Secretion of the HrpN harpin via the type III secretion system may promote this necrotroph-associated form of disease development [49]. The disease caused byPectobacterium carotovorumonPhyscomitrella patensclosely resembles that

caused by the necrotrophic see more fungusBotrytis cinerea[75]. The pectolytic enzymes in these pathogens could be described by “”GO: 0052042 positive regulation by symbiont of host programmed cell death”" (Figure2) as well as “”GO: 0052011 catabolism by symbiont of host cell wall pectin”". Hemibiotrophic fungal and oomycete pathogens Hemibiotrophic plant pathogens initially suppress or avoid triggering PCD during the biotrophic phase of infection, but then actively promote cell death during the GW3965 chemical structure transition to necrotrophy [33]. The mechanism(s) underlying the switch QNZ chemical structure from biotrophy to necrotrophy remain largely unknown [2]. InP. sojae, expression of the protein

toxin PsojNIP is associated with the transition to necrotrophy, and has been hypothesized to be responsible for the switch [33]. In wheat infected with the host-specific fungal pathogenMycosphaerella graminicola, disease symptoms often do not appear for several weeks. Once the necrotrophic stage begins, however, the host exhibits PCD-like characteristics, along with increased cell membrane leakage and apoplastic metabolite levels, which correlate with increased fungal growth, membrane transport, and metabolism [76]. A similar situation exists inFusarium graminearum, which lives biotrophically before switching to necrotrophy; following exposure toF. graminearum-derived trichothecene mycotoxins, multiple 2-hydroxyphytanoyl-CoA lyase barley transcripts were

detected including a PCD-related pirin [77], which may signify pathogen-triggered PCD. The effector Avr3a ofPhytophthora infestans, expressed during early infection of potato, can suppress the PCD triggered by the MAMP elicitin [78], i.e. “”GO: 0034054 negative regulation by symbiont of host defense-related programmed cell death”" (Figure2). Similarly, several effectors fromP. sojae, including Avr1b, could suppress BAX-triggered PCD, and were hypothesized to have a physiological role of suppressing defense-associated PCD [79].P. infestansAvr3a andP. sojaeAvr1b also can be described with “”GO: 0034055 positive regulation by symbiont of host defense-related programmed cell death”" (Figure2) as they trigger the host HR when the host resistance genesR3a orRps1b, respectively, are present [78,79], which underscores the complex roles of effectors and the need for careful annotation of them.