Top 11 Inhibitors of Plant Growth Regulators

In this article we will discuss about the top eleven inhibitors of plant growth regulators. The inhibitors are: 1. Ascorbic Acid 2. Heliangine and Portulal 3. Xanthoxin 4. Lunularic Acid 5. Asparagusate 6. Brassinolide 7. Pestalotin 8. Dihydroconiferyl Alcohol 9. Germination Inhibitors 10. Fusicoccin 11. Brassins.

Inhibitor # 1. Ascorbic Acid:

It is also called vitamin C and is a part of plant redox systems. In the plants it occurs in small amounts and is known to regulate the rate and extent of different growth processes. Some workers including Nanda and Kochhar regard it is a growth regulatory substance. It was in 1928 that Szent-Gyorgyi isolated it from animal and plant tissues and called it hexuronic acid. Its molecular formula was C₆H₈O₆.

Subsequently it was renamed as ascorbic acid and was shown reduce 2, 6-dichlorophenol indophenol. Vitamin C is made up of ascorbic acid and dehydroascorbic acid. Ascorbic acid molecule is unusually flat and its absorption spectrum shows an intense band at 245 µ.

This grouping is also responsible for its acidic character. Under neutral or alkaline condition ascorbic acid easily opens into dehydroascorbic acid. Both ascorbic acid and dehydroascorbic acid are present in plants but their amount is very low.

Late Professor J.J. Chinoy and his associates developed a method to extract free and bound ascorbic acids, using 2, 6-dichlorophenol indophenol. Ascorbic acid present in the extract reduces the dye to its colourless form.

Methods for the extraction and estimation of free and bound ascorbic acid were formulated. For details a reference may be made to the assay and application methods in plant growth regulators. Bound ascorbic acid is also called ascorbigen.

The other roles of ascorbic acid have also been brought out and these include differentiation through their influence on RNA synthesis, removal of histones from infusible chromatin. Induction of DNA activity and its action as derepressor of the ‘nucleone’ by the removal of the complex inhibitory histones from the DNA.

The role of ascorbic acid in energy transfer processes has been discussed in details in chapter on Flowering. They increase energy flow during floral induction and differentiation was attributed to charged transfer complexes and free radical mechanism. Ascorbic acid has also been shown to increase yield, stimulate nodulation in legumes, provide protection against drought and temperature stress.

Inhibitor # 2. Heliangine and Portulal:

These are the inhibitors of stem elongation. The former was isolated from the leaves of Helianthns tuberosus. The leaves were extracted with methanol and the extract was chromatographed for the analysis of inhibiting zone. It was a sesquiterpenic lactone. This compound inhibited the elongation of Avena coleoptile but promoted the rooting of Phaseolus cuttings.

From the leaves of Portulaca grandiflora portulal was extracted which promoted rooting from cuttings but inhibited elongation of Avena coleoptiles. Leaves were extracted with methanol at room temperature and analysed by TLC using ethyl acetate and benzene as solvents. Precise structure of portulal is still to be determined.

Inhibitor # 3. Xanthoxin:

This plant growth regulator may exert its biological effects via ABA transformation or per se. The first possibility seems most acceptable since it was found inactive in closing stomata of Vicia and Commelina isolated strips.

However, when it was applied to detached leaves it was active. It is more inhibitory than ABA in lateral root formation. Using GCM it has been shown that the xanthoxin conncentration in roots decreased in response to decapitation prior to the lateral root formation.

Apparently this growth regulator controls laterial root formation. However, level of xanthoxin increases in light in the shoots. The role of xanthoxins in phototropism of dicots still remains to be verified. Root tip and not root cap of Zea mays has been shown to contain xanthoxin. Obviously its role in geotropism remains to be fully supported.

From rice several compounds have been isolated which are germination inhibitors and these are ineketone, momilactone, and S-(+)-dehydrovomifolial. Structurally the latter compound resembles ABA. Another powerful germination inhibitor has been isolated from the fruits of Aegle marmelos. It was furocoumarin psoralen and had photodynamic action. The photodynamic action differed from the inhibitory effect.

Inhibitor # 4. Lunularic Acid:

The inhibitor though reported from plant species has not been isolated from algae. Conocephalum has been widely used for the isolation and characterization of its synthetic pathway which comprises at least ten steps.

Inhibitor # 5. Asparagusate:

This inhibitor has been isolated from Asparagus whereas inhibitor batatasin III is the dormancy regulator of Dioscorea batatas. One of the newly synthesized inhibitors is dikegulac.

Inhibitor # 6. Brassinolide:

Extracts of Brassica napus pollen produce novel growth promotory effect when applied to bean plants. It also stimulated cell elongation and cell division. This substance has been isolated and characterized as C₂₈H₄₈O₆. 40 kg of pollen yielded 4 mg of the hormone. Structurally the hormone is comparable with ecdysome which is an insect moulding hormone.

Inhibitor # 7. Pestalotin:

It was isolated from a phytopathogenic fungus Pestalotia. It is a gibberellin synergist active in increasing the effect of GA on shoot elongation and a-amylase synthesis. It mimics dihydro-coniferyl alcohol in some of the characters.

One of the possibilities of its stimulating lettuce hypocotyl elongation is increase of GA effect on cell wall. It has a heterocyclic pyrone ring. It is the side chain structure which is important for biological activity.

Inhibitor # 8. Dihydroconiferyl Alcohol:

It is regarded as a chemical messenger responsible for the correlation phenomena between cotyledon and GA induced lettuce hypocotyl elongation. It has an aromatic ring. The aromatic ring structure of this compound is not necessarily pre-requisite for the biological activity of the lettuce cotyledon factor.

Inhibitor # 9. Germination Inhibitors:

Several different kinds of compounds have been identified which are inhibitory for seed germination and have been distributed in seed coats or other parts. These may be unsaturated lactones (e.g., coumarins, parascorbic acid, scopoletin).

Some of the ripened fruits also cause inhibition of seed germination due to high level of unsatureated lactone. Some fruits also have high content of ABA. In some desert species seed coat contains some useful compounds also even though they are germination inhibitors.

They help in regulating the seed germination when the rains set in. As will be discussed later effect on bud dormancy may be due to the presence of high level of growth inhibitors. In the tubers and bulbs of some species inhibitors are present which decline after germination.

In some these inhibitors are ABA like compounds. The inhibitor could be naringenin as well. This flavonoid is present in the seeds of peaches also. In some plant tissues p-inhibitor complex is present. This complex inhibits growth the checks sprouting.

Leaves, stems and fruits of some plants also contain some inhibitors which leach in the rain or water and inhibit the growth of the plants growing in the vicinity. Leaves of walnut are shown to contain juglone.

Inhibitor # 10. Fusicoccin:

This is a fungal toxin isolated from Fusicoccun amygdali. This chemical induces K⁺ dependent H⁺ excretion and is accompanied by malate accumulation and H⁺ dependent cell wall elongation, comparable with IAA. This chemical acts on several plant tissues and hyperpolarizes the membrane potential.

One view is that fusicoccin acts directly on plasmalemma receptor sites whereas IAA acts via protein synthesis is having primary receptor sites at the ER. Some workers believe that this chemical simulates some steps in the malic acid synthesis. In any case it does not seem to act on enzymes directly but affects metobolism indirectly by stimulating ion transport.

Inhibitor # 11. Brassins:

These are recently discovered group of plant growth regulators isolated from the lipid ether of rape pollen, Brassica napus. Subsequently, it was also isolated from Alnus glutinosa pollen as well. These compounds are suggested as unsaturated glycosides or glucose festers of fatty acids.

Fatty hormones have also been extracted from pollen of Phaseolus vulgaris. Brassins cause both elongation and cell division of tested bean stem internodes. However, pollen hormone only induces elongation. Light and magnetic resonant spectral patterns shoed that the fatty hormones were free of indole or gibberellin- like ring components in their long chain.