Nice question Kunti. One of the reasons for frequent use of NAA instead of 2,2-D could be options available with latter to act as retardant as welll, which is distinctly  missing  with former , is effective only in triggering the growth with former.

Dear Azizah

There are many factors affecting callus formation and regeneration. Auxin type and concentration are critical factor besides genotype , explant type and culture environment. In monocots, 2,4-D is very important for embryogenic competence in early stage, followed by NAA . both are as only trigger of the cell fate swtich. but this is not a general role, for more information in this concern, (E.F. Gerorge  et al., (eds), plant propagation by tissue culture 3rd Edition,vol.1) 

Dear Azizah,

There could be many reasons for the choice of NAA over 2,4-D. One of which is that somaclonal variation increases with the use of 2,4-D.

please see   "Electronic Journal of Biotechnology Vol.13, No. 1, January 15, 2010" For better understanding of somatic embryogenesis activities in cultured tiossues., 

I worked in my past study on callus induction from fennel and I found that 2,4-D encoureged callus induction only without any other growth formation but NAA encoureged somatic embryogenesis and rooring.

2,4-D is used for callus induction more than anything.  NAA is using for other purposes, including regenerating plantlets, in combination with other hormones.  Also, as someone mentioned, 2,4-D does increase somatic variation.

2,4-D i s usually used for callus production, but for plant let regeneration ,NAA  is preferred for various reasons.

2,4-D is used for callus induction and NAA is used in regeneration media for shoot induction. Thus, NAA can b used in regeneration media for shooting and rooting but  not necessarily as I am doing tissue culture from past 3 years in rice and wheat, without adding NAA, instead addition of BAP is the prime necessity for the regeneration.  

2, 4-D is very strong auxin widely used as herbiside, and in case of in vitro culture it stimulate de-differentiation to induce callus. Whereas NAA is comparatively weak auxin and in combination with cytokinin or even alone can stimulate re-differentiation to induce shoot/root/somatic embryos.

Together with cytokinin, 2,4-D is used primarily for callus induction and suspension cultures, being replaced by NAA and IBA when morphogenesis is required, especially for inducing rooting.

Apart to act as auxin,bc\, the 2,4-D also generate stress condition which is beneficial to initiate embryogenesis.

thank you very much for all answers.... :)

kalo mau induksi kalus aja emang bagus pake 2,4-D karena punya fungsi baik dalam dediferensiasi sel tanaman, konsentrasi paling ampuh 1 ppm. Namun yang perlu diingat 2,4-D itu bukan auksin, melainkan herbisida, tapi fungsinya buat induksi kalus lumayan ok. Kalo jadi penggunaan senyawa ini terlalu lama ditakutkan bisa menyebabkan variasi somaklonal. kalo di padi biasanya induksi kalus sebulan, lalu setelahnya menggunakan NAA dll. Namun kalo NAA pada biji biasanya kalus agak lama terbentuk namun kalo 2,4-D bisa lebih cepat. Mungkin karena itu banyak yang make 2,4-D, dan penggunaan biji untuk transformasi pada padi lebih mudah dalam proses sterilisasi dan efisiensi untuk menginduksi kalusnya lebih baik. 

Jadi kalo punya kalus padi jangan kelamaan make 2,4-D, karena kalo tujuan akhirmu untuk somatik embriogenesis maka kemungkinan besar variasi somaklonalnya bisa tinggi. 

Kunti, pergunta interessante. Ambas auxinas atuam de forma muitas vezes similares normalmente o 2,4-D é usado para indução de calos embriogênicos e o ANA simplesmente na formação de calos sem entretanto visar a embriogenia. Porém, a espécie utilizada será um fator a ser considerado.