14.4.2 Selectable Markers
The most commonly utilized plant selectable marker genes include thenptIIandhptgenes that
confer antibiotic resistance as a basis to select for cell transformation (Miki and McHugh 2004)
(see also Chapter 9). Several other selectable markers conferring herbicide resistance or posi-
tive selection on the basis of novel carbon utilization pathways provide important alternativesto
the antibiotic-based selection strategies (Roa-Rodriguez and Nottenburg 2003). Broad patents
cover all of these selectable markers (Rogers and Fraley 2001; Santerre and Rao 1988; Bojsen
et al. 1998). Selection strategies do not appear to have been the topic of public-sector research
programs, and there are just a few examples of either public-domain or public-sector-patented
selectable markers for use in plant transformation (Dirk et al. 2001, 2002; Hou et al. 2006;
Mentewab and Stewart 2005). While there is potential to invent new selectable markers for
plant transformation, at this point this represents another key enabling technology where
patents have the potential to block new innovations.
14.4.3 Constitutive Promoters
Genetic regulatory elements are required to drive the expression of selectable marker genes
and of specific transgenes. Selectable marker genes are typically driven by high-level con-
stitutive promoters, with the most common constructs utilizing the CaMV 35Spromoter
derived from a viral genome and owned by Monsanto (Odell et al. 1985; Fraley et al.
1994). Many alternative promoters that confer constitutive gene expression were developed
in public-sector organizations and are either in the public domain or can be licensed for
nominal fees. These alternatives include monocot and dicot actin promoters (McElroy
et al. 1990; McElroy and Wu 2002; An et al. 1996; Huang et al. 1997), a FMV 34Spro-
moter (Comai et al. 2000), mannopine/octopine synthase (Gelvin et al. 1999), or FMV
and PCLVS FLt promoters (Maiti and Shephard 1998, 1999). The FMV 34Shas been
used to drive constitutive gene expression and reported to be essentially equivalent to the
CaMV 35Spromoter (van der Fits and Memelink 1997; Romano et al. 1993), but has
not been widely distributed to the public-sector research community. Each of these promo-
ters provides a strategy for driving constitutive transgene expression using public-sector-
derived or public-domain components.
14.4.4 Tissue- or Development-Specific Promoters
Although many genes can be expressed under the control of constitutive promoters, target-
ing of expression to plant organs or tissues is typically desirable to minimize nonspecific
effects of the introduced gene. For example, seed-specific promoters (Blechl et al. 1999;
Harada et al. 2001) have been patented with claims directed toward their use to drive
expression of heterologous genes in developing seeds. Public-sector institutions have
also patented a relatively large number of tissue- and/or developmental-specific promoters.
Examples include the root-specific CaMV 35Sfragment A promoter (Benfy and Chua
1992), a root cortex–specific promoter (Conkling et al. 1998), the Pyk10 root-specific
promoter (Grundler et al. 2001), an epidermal cell–specific Blec promoter (Dobres
and Mandaci 1998), and a vascular tissue–specific promoter RTBV (Beachy and
Bhattacharyya 1998). In addition, there exists a large number of tissue and developmental
specific promoters that have been characterized and placed in the public domain through
publication. A wide range of constitutive and regulated promoters have been tabulated in
332 INTELLECTUAL PROPERTY IN AGRICULTURAL BIOTECHNOLOGY