Subcellular localization of endogenous IAA during poplar leaf rhizogenesis revealed by in situ immunocytochemistry

Poplar 741 Populus alba × (P. davidiana + P. simonii) × P. tomentosa] leaves were rooted within 8 days when cultured on 1/2 MS medium. The subcellular localization of endogenous indole-3-acetic acid (IAA) in the rhizogenesis was investigated, using an immunocytochemical approach. The results of IAA subcellular localization revealed organelle-specific distribution. Three days after root induction, IAA in vascular cambium cells of the basal region of the petiole was distributed mainly in the plasma membrane, endoplasmic reticulum (ER), and nucleus, with a lesser amount in the cytoplasm. In phloem of the basal region of the petiole, IAA was detected in the plasma membrane and ER of the companion cell and in the plasma membrane of the sieve element. In xylem of the basal region of the petiole, no IAA gold particles were labeled. In mesophyll cells IAA was distributed in the chloroplast starch grains before root induction, and the amount in the chloroplast starch grains increased after 3 days after root induction. This suggests that the plasma membrane and nucleus of cambium cells may be the target sites where IAA performs its physiological activities during poplar leaf rhizogenesis. IAA polar transport from lamina mesophyll to the basal region of the petiole during rhizogenesis is mediated by phloem. The starch grains of mesophyll chloroplasts appeared to accumulate IAA and may be a source of IAA during poplar leaf rhizogenesis. Novel and direct evidence regarding the function of IAA during rhizogenesis is provided in this study.