LpWRKY28 promotes embryonic callus formation in Lilium pumilum by activating LpABI5 transcription

Somatic embryogenesis (SE) is widely used in agricultural production and genetic improvement. Lilium spp. has great ornamental and economic value, and SE provides an important basis for the conservation of its germplasm and its efficient propagation and genetic improvement. However, plant SE regulators have not been fully explored, and their regulatory mechanisms still need to be fully elucidated. Here, we found that the WRKY family gene LpWRKY28 is highly expressed during embryonic callus (EC) formation induced by the auxin-like substance picloram (PIC). In a stable Lilium genetic transformation system, RNA interference (RNAi) of LpWRKY28 significantly decreased the EC induction rate and EC fresh weight, whereas LpWRKY28 overexpression accelerated EC formation and significantly increased the EC fresh weight. Endogenous abscisic acid (ABA) levels increased significantly during PIC-induced EC formation, and EC formation did not occur after the inhibition of ABA biosynthesis in scales by fluridone (FLU). The endogenous ABA levels were higher in the scales of the LpWRKY28-overexpressing lines and lower in the scales of LpWRKY28 RNAi-silenced lines after PIC induction than in the wild-type lines. Further investigation revealed that LpWRKY28 regulates EC formation by activating the expression of ABA insensitive 5 (LpABI5), a key response factor in the ABA pathway. This study highlights the functions of WRKY transcription factors (TFs) in regulating plant development, establishes a connection between EC formation and the ABA pathway, and provides new insights into the molecular mechanisms of SE in Lilium.