Van den Ackerveken G F, Dunn R M, Cozijnsen A J, Vossen J P, Van den Broek H W, De Wit P J
Department of Phytopathology, Agricultural University Wageningen, The Netherlands.
Mol Gen Genet. 1994 May 10;243(3):277-85. doi: 10.1007/BF00301063.
The avirulence gene avr9 of the fungal tomato pathogen Cladosporium fulvum encodes a race-specific peptide elicitor that induces the hypersensitive response in tomato plants carrying the complementary resistance gene Cf9. The avr9 gene is not expressed under optimal growth conditions in vitro, but is highly expressed when the fungus grows inside the tomato leaf. In this paper we present evidence for the induction of avr9 gene expression in C. fulvum grown in vitro under conditions of nitrogen limitation. Only growth medium with very low amounts of nitrogen (nitrate, ammonium, glutamate or glutamine) induced the expression of avr9. Limitation of other macronutrients or the addition of plant factors did not induce the expression of avr9. The induced expression of avr9 is possibly mediated by a positive-acting nitrogen regulatory protein, homologous to the Neurospora crassa NIT2 protein, which induces the expression of many genes under conditions of nitrogen limitation. The avr9 promoter contains several putative NIT2 binding sites. The expression of avr9 during the infection process was explored cytologically using transformants of C. fulvum carrying an avr9 promoter-beta-glucuronidase reporter gene fusion. The possibility that expression of avr9 in C. fulvum growing in planta is caused by nitrogen limitation in the apoplast of the tomato leaf is discussed.
番茄病原菌fulvum的无毒基因avr9编码一种小种特异性肽激发子，可在携带互补抗性基因Cf9的番茄植株中诱导超敏反应。avr9基因在体外最佳生长条件下不表达，但当真菌在番茄叶片内生长时会高度表达。在本文中，我们提供了证据表明，在氮限制条件下体外培养的fulvum中，avr9基因表达被诱导。只有含极少量氮（硝酸盐、铵、谷氨酸或谷氨酰胺）的生长培养基能诱导avr9的表达。其他大量营养素的限制或植物因子的添加均不能诱导avr9的表达。avr9的诱导表达可能由一种正向作用的氮调节蛋白介导，该蛋白与粗糙脉孢菌NIT2蛋白同源，在氮限制条件下可诱导许多基因的表达。avr9启动子包含几个假定的NIT2结合位点。利用携带avr9启动子-葡糖醛酸糖苷酶报告基因融合体的fulvum转化体，从细胞学角度探究了感染过程中avr9的表达情况。本文还讨论了番茄叶片质外体中的氮限制导致植物体内生长的fulvum中avr9表达的可能性。
