Agriculture Reference
In-Depth Information
the Rpg 1 gene coding for durable resistance to stem rust in barley has a different type of
resistance gene structure, in that it has close homology to receptor kinases, but has no
NBS or LRR region (Brueggeman et al., 2002).
Xa 21 was sequenced and cloned and when transformed into rice found to stably
express resistance against a suite of pathogen isolates in 1995 (Wang et al., 1996). It
was subsequently transformed into the variety IR72 and the fi rst fi eld tests conducted
in 2000. In this particular case, the use of GM technology did not provide an economic
breakthrough, as the gene had already been bred into an adapted variety, IRBB21, using
non-GM technologies. It has, however, been a pioneering example demonstrating the use
of the transgenic technologies in a major crop. At this time, transgenic rice lines with
Xa 21 are still being fi eld tested and their future debated, whilst other transgenic lines are
being developed that carry cloned resistance genes for sheath blight and stem borer (Brar
& Khush, 2006). As with other major genes, new strains of bacterial blight with virulence
on Xa 21 have been found in Korea and the Philippines, subsequent to the gene being
deployed using non-transgenic technologies.
It is envisaged however that once GM rice becomes widely accepted, this technology
will be used to pyramid multiple different resistance genes into a single variety provid-
ing, it is hoped, more durable resistance. Success for this strategy, as with the use of other
major genes, will require agreement and coordination amongst plant breeders and the
industry to prevent varieties being grown together that will allow the pathogen to mutate
stepwise, one gene at a time, to overcome these gene pyramids.
The successful transfer of major resistance genes may be largely limited to closely
related species which have related resistance signalling pathways. Transfer of effec-
tive resistance from tomato to tobacco, both Solanaceous species, has been clearly
demonstrated (Rommens et al., 1995), but to date there has been no reported success
of resistance genes being effective when transferred to different families, except where
either the corresponding avirulence product was artifi cially provided or where a race
of the pathogen that affected both donor and recipient species was involved (Ayliffe &
Lagudah, 2004).
Another possible approach would be to use cloned genes that code for proteins involved
in the immune response or non-host resistance. Eukaryotes are resistant to almost all
microbes other than a very few which are generally specifi c to that species or genus.
Resistance to all the others is known as non-host resistance or innate immunity and oper-
ates through the recognition of pathogen-associated molecular patterns (PAMPs), a term
developed by mammalian scientists (Medzhitov & Janeway, 1997). PAMPS, as the name
suggests, are conserved molecules that are common across a broad range of potential
pathogens and which help to distinguish them as potentially harmful to a host. A more
appropriate general term to use perhaps is MAMPs (for microbe- associated molecular
patterns), as non-pathogenic microorganisms also elicit the defence response (see review
by Bent & Mackey, 2007). Examples of many different known MAMPs are fl agellins in
bacteria, chitins in fungi and double-stranded RNA in viruses. Recognition of MAMPs
occurs through plant surface pattern recognition receptors which elicit the immune or
non-host resistance response (MAMP-triggered immunity). Microbes that have become
pathogens are those that have evolved the ability to suppress MAMP-triggered immunity
by interfering with either MAMP recognition or with the subsequent host response path-
way through the production of 'effector' proteins secreted by the pathogen into the host
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