RESISTANCE AND SUSCEPTIBILITY OF CARROT ROOTS TO CROWN-GALL TUMOR FORMATION

Abstract

Disks of young secondary phloem were cut from radial cylinders of carrot roots, weak reactor disks (WR) from roots forming less than 20 mg of tumor tissue in 15 days and strong reactor disks (SR) from roots forming more than 80 mg of tumor tissue in 15 days. Strain B6 of Agrobacterium tumefaciens was used in suspension to inoculate the cambial-adjacent surface of the disks. One tenth ml of sterilized carrot wound juice from SR or WR carrots was applied to the cambial-adjacent surfaces of both freshly-cut and healed known SR and WR carrot disks, and after 2-3 hrs. the disks were inoculated. The juice from SR carrots had no effect on tumor formation on SR disks but shortened the time of tumor appearance and increased final tumor weight on WR disks. Juice from WR carrots had no effect on SR or WR disks. More tumor formation was produced on healed disks by SR juice than by WR juice. The resistance of WR carrots was believed due to the absence in their cells of a tumor-forming substance rather than to an inhibitor. At dilutions with water below 50% the juice factor from SR carrots had little or no effect on tumor formation by WR tissues. It is present in the wound juices from storage organs of several other plants. Various physical and chemical properties of the wound juice suggest that it may be a large, non-proteinaceous molecule. Cambial-adjacent secondary xylem tissues of SR roots had the wound-juice factor needed for tumor formation on WR phloem disks and no inhibitor for tumor formation on SR phloem disks. Both the xylem and the older secondary phloem (adjacent to the pericycle and periderm) were refractory to tumor formation; in the older phloem an inhibitor was found which masked the wound-juice factor and suppressed tumor formation on cambial-adjacent phloem from the same root. The wound-juice factor is believed to play a role in initiating tumor cells, but is not required during their growth and duplication.