Factors that Mediate Root Initiation in Plants.

When the leaf explants of Medicago truncatula are placed on auxin medium they form root initials within a week. Our studies have shown that the cells associated with the veins are stimulated into division by the added auxin, forming what we called vein-derived cells (VDCs) that grow out into callus and it is from these cells that the root meristems are formed. The way auxin interacts with other hormones is a key factor in determining the stem cell fate. In Medicago truncatula if cytokinin is added with auxin then root production is blocked and embryos are produced along with many more vascular tissues are produced throughout the callus. However, if the explants are exposed to auxin for seven days before the addition of cytokinin then only roots will form and not embryos. Thus, the commitment stage once induced is irreversible. Our study suggested that a pool of procambial or stem cells exist in the vascular tissues of the leaf and that these can be stimulated into pluripotency by auxin addition and the number of roots formed from the same leaf explants could be enhanced by added oxidized or reduced glutathione or an alteration of the ethylene sensitivity.