3.3.1. Wound Stress and Callus Initiation

In 2000, based on contract no. 6127/2000 signed with the Romanian Ministry of Research, it was possible to develop an experimental test to understand the relationship between wound stress and callus initiation for shoot formation. The expected results were to be used to improve the *Syngonium* micropropagation protocol [33,34]. In that experiment, we used fragments of in vitro petioles and leaves, and salicylic acid (SA) and the synthetic hormone 2,4-D were tested as signaling molecules. SA was recognized at the time for its role in preventing wound signaling in plants [65,66], while 2,4-D had long been recognized for its role in inducing callus formation based on the experiment conducted by John Torrey and Kenneth Thimann in 1949, who sprayed this herbicide on the surface of stumps of the tropical sicklebush tree (*Dichrostachys nutans* Benth.) [67]. They observed that a ring of callus tissue developed at the cambium level after a couple of weeks. We should also note that the use of the synthetic plant growth regulator 2,4-D for in vitro plant experiments was successfully applied after 1945 [68–70].

Today, it is already recognized and scientifically proven that wound stress is responsible for the production of callus tissue in order to seal the wounded site and to facilitate adventitious organogenesis or somatic embryogenesis [19].
