Change of fate: How do gall-inducing insects redirect plant developmental programs to create new organs?

Many insects elicit development of unique organs in their host plants called galls while feeding or ovipositing. Galls are organ-like entities that protect against biotic and abiotic stresses and provide a specialized nutrition supply. The signals used by gall-forming insects to elicit and shape galls are unknown, but phytohormones, peptides, small RNAs and arabinogalactan proteins are leading candidates. These four signal categories may work together to elicit galls and guide their development, but evidence of their transfer from insect to plant is incomplete. Galls vary in complexity from simple swellings to elaborate organs and their morphologies are tightly linked to the insect's identity. The attacked host plant tissue also shapes the gall's development, especially via physical constraint. The interaction between insect signaling and tissue constraints on development makes insect behavior important for establishing and shaping the gall. Because plant tissues are heterogeneous mosaics of cells with differing transcriptomes that change with time, galling insects must locate specific plant cells at specific points in their development to initiate their characteristic galls. Experimental evidence of signal transfer and function in planta, time series records of cell behavior during gall development, and single-cell plant tissue transcriptome maps would provide a fuller understanding of the galling phenomenon.