Plants and microbes interact in a complex relationship that can have both harmful and beneficial impacts on both plant and microbial communities. Effectors, secreted microbial molecules that alter plant processes and facilitate colonization, are central to understanding the complicated interplay between plants and microbes. Effectors in Plant-Microbe Interactions unlocks the molecular basis of this important class of microbial molecules and describes their diverse and complex interactions with host plants. Effectors in Plant Microbe Interactions is divided into five sections that take stock of the current knowledge on effectors of plant-associated organisms. Coverage ranges from the impact of bacterial, fungal and oomycete effectors on plant immunity and high-throughput genomic analysis of effectors to the function and trafficking of these microbial molecules. The final section looks at effectors secreted by other eukaryotic microbes that are the focus of current and future research efforts. Written by leading international experts in plant-microbe interactions, Effectors in Plant Microbe Interactions, will be an essential volume for plant biologists, microbiologists, pathologists, and geneticists.

Plants and microbes interact in a complex relationship that can have both harmful and beneficial impacts on both plant and microbial communities. Effectors, secreted microbial molecules that alter plant processes and facilitate colonization, are central to understanding the complicated interplay between plants and microbes. Effectors in Plant-Microbe Interactions unlocks the molecular basis of this important class of microbial molecules and describes their diverse and complex interactions with host plants.

Plants interact with microbes in many different ways and on many different levels. The most obvious interaction results in plant disease, which can be a severe threat to the global food supply. Therefore, research strives to uncover the mechanisms of host plant invasion, learn about the weapons used by pathogenic microbes, and understand the defense strategies of the affected plants. On the other hand, many interactions with the plant are indeed beneficial for the plant, increasing its ability to recruit limiting nutrients from the soil, preventing the growth of more detrimental microbes, or making the plant more resistant to abiotic stresses. Plants also serve as habitats for microbes that may colonize apoplastic spaces within leaves, may live on plant surfaces, or may prosper in the immediate vicinity of plant organs (e.g., in the rhizosoil). In this book, one editorial, two review articles, and twelve original research articles highlight the newest research endeavors on plants interacting with beneficial microbes, having to cope with detrimental microbes, or hosting complete microbiomes. Together, these articles contribute to the knowledge essential for the development of strategies that will prepare our plants to withstand the increasingly harsh conditions they will be exposed to in the coming years of climate change.

Plant pathology is an applied science that deals with the nature, causes and control of plant diseases in agriculture and forestry. The vital role of plant pathology in attaining food security and food safety for the world cannot be overemphasized.

Secretomics describes the global study of proteins that are secreted by a cell, a tissue or an organism, and has recently emerged as a field for which interest is rapidly growing. The term secretome was first coined at the turn of the millennium and was defined to comprise not only the native secreted proteins released into the extracellular space but also the components of machineries for protein secretion. Two secretory pathways have been described in fungi: i) the canonical pathway through which proteins bearing a N-terminal peptide signal can traverse the endoplasmic reticulum and Golgi apparatus, and ii) the unconventional pathway for proteins lacking a peptide signal. Protein secretion systems are more diverse in bacteria, in which types I to VII pathways as well as Sec or two-arginine (Tat) pathways have been described. In oomycete species, effectors are mostly small proteins containing an N-terminal signal peptide for secretion and additional C-terminal motifs such as RXLRs and CRNs for host targeting. It has recently been shown that oomycetes exploit non-conventional secretion mechanisms to transfer certain proteins to the extracellular environment. Other non-classical secretion systems involved in plant-fugal interaction include extracellular vesicles (EVs, Figure 1 from Samuel et al 2016 Front. Plant Sci. 6:766.). The versatility of oomycetes, fungi and bacteria allows them to associate with plants in many ways depending on whether they are biotroph, hemibiotroph, necrotroph, or saprotroph. When interacting with a live organism, a microbe will invade its plant host and manipulate its metabolisms either detrimentally if it is a pathogen or beneficially if it is a symbiote. Deciphering secretomes became a crucial biological question when an increasing body of evidence indicated that secreted proteins were the main effectors initiating interactions, whether of pathogenic or symbiotic nature, between microbes and their plant hosts. Secretomics may help to contribute to the global food security and to the ecosystem sustainability by addressing issues in i) plant biosecurity, with the design of crops resistant to pathogens, ii) crop yield enhancement, for example driven by arbuscular mycorrhizal fungi helping plant hosts utilise phosphate from the soil hence increase biomass, and iii) renewable energy, through the identification of microbial enzymes able to augment the bio-conversion of plant lignocellulosic materials for the production of second generation biofuels that do not compete with food production. To this day, more than a hundred secretomics studies have been published on all taxa and the number of publications is increasing steadily. Secretory pathways have been described in various species of microbes and/or their plant hosts, yet the functions of proteins secreted outside the cell remain to be fully grasped. This Research Topic aims at discussing how secretomics can assist the scientists in gaining knowledge about the mechanisms underpinning plant-microbe interactions.

This book, divided into 13 chapters, explores recent discoveries in the area of molecular plant-microbe interactions. It focuses mainly on the mechanisms controlling plant disease resistance and the cross talk among the signalling pathways involved, and the strategies used by fungi and viruses to suppress these defences. Two chapters deal with the role of symbionts (such as the symbiotic actinobacteria and vesicular arbuscular mycorrhizal fungi) during their interactions with plants.

The use of microbial plant protection products is growing and their importance will strongly increase due to political and public pressure. World population is growing and the amount of food needed by 2050 will be double of what is produced now whereas the area of agricultural land is decreasing. We must increase crop yield in a sustainable way. Chemical plant growth promoters must be replaced by microbiological products. Also here, the use of microbial products is growing and their importance will strongly increase. A growing area of agricultural land is salinated. Global warming will increase this process. Plants growth is inhibited by salt or even made impossible and farmers tend to disuse the most salinated lands. Microbes have been very successfully used to alleviate salt stress of plants. Chemical pollution of land can make plant growth difficult and crops grown are often polluted and not suitable for consumption. Microbes have been used to degrade these chemical pollutants.