Dear Lim,

All depends on concentration. At relatively low concentrations auxins are cell division and/or growth promoting factors (in dependence on other hormones), but at high concentrations they are herbicides. The chemical nature of the particular compund can shift dependence curves significantly. Also, plant species is important.

In field plants, 2,4-D kill dicots, but it is innocuous for monocots (it is a selective herbicide). The reason is that cell membrane receptors of monocots have low compatibility with such synthetic regulator. In plant tissue culture 2,4-D is active as auxin in both monocots and dicots, of course, in extremely low concentrations (in the order of micro-molar concentrations).

Dear Lim Wan Wan 

Pls. find the attached files

http://www.ejbiotechnology.info/index.php/ejbiotechnology/article/view/v10n1-9/198

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1067094/

http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-84041998000100009

http://link.springer.com/article/10.1007/s11240-005-0513-6

ttp://www.amjbot.org/content/91/11/1743.full

http://www.sigmaaldrich.com/catalog/product/sigma/d7299?lang=en®ion=SY

http://www.bio-world.com/productinfo/3_43_288_838/6996/Dichlorophenoxyacetic-acid-D-for-plant-tissue-culture.html

Hoping this will be helpful

Regards

Prof. Houda Kawas

The toxicity of 2,4-D depends on its chemical form (acid, salt,..). It kills plants by causing the cells in the tissue that carry water and nutrients to divide and grow without stopping. it kills broadleaf weeds as herbicides , but not most grasses ( I agree with Isidro) . Using 2,4-D as the sole synthetic auxin in low concentration, induce the formation of early-stage somatic embryos through the indirect method, callus formation as well.  while, it showing a mutagenic activity  in the concentration range higher than usually used in establishing plant tissue culture

Hey guys, thanks for all the information! Now I understand the why to it :)

You most welcome

Houda

As you know this a synthetic auxin and you might be knowing about role of auxin

2,4-Dichlorophenoxyacetic is a herbicide used to control broad leaf weeds. It also enhances storage life and quality of fruits.

2,4‐D is the first synthetic broadleaf herbicide to be commercially used in high concentration. Also, this is synthetic auxin for the induction of somatic embryos (embryoids) on cultured explants at very low agglomeration. 2,4‐D contributes to (ACX, XOD, LOX), reactive oxygen species production (ROS), ploidy level, ethylene production. Therfore, the roles mentioned depend on the concentration of this substance.