What exactly do plant parasites take up?

By attaching themselves to an unwilling host ( either through a direct connection to the phloem / xylem or indirectly via other cells such as parenchyma tissue: this remains an area of debate and may vary between species), parasites are able to access water and, in the case of holoparasites photosynthate sugars, yet what else do parasites expose themselves to by establishing this conduit? Although much evidence suggests that uptake is an active process and that the haustorium ( penetration organ) can show selective nutrient uptake, various other substances have been shown to accumulate int he parasite from the host. An intriguing example is plant toxins secreted by the host to deter herbivore predators: by taking up these compounds via the haustorium, such protection can be conferred to the parasite. The holoparasitic stem parasite Cuscuta ( dodder) supports greater numbers of feeding aphids  when grown on tomatoes compared to turnips. Turnips belong to the same family as cabbages ( Brassicas) , which are known to produce glucosinolates as a herbivore deterrent. To investigate the basis of aphid defence in Cuscuta, Smith et al. ( Pennysylvania State Univeristy) grew the parasite on wild type Arabidopsis or mutants deficient in glucosinolate production. Predation by pea aphids was greatly increased on Cuscuta grown on the glucosinolate deficient Arabidopsis, suggesting that parasite uptake of host toxins can provide protection against herbivores. Evolution however, has rendered this interchange more complex however: certain insects, such as monarch butterflies, have developed a tolerance to plant toxins and specifically feed on these to make themselves unpalatable to birds. Similarly, peach aphids are stimulated to lay eggs by glucosinolate compounds. Consequently, peach aphid fecundity was increased on Cuscuta grown on glucosinolate deficient Arabidopsis. Hence, whether these commands are actively taken up by the parasite or accumulate in a passive manner, these seem to act as a double edged sword in the battle against herbivory.

Perhaps more intriguing is the evidence that RNA* from the host can also be transferred to the parasite; there are thought to be over 2000 phloem mobile RNAs yet even RNA sequences thought to be immobile have been detected in parasites. This raises the exciting possibility that parasites could theoretically be controlled using RNA interference** using transgenic hosts expressing siRNAs or shRNAs. This has successfully been demonstrated against Triphysaria versicolour grown on the legume Medicago truncatula. The host plants were engineered to express an RNAi construct against the enzyme acetyl-CoA carboxylase, a key enzyme for plant metabolism. Triphysaria parasites that infected these hosts were unable to survive and died (Yoder et al. Univeristy of California). It is clear that understanding the flow of traffic between host and parasite could afford greater insight into potential mechanisms for control.

* RNA: an intermediate ' messenger molecule' transcribed from the DNA sequence of a gene.  The RNA then translocates from the nucleus to the min body ( cytoplasm) of the cell, where organelles called ribosomes read the RNA sequence and assemble the amino acids of the protein the original gene encoded.

** RNA interference: a mechanism to effectively silence a gene by destroying all the proteins produced from it. Unlike DNA, RNA templates are single stranded and double stranded RNAs are incompatible within the cell. By articially introducing an RNA construct that is complementary to the RNA of interest, these can anneal together to produce a double stranded molecule that is targeted to destruction by the cell. This prevents the ribosomes from manufacturing the protein encoded in the RNA.

 

Plant Scientists love to network! This example comes from the Stables Courtyard at Chatsworth House, venue for the Wednesday "cultural excursion".