Is auxin behind adventitious root formation in petunia cuttings?

If you want your Petunia hybrida cuttings to grow adventitious roots, then you incubate them in the dark. If you want to prevent that, you keep them low in nitrogen. But why does this work? Huaiyu Yang and colleagues investigated to see if auxin was an important factor in developing the roots.

Adventitious roots are roots that grow from non-root organs, usually a stem. They can form when cells near a wound site reprogramme to become root cells. Yang and colleagues wondered what hormones drive this change. It would be useful to know, as while you can incubate cuttings in the dark, keeping them in the dark adds more stress to the plant. This is a cost that horticulturalists might well want to avoid, as it can be quite a period. “Clonal propagation of many ornamental plants involves dark incubation of cuttings after harvest from the stock plant over a period of several days or weeks before planting, usually at reduced temperatures to slow down catabolic depletion of cutting,” write the authors in their paper.

What the team found was that dark incubation up-regulated genes of the auxin pathway. However the authors also said that nitrogen was critically important too. “Transcript levels of genes related to auxin revealed a decreased auxin signalling in nitrogen-limited cuttings at the levels of Ph-GH3.6, Ph-GH3.10, Ph-IAA14, Ph-ARF8, Ph-ARF10 and Ph-SAUR14 expression from 24 hpe until 48 hpe particularly under dark incubation.”

Their results show the way towards other investigations, Yang and colleagues say in their conclusion. “Based on this working model, future studies should focus in particular on the molecular and physiological mechanisms of dark-mediated auxin mobilization from the upper shoot and the functional roles of the specific Aux/IAA, ARF and SAUR genes in AR formation in cuttings.”