The Role Of Ethylene Biosynthesis And Signaling In Resistance To Fungal Diseases In Rice
Open Access
- Author:
- Helliwell, Emily Elizabeth
- Graduate Program:
- Plant Pathology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 18, 2012
- Committee Members:
- Yinong Yang, Dissertation Advisor/Co-Advisor
Kathleen Marie Brown, Committee Member
Dawn S Luthe, Committee Member
Timothy W Mcnellis, Committee Member
Seogchan Kang, Committee Member - Keywords:
- Oryza sativa
rice
ethylene
rice blast
ethylene biosynthesis
ethylene signaling - Abstract:
- Plants have evolved complex defense signaling pathways to respond to a wide variety of pathogens. Results from recent studies using rice (Oryza sativa) have suggested that the phytohormone ethylene plays a role in resistance to the rice blast pathogen, Magnaporthe oryzae. The rate-limiting step of the ethylene biosynthetic pathway is the conversion of S-adenosylmethionine (AdoMet) to 1-aminocyclopropane-1-carboxylic acid (ACC), catalyzed by ACC synthase (ACS). Of the six ACS genes in rice, OsACS1 and OsACS2 are induced within 24 hours of inoculation by M. oryzae. This induction occurs simultaneous with an increase in ethylene production that is noticeable 12 hours post inoculation. In this study, we have used a transgenic approach to further determine the role of ethylene biosynthesis and signaling in resistance to M. oryzae. In the first study, we used the RNA interference (RNAi) method, to generate transgenic rice lines with suppressed expression of the single M. oryzae inducible-OsACS2 gene. The OsACS2-RI lines exhibited reduced basal levels of ethylene, and reduced expression of ethylene signaling genes. A series of elicitation and inoculation studies were performed with the use of these ethylene-deficient OsACS2-RI lines, in conjunction with previously developed ethylene signaling mutants OsEIN2a+b-RI and OsEIL1-RI lines. We found that both ethylene-deficient and ethylene-insensitive lines showed an attenuation of mechanisms involved in basal resistance, including decreased expression of a chitin-binding receptor, lower and/or delayed expression of genes involved in basal resistance and phytoalexin biosynthesis, a reduction in early HR-like cell death, reduced callose deposition, and increased susceptibility to multiple isolates of M. oryzae. These results suggest that both ethylene production and signaling are involved in basal resistance. A second study involved generation of transgenic lines with inducible overexpression of OsACS2 (OsACS2-OX). In contrast to the ethylene-deficient and insensitive lines, the OsACS2-OX lines showed increased resistance to both a moderately and a iv highly virulent isolate of M. oryzae, as well as R. solani. These results suggest that increasing endogenous levels of ethylene may enhance host resistance to a broad spectrum of pathogens and pathogen races. A third study performed using a negative regulator of ethylene production, OsEOL1 (Ethylene-overproducer like 1) revealed more information about regulation in ethylene production involved in ethylene-mediated resistance. OsEOL1 is a BTB (broad-spectrum/tamtrac/bric a brac) domain-containing protein, and a putative E3 ligase component. Expression of OsEOL1 increased at 48 hours post-inoculation, occurring simultaneously to a decrease in ethylene production in rice. Transgenic rice lines with stress-inducible overexpression of OsEOL1 showed increased resistance to spray-inoculated, but not punch-inoculated isolates of M. oryzae. This suggests that OsEOL1 is a positive regulator of resistance to M. oryzae, depending on the time of induction. The results of these experiments suggest that ethylene production and signaling are necessary components of basal resistance, however, ethylene-mediated resistance is dependent on the timing, and dose of ethylene production.