Abstract
Ubiquitination regulates many processes in plants, including immunity. The E3 ubiquitin ligase PUB17 is a positive regulator of programmed cell death (PCD) triggered by resistance proteins CF4/9 in tomato. Its role in immunity to the potato late blight pathogen, Phytophthora infestans, was investigated here. Silencing StPUB17 in potato by RNAi and NbPUB17 in Nicotiana benthamiana by virus-induced gene silencing (VIGS) each enhanced P. infestans leaf colonization. PAMP-triggered immunity (PTI) transcriptional responses activated by flg22, and CF4/Avr4-mediated PCD were attenuated by silencing PUB17. However, silencing PUB17 did not compromise PCD triggered by P. infestans PAMP INF1, or co-expression of R3a/AVR3a, demonstrating that not all PTI- and PCD-associated responses require PUB17. PUB17 localizes to the plant nucleus and especially in the nucleolus. Transient over-expression of a dominant-negative StPUB17(V314I,V316I) mutant, which retained nucleolar localization, suppressed CF4-mediated cell death and enhanced P. infestans colonization. Exclusion of the StPUB17(V314I,V316I) mutant from the nucleus abolished its dominant-negative activity, demonstrating that StPUB17 functions in the nucleus. PUB17 is a positive regulator of immunity to late blight that acts in the nucleus to promote specific PTI and PCD pathways.
Highlights
Plants are constantly exposed to pathogenic microorganisms such as bacteria, viruses, fungi, and oomycetes
The Arabidopsis AtPUB17 protein, and its functional orthologues in tomato and tobacco, SlPUB17 (SlACRE276) and NtPUB17 (NtACRE276), respectively, were shown to be functional ubiquitin E-potato 3 (E3) ligases that act as positive regulators of the hypersensitive response (HR) triggered by R/AVR recognition events: RPM1/AvrB and RPS4/AvrRPS4 in Arabidopsis and Cf4/AVR4 in tomato and tobacco (Yang et al, 2006)
The following new observations about PUB17 function have been made: (i) whereas PUB17 is required for CF4/ AVR4 cell death, as previously shown (Yang et al, 2006), it is not required for cell death triggered by the P. infestans PAMP INF1, indicating that PUB17 is not involved in all cell death events triggered following pathogen perception by cell surface receptors; (ii) silencing PUB17 orthologues in potato, N. benthamiana or tobacco failed to attenuate R3a/ AVR3aKI HR, indicating that PUB17 is not involved in all R/ AVR-mediated cell death events; (iii) remarkably, it is demonstrated that PUB17 promotes the activation of pattern-triggered immunity (PTI) triggered by the PAMP flg22; and (iv) critically, using a dominant-negative mutant of StPUB17, it is shown that its activity requires nuclear localization
Summary
Plants are constantly exposed to pathogenic microorganisms such as bacteria, viruses, fungi, and oomycetes. The effector AVR3a, which is recognized by potato R3a (Armstrong et al, 2005), is able to suppress PCD triggered by the P. infestans PAMP INF1 (Bos et al, 2006), and a range of other pathogen elicitors (Gilroy et al, 2011), by modifying the activity of the host ubiquitin E3 ligase CMPG1 (Bos et al, 2010). Preliminary work showed that StPUB17-RNAi potato plants exhibited enhanced susceptibility to P. infestans (Ni et al, 2010), suggesting it potentially plays a role in basal immunity to P. infestans It is unknown: (i) what aspects of plant immunity of potential relevance to the host-P. infestans interaction are positively regulated by PUB17; or (ii) where within the plant cell this E3 ligase acts to promote immune responses. These results implicate PUB17 involvement in the nucleus as a positive regulator of specific defence pathways activated by P. infestans infection
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