Vascular associated death1, a novel GRAM domain-containing protein, is a regulator of cell death and defense responses in vascular tissues

NtLRP1, a tobacco leucine-rich repeat gene with a possible role as a modulator of the hypersensitive response

Plant defense responses against pathogens often involve the restriction of the pathogen to its site of penetration achieved through the combined effects of the hypersensitive response (HR) and its tightly connected localized acquired resistance (LAR). The tobacco DD9-3 expressed sequence tag was previously isolated from a screen designed to isolate genes induced early during the HR, thus potentially involved in the induction/regulation of the HR or LAR. Translation of the open reading frame of DD9-3 revealed a leucine-rich repeat (LRR) domain highly homologous with the receptor domain of a receptor kinase, suggesting a potential function in signaling pathways. The full-length cDNA was cloned. It encodes a small (232 amino acids) LRR protein, designated Nicotiana tabacum leucine-rich protein 1 (NtLRP1), containing a signal peptide, four leucine zipper repeats, five LRR repeats, and a C-terminal domain rich in proline. NtLRP1 expression is induced early during the HR initiated by elicitins, Ralstonia solanacearum, or Tobacco mosaic virus. NtLRP1 coupled with the green fluorescent protein localizes to the endoplasmic reticulum (ER). Loss-of-function through virus-induced gene silencing or through RNA interference did not modify the elicitin-induced HR or LAR. Gain-of-function experiments through transient Agrobacterium tumefaciens-mediated NtLRP1 expression in tobacco leaves caused the suppression of the HR induced by 2 nM elicitin and delayed the HR when the elicitin was applied at higher concentrations. The results suggest that NtLRP1 acts as a modulator of the HR and that retention in the ER is essential for its function.

An essential role for salicylic acid in AtMYB30-mediated control of the hypersensitive cell death program in Arabidopsis

Salicylic acid (SA) plays a central role in resistance and defense induction in response to pathogen attack, but its role in the activation of the hypersensitive response (HR), a form of programmed cell death associated with resistance of plants, remains to be elucidated. AtMYB30, a R2R3-MYB transcriptional factor which acts as a positive regulator of the HR, is a good model for studying the role of SA in programmed cell death. Here, we demonstrate that AtMYB30 expression in response to an HR-inducing bacterial pathogen is dependent on SA accumulation, but NPR1-independent. Alterations of AtMYB30 expression (overexpression, depletion by antisense strategy, T-DNA insertion mutant) modulate SA levels and SA-associated gene expression. Additionally, mutants or transgenic lines altered in SA accumulation (nahG, sid1, sid2), but not those affected in SA signalling (npr1), abolish the accelerated cell death phenotype conferred by over-expression of AtMYB30. These results suggest that AtMYB30 is involved in an amplification loop or signalling cascade that modulates SA synthesis, which in turn modulates cell death.

The Oryza sativa no pollen (Osnop) gene plays a role in male gametophyte development and most likely encodes a C2-GRAM domain-containing protein

Phenotype screens of Ds insertional lines identified a male sterile Orysa sativa no pollen (Osnop) mutant with a pollen-less phenotype at the flowering stage. The mutant phenotype showed linkage to Ds insertion into Osnop gene region. This mutant contained a deletion of 65 kb chromosomal region at the site of Ds insertion containing 14 predicted genes. Out of these deleted genes, Delegen 5-7, 9-10 were redundant, as two or three copies were present with 100% homology in other regions of rice genome. RT-PCR analysis showed that Delegen 5-7 were expressed not only in wild type plants but also in the mutant plants. In addition to this, Delegen 8-10 transcripts could not be detected under normal growth conditions, and Delegen 12 was expressed only in roots, thus deletion of these genes may not affect the pollen development. Our data and analysis also ruled out the possibility of delegen 1-4, 11, and 13 as candidates contributing to the pollen-less phenotype. Further investigation showed that the delegen 14 was expressed only in late stage of pollen development with the highest expression at the stage of pollen release and germination by RT-PCR, Northern blotting, in situ hybridization, and promoter-GUS transgenic plants. Thus, the delegen 14 gene is the best candidate for Osnop, corresponding to the pollen-less phenotype in the mutant. Our data suggest that delegen 14 may play an important role during late stage of pollen development and its germination. Since the delegen 14 gene has both C(2) and GRAM domains, it can be assumed that this gene cross-links both calcium and phosphoinositide signaling pathways. This is the first report to suggest possible functions for this gene in plant development.

Stress induces the expression of AtNADK-1, a gene encoding a NAD(H) kinase in Arabidopsis thaliana

A novel Arabidopsis thaliana gene (AtNADK-1) was identified based on its response to radiation and oxidative stress. Levels of AtNADK-1 mRNA increase eight-fold following exposure to ionising radiation and are enhanced three-fold by treatment with hydrogen peroxide. The gene also appears to be differentially regulated during compatible and incompatible plant-pathogen interactions in response to Pseudomonas syringae pv. tomato. The full-length AtNADK-1 cDNA encodes a 58-kDa protein that shows high sequence homology to the recently defined family of NAD(H) kinases. Recombinant AtNADK-1 utilises ATP to phosphorylate both NAD and NADH, showing a two-fold preference for NADH. Using reverse genetics, we demonstrate that AtNADK-1 deficient plants display enhanced sensitivity to gamma irradiation and to paraquat-induced oxidative stress. Our results indicate that this novel NAD(H) kinase may contribute to the maintenance of redox status in Arabidopsis thaliana.