39 
that the normal virions can carry probably cannot exceed 
2 x 10 6 dal tons, we urge that consideration be given 
to the development of systems in which recombinants can 
be cloned and propagated purely in the form of DNA, rather 
than in the coats of infectious agents. Plasmid forms 
of viral genomes or organelle DNA should be explored as 
possible cloning vehicles in eukaryotic cells. 
<b> Plant host-vector systems 
Cells in tissue cultures, seedlings, or plant parts (e.g., tubers, 
stems, fruits, and detached leaves) or whole mature plants of small 
species (e.g., Arabidopsis ) can be handled under the P1-P4 containment 
conditions that we have specified previously. However, work in most 
plants poses additional problems. P2 physical containment conditions 
can be provided by: (i) the best insect-proof greenhouses, (ii) appro- 
priate disinfection of contaminated plants, pots, soil, and runoff 
water, and (iii) adoption of the other standard practices for microbio- 
logical work. P3 physical containment can be sufficiently approximated 
by confining the operations with whole plants to growth chambers like those 
used for work with radioactive isotopes, provided that (i) such chambers 
are modified to produce a negative pressure environment with the exhaust 
air appropriately filtered, and (ii) that other operations with infectious 
materials are carried out under the specified P3 conditions. The P2 
and P3 conditions specified earlier are therefore extended to include 
these cases for work on higher plants. 
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