Abstract
The mode of action of 1-naphthylphthalamic acid (NPA) to induce conspicuous local stem swelling in the area of its application to the growing internode in intact Bryophyllum calycinum was studied based on the aspects of histological observation and comprehensive analyses of plant hormones. Histological analyses revealed that NPA induced an increase in cell size and numerous cell divisions in the cortex and pith, respectively, compared to untreated stem. In the area of NPA application, vascular tissues had significantly wider cambial zones consisting of 5–6 cell layers, whereas phloem and xylem seemed not to be affected. This indicates that stem swelling in the area of NPA application is caused by stimulation of cell division and cell enlargement mainly in the cambial zone, cortex, and pith. Comprehensive analyses of plant hormones revealed that NPA substantially increased endogenous levels of indole-3-acetic acid (IAA) in the swelling area. NPA also increased endogenous levels of cytokinins, jasmonic acid, and its precursor, 12-oxo-phytodienoic acid, but did not increase abscisic acid and gibberellin levels. It was shown, using radiolabeled 14C-IAA, that NPA applied to the middle of internode segments had little effect on polar auxin transport, while 2,3,5-triiodobenzoic acid substantially inhibited it. These results strongly suggest that NPA induces changes in endogenous levels of plant hormones, such as IAA, cytokinins, and jasmonic acid, and their hormonal crosstalk results in a conspicuous local stem swelling. The possible different mode of action of NPA from other polar auxin transport inhibitors in succulent plants is extensively discussed.
Highlights
Plant growth, such as cell elongation, cell division, and cell enlargement, is primarily regulated by plant hormones and other plant growth substances
The application of triiodobenzoic acid (TIBA) [11] and HFCA (Saniewski et al [11], unpublished results) in the same manner to B. calycinum plants did not induce conspicuous local stem swelling, whereas slight swelling along the entire internode above the treatment area was observed. These results suggest that the mode of actions of naphthylphthalamic acid (NPA) is different from that of TIBA and HFCA in species of the family Crassulaceae, and that stem swelling in B. calycinum is caused by the interaction with NPA and indole-3-acetic acid (IAA)
As described in the Introduction, we have reported that NPA induced conspicuous local stem swelling only at the site of its application in intact plants and in the decapitated internode of B. calycinum when IAA was applied simultaneously on the top of the internode [11]
Summary
Plant growth, such as cell elongation, cell division, and cell enlargement, is primarily regulated by plant hormones and other plant growth substances. Naturally occurring auxin, indole-3-acetic acid (IAA), mainly synthesized in the apical part of shoots and young leaves, shows a specific basipetal movement known as polar auxin transport. In this process, auxin is transported basipetally between cells toward the root tip through the stele in the aboveground parts, and redirected symmetrically in specific root tip cells, and transported back towards the root elongation zone [1,2,3,4,5,6,7]. We found that NPA induced conspicuous local stem swelling only in the area of its application in intact plants of B. calycinum, and in the decapitated ones when IAA was applied simultaneously on the top of the internode [11]
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call