Figure 8.

Model of participation of PbrBZR1-PbrARI2.3 complex in BR-mediated pollen tube growth during SI in pear. A) Under normal conditions, PbrARI2.3 barely accumulates, leading to the inability to degrade the BR-activated dephosphorylated (active) form of PbrBZR1. Dephosphorylated PbrBZR1 promotes pollen tube growth by activating PbrEXLA3 transcription. B) Under SI conditions, PbrARI2.3 accumulates and captures dephosphorylated PbrBZR1 to mediate its degradation through the 26S proteasome pathway. Thus, little PbrBZR1 is available to bind to the PbrEXLA3 promoter and regulate its transcription. As a result, pollen tube growth is inhibited. C) Under BR-sufficient and SI conditions, most PbrBZR1 protein exists in the dephosphorylated (active) form, so it cannot be completely degraded by PbrARI2.3. Thus, the dephosphorylated PbrBZR1 can act on PbrEXLA3 to alleviate the SI-induced inhibition of pollen tube growth. Circle “P” represents phosphorylation modification; Circle “U” represents ubiquitination modification; plus sign represents the ability of PbrBZR1 to activate PbrEXLA3 expression (the more plus signs, the stronger the ability); Cross symbol represents the inhibition of PbrEXLA3 transcription.