Transcriptional responses of Solanum nigrum to methyl jasmonate and competition:: a glasshouse and field study

1. Field studies are needed to understand how plants respond to environmental stresses. However, transcriptional responses of plants interacting with biotic stressors, such as herbivores or competitors, have been measured primarily in controlled environments. Using oligonucleotide microarrays (with 60 and 568 genes), we determined how accurately glasshouse (GH) studies of transcriptional responses to methyl jasmonate (MeJA) elicitation and competition reflect the responses of Solanum nigrum L. plants grown under field conditions. We used MeJA to elicit herbivore resistance in a standardized manner. 2. In field-grown plants (experiment I), 13 genes (from the 60-gene microarray) involved in defence and primary metabolism were significantly up- or downregulated by MeJA elicitation, which also increased the activity of trypsin proteinase inhibitors. 3. We also grew S. nigrum with competitors in GH (experiment IIa) and field mesocosms (experiment IIb), and used the 60-gene microarray to examine the MeJA-elicited transcriptional changes. Both GH- and field-grown plants consistently regulated genes involved in oxylipin signalling and primary metabolism. However, the larger number of downregulated genes in GH-grown compared with field-grown plants suggests that field-grown plants are less sensitive to MeJA elicitation, probably because they are exposed to more abiotic stresses. 4. The 60-gene microarray did not reveal any effects of competition, but the analysis with the 568-gene array identified many genes, some of which are involved in stress-signalling pathways and significantly altered only in plants grown with competitors. These alterations were associated with a significant decrease in fitness (number of fruits). 5. We conclude that microarrays can be used successfully to monitor the gene expression of plants growing in natural environments, and we identify key genes that are regulated in plants interacting with competitors and herbivores. Whether these transcriptional responses mature into phenotypic changes should be determined by additional phenotypic analysis.