Dr. Mohit Gupta
Plant Physiology 

Questions asked in Section - C of  CSIR-NET December 2014.

Q101.  A transgenic lettuce plant was generated by over-expressing isopenteny׀ transferase (IPT) gene under the control of promoter of senescence activator gene (SAG12). Following are some statements regarding this transgenic plant. The transgenic plants
A. Exhibit delayed senescence
B. Exhibit fast senescence.
C. Have higher amount of cytokinin during senescence.
D. Have higher amount of gibberellins during senescence.

Which one of the following combinations of above statements is correct
1.  A and B                           2. A and C                            3. B and D                            4.  C and D

Ans (2)  Phytohormones is one of the most wide topics in Plant Physiology and most important as well.

The above is a direct question from Cytokinin synthetic pathway. Infact cytokinin is synthesized from condensation of  DMAPP (which is an isoprene i.e Terpenoid) and Adenosine group (contributed by either ATP or ADP)........and the reaction is catalyzed by Isopentenyl transferase (IPT), so overexpression of IPT shall result in enhanced synthesis of cytokinin. 

Infact cytokinin synthesis takes place in plastids and site of cytokinin production is mainly root tips, developing embryos and young leaves. First discovered cytokinin is KINETIN, which is a synthetic cytokinin and is 6-Amino purine. First natural cytokinin to be discovered is ZEATIN, from maize endosperm. Infact all cytokinins are aminopurines.

Receptors for cytokinin signalling e.g. CRE-1, AHK-2, AHK-3 are found in Plasma Membrane and for students information, cytokinin signalling is very similar to bacterial two component signalling system and it involves a phospho-relay system i.e. phospate is transferred from one signalling component to another.

Infact one of cytokinins role is that it delays senescence specially in leaves, as it increases the longevity of chloroplast and enhances the chlorophyll content called as RICHMOND LANG EFFECT.

Some other roles of cytokinin are:
a) They break / modify Apical dominance and promote growth of lateral buds.
b) They regulate cell cycle along with auxins.
c) They inhibit root growth. 
d) They are involved in root nodule formation in legumes.
e) They also influence red light responses via its interaction with phytochrome signalling, which is a red light receptor.

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