The bean pathogen Pseudomonas syringae pv. syringae B728a expresses homologs of the type III effectors AvrPto and AvrPtoB, either of which can trigger resistance in tomato cultivars expressing Pto and Prf genes. We found that strain B728a also elicits nonhost resistance in tomato cultivars VFNT Cherry and Moneymaker that lack Pto but express other members of the Pto family (e.g., SlFen and SlPtoC). Here, we show that the AvrPtoB homolog from B728a, termed AvrPtoBB728a (also known as HopAB1), is recognized by 'VFNT Cherry' and 'Moneymaker' when the effector is expressed in P. syringae pv. syringae 61, a strain lacking the avrPto or avrPtoB homolog. Using a gene-silencing approach, this recognition was shown to involve one or more Pto family members and Prf. AvrPtoBB728a interacted with SlFen, SlPtoC, and SlPtoD, in addition to Pto, in a yeast two-hybrid assay. In P. syringae pv. tomato DC3000, the C-terminal domain of AvrPtoB is an E3 ubiquitin ligase that ubiquitinates Fen, causing its degradation and leading to disease susceptibility. Although the C-terminal domain of AvrPtoBB728a shares 69% amino acid identity with that of AvrPtoB, we found that it has greatly reduced E3 ligase activity and is unable to ubiquitinate Fen in an in vitro ubiquitination assay. Thus, the nonhost resistance of 'VFNT Cherry' and 'Moneymaker' to B728a appears to be due to recognition of AvrPtoBB728 as a result of the effector's reduced E3 ligase activity, which prevents it from facilitating degradation of a Pto family member. We speculate that the primary plant host of B728a lacks a Fen-like protein and that, therefore, the E3 ligase of AvrPtoBB728 was unnecessary for pathogenicity and has diverged and become ineffective.

译文

:豆病原体丁香假单胞菌PV。丁香香脂素B728a表达III型效应子AvrPto和AvrPtoB的同源物,它们两者均可触发表达Pto和Prf基因的番茄品种的抗性。我们发现菌株B728a在缺乏Pto但表达Pto家族其他成员的番茄品种VFNT Cherry和Moneymaker中也引起了非寄主抗性。在这里,我们显示了当在丁香假单胞菌pv中表达效应子时,来自B728a的AvrPtoB同源物,称为AvrPtoBB728a(也称为HopAB1),被“ VFNT Cherry”和“ Moneymaker”识别。丁香61,缺乏avrPto或avrPtoB同源物的菌株。使用基因沉默方法,该识别显示涉及一个或多个Pto家族成员和Prf。在酵母双杂交试验中,AvrPtoBB728a除Pto外还与SlFen,SlPtoC和SlPtoD相互作用。在丁香假单胞菌pv。番茄DC3000,AvrPtoB的C末端域是E3泛素连接酶,泛素化Fen,导致其降解并导致疾病易感性。尽管AvrPtoBB728a的C末端结构域与AvrPtoB具有69%的氨基酸同一性,但我们发现它具有大大降低的E3连接酶活性,并且在体外泛素化测定中无法泛素化Fen。因此,“ VFNT Cherry”和“ Moneymaker”对B728a的非宿主抗药性似乎是由于效应子E3连接酶活性降低导致对AvrPtoBB728的识别,从而阻止了它促进Pto家族成员的降解。我们推测B728a的主要植物宿主缺乏Fen样蛋白,因此,AvrPtoBB728的E3连接酶对于致病性而言是不必要的,并且已经分化并且变得无效。

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