USE OF ARABIDOPSIS RECOMBINANT INBRED LINES REVEALS A MONOGENIC AND ANOVEL DIGENIC RESISTANCE MECHANISM TO XANTHOMONAS-CAMPESTRIS PV CAMPESTRIS

Infiltration of the Arabidopsis thaliana accession Landsberg erecta (L er) with Xanthomonas campestris pv campestris isolate 2D520 results in extensive necrosis and limited chlorosis within 5-6 days post-inocula tion (d.p.i.), which can lead to systemic necrosis within 23 d.p.i. In contrast, the accession Columbia (Col) remains asymptomatic after inf iltration. Although both accessions support bacterial growth, 5-28-fol d more bacteria are present in Ler than in Col leaf tissue. Inheritanc e studies indicate that three independent, dominant or partially domin ant, nuclear genes condition resistance to X. c. campestris 2D520. The major gene, termed RXC2, conditions monogenic resistance to X. c. cam pestris and was mapped to a 5.5 cM interval of chromosome V. Segregati on data indicate that the locus RXC3 in conjunction with RXC4 confers digenic resistance to X. c. campestris. The combined action of RXC3 an d RXC4 is correlated with a suppression of in planta bacterial levels and a suppression of symptoms relative to Ler. The RXC3 + RXC4-mediate d resistance is novel in that although the Col allele of RXC4 contribu tes positively to resistance, it is the Ler and not the Col allele of RXC3 that contributes positively to resistance. RXC3 was mapped to the bottom arm of chromosome II in a 2.7 cM interval within the major rec ognition gene complex MRC-J, a cluster of genes involved in disease re sistance. RXC4 was mapped to a 12 cM interval on chromosome II that al so contains RXC1, a gene conferring tolerance to X. c. campestris.