Regeneration of active endogenous IAA in rice calli following acclimation to 2,4-D free medium

The habituated rice calli grown on an auxin-free medium were isolated from rice embryo-derived calli previously cultivated on medium supplemented with 2,4-D. The habituated rice calli grow faster than the non-habituated ones, particularly during the first week. We performed microarray analysis to unravel the changes in the transcriptomic level of rice calli grown on hormone-free medium. Several genes related to hormone metabolism and signaling were differentially expressed between the two calli types. Especially, the expressions of two amido-transferase enzymes genes Gretchen Hagen 3s (GH3.1 and GH3.8) were down-regulated and one indole-3-acetic acid (IAA)-amino acid (IAA-AA) hydrolase gene ILR1-like 3 was up-regulated in habituated calli compared to non-habituated. We further assessed the contents of different hormones in these two calli kinds. The IAA content was indeed significantly increased by about 7 times in habituated calli compared to non-habituated, initially grown in plant hormone-free medium. In parallel to the increase in the endogenous auxin, IAA-AA conjugates, abscisic acid (ABA), ethylene and GA(12), GA(19), GA(29) were decreased in the habituated calli. Our results indicate that the regeneration of endogenous auxin is essential and required for the habituation of callus cells. The changes in the endogenous hormones might be coordinated and monitored during the habituation process by various biological pathways. Eventually, this study debates the interrelated changes and adjustment in endogenous plant hormones occurred (happened) in the habituated calli. This report exhibits also a completely different hormonal composition status for each rice calli, which might potentially be responsible for the calli growth regulation and its acclimation process.

Automated summary

Habituated rice calli grown on auxin-free medium exhibit faster growth and differential gene expression related to hormone metabolism compared to non-habituated calli. Increased levels of IAA and decreased levels of other hormones such as ABA and ethylene are associated with habituation process. The changes in endogenous hormone levels play a crucial role in the acclimation and growth regulation of rice calli.