Transplastomics – a convergence of biotechnology and evolution

November 16, 2008

One of the going concerns in plant biotechnology is the matter of containment of transgenes in the field.  This concern arises from the inescapable fact that genetically-modified crop plants may, depending on the specific species involved, “disseminate” transgenes via hybridization with nearby plants (of the same species or closely-related ones).  A number of strategies have been devised to reduce or eliminate this possibility.  Among these is the approach of placing transgenes in the chloroplast genome of a recipient crop plant.  The rationale behind this approach lies in the fact that the chloroplast genome is inherited in a maternal fashion (much as are mitochondrial genomes in animals).  Consequently, pollen shed by a transplastomic plant (the jargon shorthand term for the plant that has one or more transgenes resident in the plastid genome, as opposed to the nuclear genome) should not carry or transmit the transgene, since transmission is through the female gamete.

Expressing foreign genes in the chloroplast comes with some other advantages.  Since the chloroplast is a prokaryotic genetic system, it is not “encumbered” by the presence of an elaborate and hard-to-control gene silencing system, one that affects nuclear-sited transgenes in a haphazard fashion.  This means that expression of chloroplast-situated transgenes is more consistent (and often attains higher levels) that that of similar nuclear transgenes.  The chloroplast is as well the location for some very highly-expressed proteins (plant physiology students learn early on that the chloroplast enzyme rubisco aka ribulose-1,5-bisphosphate carboxylase is the most abundant protein on earth), which means that it is feasible to attain higher protein levels in such systems than from nuclear transgenes.  (Of course, there are controls on mRNA and protein accumulation in the chloroplast, so that it is necessary to test and manipulate the specific transgene and its protein product to achieve the desired results.)

These considerations aside, the possibility of transmission of chloroplast-sited transgenes remains something of an open issue.  One matter should be familiar to readers who follow the field of human ancestry; work in this field has been complicated by the observation that mitochondrial genomes, typically assumed to be maternally-transmitted, may on occasion be inherited through the paternal gamete.  A similar concern applies to those plant species that are assumed to transmit chloroplast genomes maternally; for example, recent studies (5, 8 ) show that paternal inheritance of chloroplast-localized transgenes does occur. Read the rest of this entry »

Hope for school cafeterias, parents, …

November 16, 2008

…. and others who must deal with allergies to peanuts. A commodity that would be of considerable utility and value, a hypoallergenic peanut, is the goal of several laboratories. One group, consisting of labs at a USDA research unit in New Orleans and the campus of the University of Georgia in Tifton, recently published an interesting and promising approach to develop such a commodity. These researchers set out to screen wild relatives of the peanut (Arachis hypogaea L.) for variants of the principle allergen in peanuts, the seed storage protein Ara h 2.01, that did not possess the allergenic properties of the peanut protein. They used an approach called EcoTILLING (1) to screen several accessions of the wild relative Arachis duranensis for those with point mutations in the gene homologous to that encoding Ara h 2.01. In so doing, they identified several such variants, including one that had a significant reduction in IgE binding but otherwise seemed to cause a minimal structural change in the protein. This result is encouraging, as it may provide a way to replace, by standard breeding or by TILLING-assisted mutagenesis, the offending seed storage protein with one that is less allergenic. Read the rest of this entry »

An interesting take on poly(A) signals

November 16, 2008

One interesting facet of RNA biology is the matter of the occurrence and function of structural RNA units within RNAs inside of living cells.  Excellent examples of these are the many so-called riboswitches, motifs that bind metabolites and alter the functionalities of RNAs in which they reside.  As more and more examples of RNAs with catalytic activities become known, questions naturally arise as to whether such activities might impact RNA function in vivo.  A recent study poses just such a question for what is among the simplest of known catalytic RNAs, namely the manganese-dependent ribozyme.  This enzyme consists of little more that a GAAA-UUU complex.  These two motifs, that need not be adjacent in the RNA, are expected to occur at a high frequency in natural RNAs.  A recent report indeed finds that RNAs that possess such motifs indeed may be Mn-dependent ribozymes.  The physiological signifiance of this finding is, in my opinion, a bit of an open issue.  But the possibilities are fascinating.  The last sentence of the abstract (that follows) is especially provocative. Read the rest of this entry »