A Tobacco S-like RNase Inhibits Hyphal Elongation of Plant Pathogens

Abstract

Ribonuclease (RNase) NE gene expression is induced in tobacco leaves in response to Phytophthora parasitica. Using antibodies directed against RNase NE, we demonstrate that RNase NE is extracellular at the early steps of the interaction, while the fungal tip growth is initiated in the apoplastic compartment. After production in Pichia pastoris and biochemical purification, we show that the S-like RNase NE inhibits hyphal growth from P. parasitica zoospores and from Fusarium oxysporum conidia in vitro. Conversion into an enzymatically inactive form after mutagenesis of the active site-histidine 97 residue to phenylalanine leads to the suppression of this activity, suggesting that RNase NE inhibits the elongation of germ tubes by degradation of microbial RNAs. Exogenous application of RNase NE in the extracellular space of leaves inhibits the development of P. parasitica. Based on its induction by inoculation, its localization, and its activity against two plant pathogens, we propose that RNase NE participates in tobacco defense mechanisms by a direct action on hyphal development in the extracellular space. The RNase activity-dependent antimicrobial activity of the S-like RNase NE shares similarities with the only other biological activity demonstrated for plant RNases, the inhibition of elongation of pollen tubes by the S-RNase in gametophytic self-incompatibility, suggesting a functional link between self and nonself interactions in plants.

Studies of self-incompatibility in wild tomatoes: I. S-allele diversity in Solanum chilense Dun. (Solanaceae)

“…Such genes likely originate as paralogous copies of S-alleles. Many more-distantly related genes in the same RNase superfamily exist, but with much lower amino-acid identities, and they are expressed in other plant tissues (for example, Hugot et al, 2002). We found that the putative S-alleles we sequenced co-segregated with S-allele phenotypes by performing a series of half-compatible crosses to test the association between sequence and functional phenotype.…”

A coupled yeast signal sequence trap and transient plant expression strategy to identify genes encoding secreted proteins from peach pistils

“…The function and mode of action of the non-S-RNase PA1 is still unknown, although there are clearly some similarities with the S-RNases, in that they are pistil-specific basic RNases (Yamane et al, 2003). It is known that some acidic non-S-RNases are induced by wounding and pathogen infection (Ye and Droste, 1996;Galiana et al, 1997;Kariu et al, 1998;Hugot et al, 2002;LeBrasseur et al, 2002); however, PA1 mRNA levels are neither induced by wounding nor by salicylic acid application (data not shown). Since PA1 is a pistil-specific protein, the suggestion is that its function is more likely to be related to reproduction, as with S-RNases (Murfett et al, 1996;Hancock et al, 2003), rather than part of a wounding or disease response.…”

RNase T2 genes from rice and the evolution of secretory ribonucleases in plants

“…Tobacco RNase NE expression is induced in response to Phytophthora parasitica, and purified RNase NE can inhibit hyphal elongation of this pathogen (Hugot et al 2002). Other Nicotiana genes, RNase NW, and RNase Rk1, are induced in response to tobacco mosaic virus and cucumber mosaic virus infections, respectively (Kurata et al 2002;Ohno and Ehara 2005).…”