Plant viruses contain single-stranded or double-stranded RNA or DNA genomes and vary substantially in their genome structure and organization. Moreover, limited viral genome sizes and coding capacities have resulted in evolution of multifunctional proteins that are involved in different steps in the virus life cycle, including replication, movement, encapsidation and transmission. On the other hand, as obligate intracellular parasites, plant viruses absolutely depend on the host cell machinery to multiply, move throughout the plant and spread to susceptible hosts. During infection, viruses consume a substantial amount of host resources; subsequently, disease symptoms develop as a consequence of disruptions of the cellular machinery required for plant physiology and natural growth, and these disruptions eventually result in developmental abnormalities and other phenotypic manifestations. Viruses can be critical players in pathogenesis through direct or indirect interactions. However, in some plant species or varieties, virus-encoded proteins can sometimes act as determinants in plant defense responses and as host-controlled pawns to elicit extreme resistance (ER).

Over the past three decades, our knowledge about plant virus resistance genes and corresponding Avr factors has advanced dramatically. Compared to other pathogens (i.e., bacterial, fungal and oomycetes), viral Avr gene identification is relatively easy due to small virus genome sizes and limited numbers of gene products. Most virus Avr genes have been matched with an R-gene type NB-LRR receptor (Table1). In this review, we intend to provide a brief overview of virus-encoded proteins and their roles in triggering plant resistance, and we also summarize current progress in understanding plant resistance against virus Avr genes. Moreover, we present applications of Avr gene-mediated phenotyping in R gene identification and screening of segregating populations during breeding processes^[1].

This entry is adapted from the peer-reviewed paper 10.3390/v13040688