Distinct roles of the pepper hypersensitive induced reaction protein gene CaHIR1 in disease and osmotic stress, as determined by comparative transcriptome and proteome analyses

A Capsicum annuum hypersensitive induced reaction protein1 (CaHIR1) was recently proposed as a positive regulator of hypersensitive cell death in plants. Overexpression of CaHIR1 in transgenic Arabidopsis plants conferred enhanced resistance against the hemi-biotrophic Pseudomonas syringae pv. tomato (Pst) and the biotrophic Hyaloperonospora parasitica. Infection by avirulent Pseudomonas strains carrying avrRpm1 or avrRpt2 caused enhanced resistance responses in transgenic plants, suggesting that CaHIR1 is involved in basal disease resistance in a race-nonspecific manner. H. parasitica exhibited low levels of asexual sporulation on CaHIR1 seedlings. In contrast, transgenic plants were susceptible not only to the necrotrophic fungal pathogen Botrytis cinerea but were also sensitive to osmotic stress caused by high salinity and drought. To identify proteins whose expression was altered by CaHIR1 overexpression in Arabidopsis leaves, a quantitative comparative proteome analysis using two-dimensional gel electrophoresis coupled with mass spectrometry was performed. Of about 400 soluble proteins, 11 proteins involved in several metabolic pathways were up- or down-regulated by CaHIR1 overexpression. Genes encoding glycine decarboxylase (At2g35370) and an unidentified protein (At2g03440), which were strongly upregulated in CaHIR1-overexpressing Arabidopsis, were also differentially induced at the transcriptional level by Pst infection. Arabidopsis carbonic anhydrase (At3g01500), highly similar to tobacco salicylic acid-binding protein 3, was up-regulated by CaHIR1 overexpression. The activity of an anti-oxidant enzyme, cooper/zinc superoxide dismutase (At2g28190), was also attenuated in transgenic Arabidopsis by CaHIR1 overexpression. Together, these results suggest that CaHIR1 overexpression in Arabidopsis mediates plant responses to biotrophic, hemi-biotrophic and necrotrophic pathogens, as well as to osmotic stress in different ways.