Write a short note on Auxins.

(N/A) Auxin (Greek 'auxein': to grow) was initially isolated from human urine,but later its presence was also found in plants,proving it to be the first $PGR$ (Plant Growth Regulator) ever known.
The primary natural plant auxin is chemically known as Indole-$3$-Acetic Acid $(IAA)$.
The term 'auxin' is also applied to other natural and synthetic compounds having various growth-regulating properties.
Production of auxin generally takes place in the growing apices of stems and roots,from where they migrate to the site of action.
Types of Auxin:
$(a)$ Natural Auxins: These occur naturally in plants and fungi,e.g.,Indole Acetic Acid $(IAA)$ and Indole Butyric Acid $(IBA)$.
$(b)$ Synthetic Auxins: These are prepared from synthetic compounds that cause several responses similar to $IAA$,e.g.,Naphthalene Acetic Acid $(NAA)$ and $2,4$-Dichlorophenoxyacetic acid $(2,4-D)$.
Functions of Auxins:
$(a)$ Cell Elongation: Auxin stimulates the elongation of cells in shoots.
$(b)$ Initiation of Roots: In contrast to stems,higher concentrations of auxin inhibit shoot elongation but initiate lateral root formation.
$(c)$ Inhibition of Abscission: Natural auxins delay the abscission of young fruits and leaves and are used to control pre-harvest fruit drop.
$(d)$ Apical Dominance: The presence of auxin in higher concentrations in the shoot apex promotes apical dominance,where the apical bud inhibits the growth of lateral buds. Lateral buds only develop into branches when the apical bud is removed (decapitation).
$(e)$ Promotes Flowering: Auxin helps in promoting flowering in plants like pineapple and litchi.
$(f)$ Parthenocarpy: Auxins are used to induce parthenocarpy in unpollinated pistils,which increases market value.
$(g)$ Metabolism: Application of auxin can enhance metabolism due to the mobilization of plant resources.
$(h)$ Eradication of Weeds: Auxins are used as weedicides. For example,$2,4-D$ is widely used to kill dicotyledonous weeds without affecting mature monocotyledonous plants.
$(i)$ Cell Division: Besides cell elongation,auxin may also be active in cell division.