Table 7.

Overview of the three main strategies for plant gene editing: ZFN, TALEN, and CRISPR/Cas9.

Technology	Acronym	Type	System components	Mechanism of action	Sensitivity to methylation	Off-target effects	Design difficulty	Scaling up for library production	Studies in tomato and grape
Zinc finger nuclease	ZFN	Protein-DNA	Zinc finger DNA binding domain fused with an endonuclease (usually FokI); specific recognition of 3 bp sequences	A DNA-cutting/DNA-grabbing-based system, able to recognize target genes	Yes	High	Difficult	No (custom protein selection for each gene)	Hilioti et al., 2016
Transcription activator-like effector nuclease	TALEN	Protein-DNA	Endonuclease (usually FokI) catalytic domain fused to Xanthomonas spp. DNA binding domain of transcription activator-like effectors. Composed by 33–35 amino acid (aa) multiple repeats containing a repeat variable diresidue (RDV; usually, the aa 12 and 13)	Same as ZFN	Yes	Low	Medium	Possible but complicate	Lor et al., 2014
Clustered regularly-interspaced short palindromic repeat/CRISPR-associated	CRISPR/Cas9	RNA-DNA	20 nt crRNA fused to tracrRNA and Cas9 endonuclease	A DNA-cutting protein associated to a guided RNA which can specifically recognize target genes	No	Low	Easy	Easy (generation of 20 nt adapter/s for each gene)	Brooks et al., 2014; Ron et al., 2014; Čermák et al., 2015; de Toledo Thomazella et al., 2016; Jacobs and Martin, 2016; Ito et al., 2015; Klap et al., 2016; Pan et al., 2016; Xu et al., 2016; Ren et al., 2016; Wang et al., 2016

Technical characteristics and a survey of all the studies described, to date, in tomato and grape are also provided.