cDNAs Generated from Individual Epidermal Cells Reveal that Differential Gene Expression Predicting Subsequent Resistance or Susceptibility to Rust Fungal Infection Occurs Prior to the Fungus Entering the Cell Lumen

As the cowpea rust fungus penetrates the wall of a cowpea epidermal cell, resistant and susceptible plants exhibit different ultrastructural and cytochemical changes within the epidermal protoplast. To examine plant gene expression at this stage of infection, cytoplasm was extracted from individual inoculated or uninoculated epidermal cells before the fungal penetration peg reached the cell lumen. Initial differential colony hybridization screening of an expressed sequence tag library constructed from globally amplified cDNAs generated from the inoculated resistant cells resulted in 80 clones (out of 835) with a differential hybridization pattern. Further slot-blot screening and screening of the amplified cDNAs generated from inoculated or uninoculated, resistant or susceptible cells revealed 28 separate genes, mostly with no matching sequences in the databases, that were up-regulated in response to the growth of the fungus through the wall of resistant or susceptible cells. Five genes, including those coding for betaand alpha-tubulin, were found to be down-regulated specifically in inoculated, susceptible cells, and five were specifically up-regulated in inoculated, resistant cells, including a PR-10 homolog and a phenylalanine ammonia-lyase gene. Probing the amplified cDNAs from each cell type for the expression of cell death-related genes revealed that an LLS1 homolog (vuLLS1), cloned from cowpea, was up-regulated by infection in both resistant and susceptible cells and that a homolog of HSR203J was differentially up-regulated in resistant cells. These data show that changes in gene expression predicting the subsequent expression of susceptibility or hypersensitive resistance to fungal infection occur prior to the fungus entering the cell lumen.