A role for inositol hexakisphosphate in the maintenance of basal resistance to plant pathogens

Phytic acid (myo-inositol hexakisphosphate, InsP(6)) is an important phosphate store and signal molecule in plants. However, low-phytate plants are being developed to minimize the negative health effects of dietary InsP(6) and pollution caused by undigested InsP(6) in animal waste. InsP(6) levels were diminished in transgenic potato plants constitutively expressing an antisense gene sequence for myo-inositol 3-phosphate synthase (IPS, catalysing the first step in InsP(6) biosynthesis) or Escherichia coli polyphosphate kinase. These plants were less resistant to the avirulent pathogen potato virus Y and the virulent pathogen tobacco mosaic virus (TMV). In Arabidopsis thaliana, mutation of the gene for the enzyme catalysing the final step of InsP(6) biosynthesis (InsP(5) 2-kinase) also diminished InsP(6) levels and enhanced susceptibility to TMV and to virulent and avirulent strains of the bacterial pathogen Pseudomonas syringae. Arabidopsis thaliana has three IPS genes (AtIPS1-3). Mutant atips2 plants were depleted in InsP(6) and were hypersusceptible to TMV, turnip mosaic virus, cucumber mosaic virus and cauliflower mosaic virus as well as to the fungus Botrytis cinerea and to P. syringae. Mutant atips2 and atipk1 plants were as hypersusceptible to infection as plants unable to accumulate salicylic acid (SA) but their increased susceptibility was not due to reduced levels of SA. In contrast, mutant atips1 plants, which were also depleted in InsP(6), were not compromised in resistance to pathogens, suggesting that a specific pool of InsP(6) regulates defence against phytopathogens.