# Fusing _Fusarium_ Pathogens in the Era of Climate Change: A Conceptual Approach
## 4 The Phylosphere  
Plants harbor different species- and cultivar-specific microbial communities that grow as epiphytes and endophytes in the rhizosphere and phyllosphere [43]. The phyllosphere microbiome is composed of various microorganisms but bacteria are the most abundant cellular organisms in the phyllosphere community, and the most efficient colonizers are present in numbers between \(10^{6}\) and \(10^{7}\) cells cm\({}^{-2}\) of leaf tissue [43; 44; 45; 46]. The bacteria in phyllosphere are often vertically transmitted as endophytes and they form patchy assemblages on plant surface called biofilms [39; 47; 48; 49] or microbial consortia [39; 45; 46]. While the rhizosphere and endo-rhizosphere have been thoroughly studied for centuries, less is known about the drivers that influence the phylosphere, i.e., any aerial part of the plant, such as stems, leaves, leaf ears, or flowers [50; 51]. This is a significant limitation as phyllosphere microbiome is of critical importance for dispersal of _Fusarium_ pathogens. Although _Fusarium_ spp. can survive several years on plant debris, even if located deep in the soil, the pathogens themselves, however, develop only on the upper parts of living plants [20]. Hence, the review focuses on the phyllosphere, which has a large potential for the design of beneficial microbial biofilm layers [52; 53]. The phyllosphere is a highly nutrient-limited habitat compared to the rhizosphere [50; 51; 54]. Phyllosphere is characterized by permanently changing temperature and humidity regimes, whereas the phyllospheric environmental conditions are predicted to become aggravated in warming climates [50; 51; 54]. However, in cooler climates such as boreal and temperate biomes, current global warming patterns generally stimulate biofilm formation primarily through the increased winter survival [3]. Thus, the expected changes in cool biomes can lead to a higher phyllosphere biofilm formation [50; 51; 54]. Biofilms are considered as micronuclehes which allow the bacteria to operate as a functional unit and enable them to accomplish tasks that are not possible at their planktonic stage. Biofilms consist of cells and matrices where complex exopolysaccharides (EPS) and proteins are major components [55; 56; 57; 58]. Biofilm environment provides an important means for survival of phyllospheric microobia in natural ecosystems [55; 56; 57; 58]. Several studies have revealed the role of quorum sensing in biofilm formation [59; 60; 61] where microbial cells use chemical signals for coordination prior to biofilm formation. Quorum sensing permits the microbes to determine the environmental conditions of the actual population [59; 61; 62]. N-acyl-homoserine lactones together with oligopeptide autoinducers are the main diffusible signals employed in bacterial communication (Figure 1) [59; 61; 62]. Biofilm formation is therefore a dynamic balanced process to promote planktonic cells attachment until microcolony formation allowing cell migration and new colony formation (Figure 2) [59; 61; 62]. Apart from quorum sensing, the microorganisms communicate with each other, with the plant, and other organisms through emission of volatile organic compounds (VOC) [63]. VOCs are of great importance as signaling molecules and in the plant chemical defense by either directly suppressing pathogens [64] or serving as volatile cues eliciting defense responses in neighboring not yet infected leaves [65; 66]. Biological control in the phyllosphere can be accomplished through direct antagonism via production of antibiotics or via the induction of plant resistance [67]. The advantage of biological control pathways that involve the plant own immunity is that these defense mechanisms can stay active even after the infecting agent is no longer present and therefore can potentially provide a longer-term benefit [23]. A large set of plant compounds, that are activated by phyllospheric microorganisms can initiate transcriptional reprogramming of the infected plant tissue [23; 61].