Ethylene Biosynthesis Physiological effects

Biosynthesis of Ethylene:- Ethylene is known to be synthesized in plant tissues from the amino acid methionine. A non-protein amino acid, 1-amino cyclopropane-l-carboxylic acid (ACC) is an important intermediate and also immediate precursor of ethylene biosynthesis. The two carbons of ethylene molecule are derived from carbon no. 3 and 4 of methionine.

a. First Step:- In the first step, an adenosine group (i.e., adenine + ribose) is transferred to methionine by ATP to form S-adenosylmethionine (SAM). This reaction is catalysed by the enzyme SAM-synthetase (methionine adenosyl transferase).

b. Second Step:- In the second step, SAM is cleaved to form 1-aminocyclopropane-l- carboxylic acid (ACC) and 5′-methylthioadenosine (MTA) by the enzyme ACC-synthase.

i. Synthesis of ACC is rate limiting step in ethylene biosynthesis in plant tissues.

ii. Exogenously supplied ACC greatly enhances production of ethylene in plant tissues.

c. Third Step:- In the third and last step of ethylene biosynthesis, ACC is oxidised by the enzyme ACC-oxidase (previously called ethylene forming enzyme i.e., EFE) to form ethyl­ene. Two molecules, one each of HCN and H2O are eliminated.

i. ACC oxidase activity can be rate limiting step in ethylene biosynthesis in plant tis­sues which show high rate of ethylene production such as ripening fruit.

ii. The enzyme ACC oxidase requires ferrous iron (Fe2+) and ascorbate as cofactors.

iii. ACC can be conjugated to give N-malonyl ACC and thus, may play an important role is regulation of ethylene biosynthesis.

Yang Cycle:-

- There is only limited amount of free methionine (which is a sulphur containing amino acid) in plant tissues. Therefore, to sustain normal rate of ethylene biosynthesis, the sulphur released during ethylene biosynthesis is recycled to methionine again through methionine cycle or Yang cycle (so named after the pioneer worker S.F. Yang on ethylene biosynthesis). 

- The CH3-S group is salvaged and reappear in methionine as a unit. The remaining 4C atoms of methionine are supplied from ribose moiety of ATP which was originally used to form SAM. A transamination reaction provides the amino group.

Physiological effects of Ethylene:- 

i. Fruit Ripening:- Its main function is to ripen fruits. The most commonly used chemical is called ethephon (2-chloro ethylphosphonic acid). It penetrates into the fruit and decomposes ethylene.

ii. Triple Response:- Ethylene  causes plants to have – 

    i. Short shoots:- Inhibition of stem elongation.

    ii. Fat shoots:- Stimulation of radial swelling of stems.

    iii. Diageotropism:- Increased lateral root growth and horizon­tal growth of stems with respect to gravity.

iii. Formation of Adventitious Roots and Root Hairs:- Ethylene induces formation of adventitious roots in plants from different plant parts such as leaf, stem, peduncle and even other roots. In many plants especially Arabidopsis, ethylene treatment promotes initiation of root hairs.

iv. Inhibition of Root Growth:- Ethylene is known to inhibit linear growth of roots of dicotyledonous plants.

v. Leaf Epinasty:- When upper side (adaxial side) of the petiole of the leaf grows faster than the lower side (abaxial side), the leaf curves downward. This is called as epinasty. Ethylene causes leaf epinasty in tomato and other dicot plants such as potato, pea and sunflower. Young leaves are more sensitive than the older leaves. However, monocots do not exhibit this response.

vi. Flowering:- Ethylene is known to inhibit flowering in plants.

vii. Sex Expression:- In monoecious species especially some cucurbits like cucumber, pumpkin, squash and melon; ethylene strongly promotes formation of female flowers thereby suppressing the number of male flowers considerably.

viii. Senescence:- Ethylene enhances senescence of leaves and flowers in plants. In senescence, concentra­tion of endogenous ethylene increases with decrease in conc. of cytokinins and it is now generally held that a balance between these two phytohormones controls senescence.

ix. Abscission of Leaves:- Ethylene promotes abscission of leaves in plants. Older leaves are more sensitive than the younger ones.

x. Breaking Dormancy of Seeds and Buds:- Ethylene is known to break dormancy and initiate germination of seeds in barley and other cereals. Seed dormancy is also overcome in strawberry, apple and other plants by treatment with ethylene. Non-dormant varieties of seeds produce more ethylene than those of dormant varieties.