Epidermal anthocyanin production as an indicator of bacterial blight resistance in cotton

Among near-isogenic upland cotton ( Gossypium hirsutum L.) lines, remarkable variation exists in the ability of the leaves to produce a red accessory pigment anthocyanin when challenged by selected isogenic races of the bacterial pathogen, Xanthomonas campestris pv. malvacearum (Smith) Dye. Inoculated areas that developed visible water-soaking that progressed beyond pinpoint-sized dots, a sign of susceptibility, very rarely produced anthocyanin, and on the couple of occasions in which anthocyanin was seen, it was borderline. Inoculated areas that exhibited resistance by developing little or no water-soaking produced various, but obvious, amounts of anthocyanin, with the exception of the cotton line whose only resistance gene is B In . These results indicate that anthocyanin production by cotton leaves in response to an unsuccessful challenge by the bacterium is a bacterial blight resistance response, but is not essential for resistance. The epidermis of the adaxial surface of the leaves that received direct illumination was the tissue involved in anthocyanin production. The subsidiary cells of the stomatal complex were the initial participants in anthocyanin accumulation. A protective role of anthocyanin-containing cells against damage by infection-related reactive oxygen species and light-activated phytoalexins to the healthy tissues surrounding infection sites is suggested.