GENE	TRANSFER	METHODS	IN	                  1.  A	promoter	which	determines	the	
                  PLANTS	                                       site	 and	 timing	 of	 expression	 of	
                  	                                             the	introduced	gene		
                  Gene	 transfer	 methods	 differ	 for	      2.  A	terminator	to	identify	the	end	of	
                  both	 animals	 and	 plants.	 In	 plants,	     transcription	and		
                  the	uptake	of	genes	by	cells	is	often	     3.  A	 marker	 gene,	 which	 allows	
                  described	 as	 transformation,	 while	        selection	 of	 plants	 having	 the	
                  in	 animals	this	term	is	replaced	by	         introduced	gene.		
                  the	term	transfection,	leading	to	the	     Various	 desirable	 traits	 have	 been	
                  production	of	transgenic	plants	and	       efficiently	 introduced	 and	 stably	
                  animals.	 Plant	 transformation	 thus	     expressed	 in	 almost	 150	 plant	
                  depends	 on	 two	 events:	 successful	     species.	  Different	  methods	 are	
                  introduction	 of	 foreign	 DNA	 into	      available	   to	   achieve	   genetic	
                  target	 plant	 cells,	 and	 subsequent	    transformation	 of	 plants	 i.e.	 the	
                  development	 of	 a	 complete	 plant	       delivery	of	the	foreign	DNA	into	the	
                  derived	from	the	transformed	cells.		      host	 plant	 (Figure	 1).	 These	 are	
                  Plant	   transformation	    methods	       divided	into	two	main	groups		
                  therefore	require	an	efficient	way	to	     1. Indirect	 methods:	 In	 this	 case	
                  introduce	 DNA	 into	 cell	 and	 the	         vector	is	needed	for	insertion	of	the	
                  regeneration	 of	 the	 transformed	           foreign	DNA	into	the	host	genome.		
                  cells	or	tissues	into	whole	plants.		      2. Direct	 methods:	 This	 method	 is	
                  The	   DNA	   segment	    which	   is	        vector	 independent.	 The	 DNA	 is	
                  introduced	 in	 this	 process	 contains	      directly	 inserted	 into	 the	 host	
                  the	 gene	 of	 interest	 and	 a	 cassette	    genome.	
                  containing	    additional	    genetic	
                  material.	Additional	genetic	material	
                  includes		
                  	
                                                                                                  	
                  INDIRECT	METHODS:		                          Agrobacterium	can	only	infect	plants	
                  Agrobacterium-mediated           plant       through	wounds.	When	a	plant	root	
                  transformation                               or	 stem	 is	 wounded	 it	 gives	 off	
                                                               (produce)	 certain	 chemical	 signals.	
                  A.	tumefaciens	are	soil	bacteria	that	       In	  response	   to	  these	  signals,	
                  have	the	ability	to	infect	plant	cells	      agrobacterial	  vir	  genes	  become	
                  and	 transfer	 a	 defined	 sequence	 of	     activated	 and	 direct	 a	 series	 of	
                  their	 DNA	 to	 the	 plant	 cell	 in	 the	   events	necessary	for	the	transfer	of	
                  infection	 process.	 Upon	 integration	      the	T-DNA	from	the	Ti	plasmid	to	the	
                  of	 the	 bacterial	 DNA	 into	 a	 plant	     plant	cell	through	the	wound.		
                  chromosome,	it	directs	the	synthesis	        To	harness	A.	tumefaciens	and	the	Ti-
                  of	 several	 proteins,	 using	 the	 plant	   plasmid	 as	 a	 transgene	 vector,	 the	
                  cellular	 machinery,	 that	 ensure	 the	     tumor	inducing	section	of	T-DNA	is	
                  proliferation	   of	  the	   bacterial	      removed,	 while	 the	 T-DNA	 border	
                  population	within	the	infected	plant,	       regions	 and	 the	 vir	 genes	 are	
                  Agrobacterium	 infections	 result	 in	       retained.	 The	 desired	 transgene	 is	
                  crown	gall	disease	(Figure	2).		             inserted	between	the	T-DNA	border	
                  In	 addition	 to	 its	 chromosomal	          regions,	 applying	 recombinant	DNA	
                  genomic	DNA,	an	A.	tumefaciens	cell	         technology.	 Thus,	 in	 the	 infection	
                  contains	a	plasmid	known	as	the	Ti	          process,	 the	 transgene	 DNA	 is	
                  (tumour-inducing)	 plasmid.	 The	 Ti	        transferred	 to	 the	 plant	 cell	 and	
                  plasmid	 contains	 a	 series	 of	 vir		      integrated	    into	   the	    plant’s	
                  (virulence)	 genes	 that	 direct	 the	       chromosomes.	        To	      achieve	
                  infection	 process,	 and	 a	 stretch	 of	    transformation,	 Agrobacterium	 cells	
                  DNA	termed	T-DNA	(transfer	DNA),	            carrying	      an	      appropriately	
                  approximately	20	kb	in	length,	that	         constituted	   Ti	  plasmid	   vector	
                  is	transferred	to	the	plant	cell	in	the	     containing	the	desired	transgene	can	
                  infection	   process.	  The	   T-DNA	        be	inoculated	into	plant	stems,	leaf	
                  encodes	 proteins	 required	 for	 the	       disks	etc.,	to	allow	infection	and	T-
                  maintenance	 of	 infection.	 These	          DNA	transfer	to	the	plant	cells.	The	
                  proteins	   include	   certain	  plant	      explants	   that	  have	   been	   co-
                  hormones	that	stimulate	cell	growth,	        cultivated	 with	 Agrobacterium	 are	
                  resulting	 in	 the	 formation	 of	 galls,	   subsequently	   processed	    through	
                  and	proteins	required	for	a	certain	         various	tissue	culture	steps	resulting	
                  metabolic	pathway	that	secures	the	          in	 the	 selection	 and	 production	 of	
                  availability	 of	 nutrients	 for	 the	       transformed	    cells	  and	   plants.
                  bacteria	(Figure	3).		
                  	
                  	
                  	
                  	
                  	
                                           	
        Figure	 1|	 Depending	on	species,	plant	tissues	are	extremely	pliable	and	can	be	
        easily	interconverted	and	regenerated	in	culture.	This	diagram	shows	some	of	the	
        ways	in	which	this	flexibility	can	be	exploited	to	facilitate	gene	transfer	and	the	
        creation	of	transgenic	plants.	
        	
        	
                                           	
        Figure	 2|	 Steps	 involved	 in	 the	 generation	 of	 genetically	 transformed	 plants	
        using	 either	 the	 Agrobacterium	tumefaciens	 or	 micro-projectile	 bombardment	
        approaches	
        	
        (Following	 introduction	 of	 foreign	 DNA	 into	 the	 plant	 cell,	 successfully	
        transformed	cells	are	selected	and	used	to	regenerate	a	transgenic	plant.)