Galls are plant structures generated by gall–inducing organisms including insects, nematodes, fungi, bacteria and viruses. Those made by insects generally consist of inner callus–like cells surrounded by lignified hard cells, supplying both nutrients and protection to the gall insects living inside. This indicates that gall insects hijack developmental processes in host plants to generate tissues for their own use. Although galls are morphologically diverse, the molecular mechanism for their development remains poorly understood. To identify genes involved in gall development, we performed RNA–sequencing based transcriptome analysis for leaf galls. We examined the young and mature galls of Glochidion obovatum (Phyllanthaceae), induced by the micromoth Caloptilia cecidophora (Lepidoptera: Gracillariidae), the leaf gall from Eurya japonica (Pentaphylacaceae) induced by Borboryctis euryae (Lepidoptera: Gracillariidae), and the strawberry-shaped leaf gall from Artemisia montana (Asteraceae) induced by gall midge Rhopalomyia yomogicola (Oligotrophini: Cecidomyiidae). Gene ontology (GO) analyses suggested that genes related to developmental processes are up–regulated, whereas ones related to photosynthesis are down–regulated in these three galls. Comparison of transcripts in these three galls together with the gall on leaves of Rhus javanica (Anacardiaceae), induced by the aphid Schlechtendalia chinensis (Hemiptera: Aphidoidea), suggested 38 genes commonly up–regulated in galls from different plant species. GO analysis showed that peptide biosynthesis and metabolism are commonly involved in the four different galls. Our results suggest that gall development involves common processes across gall inducers and plant taxa, providing an initial step towards understanding how they manipulate host plant developmental systems.

Leaves represent the main resource for herbivorous insects and their performances are mainly a function of leaf nutritional quality. Two feeding strategies are known to optimize the exploitation of leaf resources: leaf-miners that selectively feed on tissues of high nutritional quality and gall-inducers that induce the development of a new tissue showing an enhanced nutritional value. Some leaf-miners are known to also manipulate their nutritional environment, but do not affect plant development. Cases of callus proliferation in leaf-mines have been reported, however, the direct role of the insect in the formation of additional plant cells and the nutritional function of this tissue have never been established. Using an experimental approach, we show that leaf-mining larvae of micromoth, Borboryctis euryae (Lepidoptera: Gracillariidae), that grow on Eurya japonica (Pentaphylacaceae), actively induce callus proliferation within their leaf-mine at the fourth instar. We experimentally demonstrated that, at this developmental stage, the larva feeds exclusively on this newly formed tissue and feeding of the tissue is essential for completing larval stage. Phenological census revealed considerable expansion and variation of fourth instar duration caused by the continuous production of callus. We propose here the “cornucopia” hypothesis which states that the newly produced callus induced by the leaf-mining larvae provides virtually unending nourishment, which in turn allows flexible larval development time. This represents the first example of a leaf-miner manipulating plant development to its benefit, like a gall-inducer. We propose to name this life style “mine-galler”.

We screened for a gene that inhibits streptomycin production in Streptomyces griseus when it is introduced on a high-copy-number plasmid pIJ702, and obtained a plasmid pKM545. The introduction of pKM545 abolished streptomycin production on all media tested including YMP-sugar and Nutrient broth. S1 protection analysis demonstrated that the introduction of this plasmid downregulated the transcriptional activity of the promoter preceding strR, the pathway-specific transcriptional regulator for streptomycin biosynthesis. The 2.8-kb BamHI fragment cloned onto pKM545 contained two coding sequences SGR_5442 and 5443. These coding sequences and the two downstream ones (SGR_5444 and 5445) constituted a possible operon structure designated to be rspABCD (regulation of streptomycin production). RspB and RspC exhibited a marked similarity with an ATP-binding domain and a membrane-associating domain of an ABC-2 type transporter, respectively, suggesting that the Rsp proteins comprise a membrane exporter. The gene cluster consisting of the rsp operon and the upstream divergent small coding sequence (SGR_5441) was widely distributed to Streptomyces genome. An rspB mutant of S. griseus produced 3-fold streptomycin of the parental strain in YMP liquid medium. The evidence implies that the Rsp translocator is involved in the export of a substance that specifies the expression level of streptomycin biosynthesis genes in S. griseus.