**5. RNA silencing and plant defence: an outlook**

Altogether, the strategic outline of Arabidopsis and maize antifungal defence against *Colletotrichum* spp. points towards the concept that sRNAs are acting as fine-tuners mediat‐ ing the balance of multiple genetic and metabolomic-defence layers. The sRNA-orchestrated fine-tuning of defensive layouts may provide a genetic flexibility allowing rapid and efficient adaptation of immune pathways. The question whether sRNA pathways are indispensable and pivotal antifungal-defence regulators remains debatable. Despite various studies showing the altered susceptibility of sRNA mutants, the general trend is that the outcome of sRNAmediated defence strongly depends on a specific pathosystem. This chapter adds further high‐ lights to this picture by showing that no miRNAs targeting classical defence pathways are de‐regulated upon *C. graminicola* infection in maize, and Arabidopsis sRNA mutants under fungal attack appear to have altered metabolomic profile compared to the wild-type situation.

Nevertheless, considering the fact that sRNA pathways are also involved in setting up proper abiotic stress responses, it might represent a multi-valuable biotechnological approach to generate crops that are more efficient and variant in expressing their sRNA repertoire. Over the past years, a transgene-based approach where pathogen-targeting sRNAs are expressed in host species was repeatedly confirmed to efficiently control fungal diseases. This host-induced gene-silencing (HIGS) approach was successfully applied to a broader range of host-pathogen systems, thus bearing a valuable industrial potential. Significant drawbacks with this technology are the restrictive acceptance of genetically modified crops, and the yet elusive question of how fast pathogens evolve tolerance or resistance. For instance, *F. graminearum* sRNA mutants are showing normal virulence in wheat infection assays [51]. A yet elusive question is the role of plant endogenous sRNAs in targeting the genes of their fungal parasites. *B. cinerea* has been demonstrated to hijack plant genes using sRNA effectors [52]; thus it could be possible that plant sRNA effectors are able to infiltrate fungal cells to act as antimicrobial molecules.

Recent studies from two different research groups demonstrate fungal control by exogenous application of sRNAs to *F. graminearum* [53] and *B. cinerea* [54]. The so‐called spray‐induced gene silencing (SIGS) might provide novel biotechnological opportunities to control fungal diseases. Although the data from both studies are promising, it remains elusive how efficient sRNAs are compared to classical biologicals, and how broadly this technology can be applied. For instance, Botrytis is one of the few fungal species known to require a functional sRNA machinery for proper infection, hence possibly representing a special situation in sRNAmediated plant-pathogen interaction. Moreover, exogenous control by sRNAs was efficient in controlling fungal growth on vegetables and fruits, and not demonstrated on leaves [54], suggesting efficacy only in a very specific infection condition. Altogether, it remains to be elu‐ cidated how efficient SIGS could work in field conditions, and in particular also the applica‐ tion spectrum of this technology. So far, this new technology lacks confirmation by additional independent studies to allow fully evaluating its industrial potential.

Prospective investigations will help in further elucidating of the full potential of sRNA‐ mediated antifungal defence. While the data presented here and in recent studies suggest that sRNAs are subtle players in the concert of mounted antifungal defence, and new approaches using exogenously applied sRNAs are promising, there remains challenging basic research to be completed first in order to truly understand sRNA trafficking and signalling in plant‐pathogen interactions.
