocitrate lyase and a few oxidases.

Some relationship between cAMP and plant hormones has also been brought out. It is generally proposed that cAMP is a mediator of hormonal stimulation of enzyme activity like that in animals. It also mimics the action of some hormones and causes some tissue responses.

Studies by Polard are very illuminating. He incubated barley aleurone layers with ¹⁴C adenine with and without GA¹ for 24 hours. Radioactivity was traced in cAMP chromatographically. The hormone treated tissues exhibited double amount of label cAMP.

He proposed that GA3 and auxin were responsible for increasing cAMP level and hormone could replace cAMP level by increasing the activity of cAMP metabolizing enzymes. They proposed two messenger hypothesis. According to this hypothesis hormone was the first messenger and triggered the synthesis of cAMP which acted as a second messenger.

Two messenger hypothesis is also supported by several other evidences like both hormone and cAMP stimulate enzyme activity; the increased responses caused by both are inhibited by ABA and lastly when theophylline is added to suboptimal hormone concentration, lettuce-seed germination was promoted and it was more than the hormone alone.

Other workers especially Keats do not support that the cAMP mediates the hormonal action. The arguments against the two messenger hypothesis are as follows One, cAMP cannot substitute hormone action completely and second, the increased enzymatic activity caused by cAMP is much more sensitive to ABA than the hormone stimulated enzyme activity. In the following the brief working of cAMP is brought out:

In order to find out whether a given sample contains pure cAMP or some contaminants are present, some workers use labelled cAMP to a given sample separately. To these cyclic phosphodiesterase is added and the percentage recovery of the product is determined.

In pure samples the recovery is 90% whereas in the unknown sample it is 5 to 10 per cent. Apparently some of the nucleotide had the same Rf value as authentic cAMP. Very recently some workers have postulated that cyclic nucleotides and hormones operate at two different cells.

Hormone……… site I enzyme stimulation

cAMP………. site II

It may be added that exact role of cAMP in plants is still conjuncture and much remains to be worked out.