Download Ecklonia maxima is a giant brown seaweed growing at the

Ecklonia maxima is a giant brown seaweed growing at the
southern tip of Africa at the astonishing speed of up to 3
cm per day. One unique feature of Ecklonia maxima is its
unusually high content of auxin in combination with a
much lower cytokinin value leading to probably the
highest auxin to cytokinin ratio known in plants.
Auxin and cytokinin are two of the 5 major classes of plant
hormones. In general, plant hormones control every
aspect of plant growth and development. Plant hormones are produced in very small
concentrations, but even a minute amount can have a profound effect. Reactions to plant
hormones always depend on their relative concentrations compared to other hormones
present. It is the hormonal balance that controls the growth and development of each plant.
Auxin was the first plant hormone detected. The most prominent auxin is IAA (Indol-3-acetic
acid). IAA is synthesised from tryptophan or indole primarily in leaf primordia, young leaves
and in developing seeds.
Auxin is the key plant hormone as it not only initiates different effects but also controls the
action of all other plant hormones like cytokinin and gibberellin.
Key effects are:
Cell enlargement and stem growth
Root initiation
Tropistic responses (gravity and light)
Apical dominance
Delay of leaf senescence
Delay (and promotion) of leaf and fruit abscission in some plants
Fruit setting and growth
Promotion of flowering
Growth of flower parts
Cytokinin is adenine derivates with the ability to induce cell division in tissues in the presence of auxin.
Cytokinin is synthesized in root tips and developing seeds.
Key effects are:
Cell division – in tissues after exogenous application in the presence of auxin, endogenous in gall tumours on plants. Present in actively dividing cells
Growth of lateral buds
Leaf extension by cell enlargement – adjustment of total leaf area to growth of root
system
Delay of leaf senescence
Enhancement of stomata opening (in some species).
The other key plant hormones are gibberellins, ethylene, and abscisic acid.
Gibberellins are synthesised in young tissues of the shoot and developing seeds and initiate
stem growth, fruit setting and growth (with exogenous application) and induce germination (instead of cold or light).
Ethylene is synthesized in many tissues in response to stress, and is the fruit ripening
hormone. In particular, it is synthesized in tissues undergoing senescence or ripening.
Abscisic acid is synthesised in roots and mature leaves, particularly in response to water
stress with the key effects of stomata closure, inhibition of shoot growth and induction of
storage protein synthesis in seeds.
Additionally, many more plant hormones have been detected. Amongst others are polyamines, brassinosteroids, jasmonates, peptides, salicylic acid which all mainly play a role in
plant defence mechanisms.
Balance of auxin and cytokinin:
In the 1950’s a series of experiments showed how the ratio
between auxin and cytokinin works. If auxin is added to a
plant then the cells grow very large but they don’t divide. If
cytokinin is added - and auxin is present - then the plant
cells divide. Therefore, the ratio of auxin to cytokinin determines how the cells will differentiate.
In young plants, a surplus of auxin will initiate root growth.
A higher concentration of cytokinin will support the development of shoots and shoot buds. If the concentrations of
both are equal then the plant cells will grow but will remain
undifferentiated.
The exogenous application of auxin will promote root
growth and initiate the synthesis of cytokinin in the root
meristem. These freshly produced cytokinin will signal the plant to grow more shoots,
leading to a naturally balanced bigger and stronger plant.