Abstract
Maize transformation is highly based on the formation of embryonic callus, which is mainly derived from scutellum cells of the immature maize embryo. However, only a few genes involved in callus induction have been identified in maize. To reveal the potential genes involved in the callus induction of maize, we carried out a high-throughput RNA sequencing on embryos that were cultured for 0, 1, 2, 4, 6, and 8 days, respectively, on a medium containing or lacking 2,4-dichlorophenoxyacetic acid. In total, 7,525 genes were found to be induced by 2,4-dichlorophenoxyacetic acid and categorized into eight clusters, with clusters 2 and 3 showing an increasing trend related to signal transmission, signal transduction, iron ion binding, and heme binding. Among the induced genes, 659 transcription factors belong to 51 families. An AP2 transcription factors, ZmBBM2, was dramatically and rapidly induced by auxin and further characterization showed that overexpression of ZmBBM2 can promote callus induction and proliferation in three inbred maize lines. Therefore, our comprehensive analyses provide some insight into the early molecular regulations during callus induction and are useful for further identification of the regulators governing callus formation.
Highlights
Tissue culture technology has been widely used in breeding programs, genetic engineering, and fundamental studies (Yadava, 2017)
Auxin signaling is transduced by auxin response factors (ARF), especially ARF7 and ARF19, which can activate the expression of the LATERAL ORGAN BOUNDARIES DOMAIN (LBD) and E2F TRANSCRIPTION FACTOR a (E2Fa) transcription factors (Fan et al, 2012; Ikeuchi et al, 2013)
Immature embryo from the maize line A188 at 0, 24, 36, 48, and 72 hours after induction were analyzed by high-throughput RNA sequencing (RNA-Seq), and the results showed that the expression of the genes involved in stress response and hormone transport was increased (Salvo et al, 2014)
Summary
Tissue culture technology has been widely used in breeding programs, genetic engineering, and fundamental studies (Yadava, 2017). Agrobacterium-mediated transformation and particle bombardment are widely used tissue culture technology in cereals, both of which require callus induction and selection (Shrawat and Lörz, 2006). Sugimoto et al (2010) suggested that callus are induced through lateral root initiation pathways in Arabidopsis thaliana. The exogenous application of auxin and cytokinin is necessary for in vitro callus induction for numerous plant species (Ikeuchi et al, 2013). Auxin signaling is transduced by auxin response factors (ARF), especially ARF7 and ARF19, which can activate the expression of the LATERAL ORGAN BOUNDARIES DOMAIN (LBD) and E2F TRANSCRIPTION FACTOR a (E2Fa) transcription factors (Fan et al, 2012; Ikeuchi et al, 2013).
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