ABA synthesized in the roots can also be transported to the shoot via the xylem. The concentration of ABA in the xylem sap of well-watered sunflower plants is between 1.0 and 15.0 nM, the ABA concentration in water stressed sunflower plants increases to as much as 3000 Nm (3.0 μM)

TLS Online TPP Program

#Id: 10173

#Unit 6. System Physiology – Plant

GAI and RGA proteins normally act as repressors of gibberellin responses. Gibberellin acts indirectly through an unidentified signaling intermediate, which is thought to bind to the regulatory domains of the GAI and RGA Proteins. 

TLS Online TPP Program

#Id: 10174

#Unit 6. System Physiology – Plant

DELLA is feedback regulator

TLS Online TPP Program

#Id: 10175

#Unit 6. System Physiology – Plant

In the absence of gibberellin, 

the DELLA proteins positively regulate GID1, GA20ox, and GA3ox, and negatively regulate GA2ox. 

TLS Online TPP Program

#Id: 10176

#Unit 6. System Physiology – Plant

Conversely bioactive gibberellin and the GID1 receptor enhance DELLA repressor degradation, while GA2ox blocks DELLA repressor degradation

TLS Online TPP Program

#Id: 10177

#Unit 6. System Physiology – Plant

DELLA degradation results in the release and activation of phytochrome interacting factor (PIF) transcription factors, such as PIF3 and PIF4, as well as other bHLH transcription factors

TLS Online TPP Program

#Id: 10178

#Unit 6. System Physiology – Plant

Binding of gibberellin receptor GID1 to DELLA repressor proteins triggers ubiquitination of the latter via the F-box protein SLY1 and subsequent degradation