LEAFY COTYLEDON2 (LEC2) promotes embryogenic induction in somatic tissues of Arabidopsis, via YUCCA-mediated auxin biosynthesis

Abstract

The LEAFY COTYLEDON2 (LEC2) transcription factor with a plant-specific B3 domain plays a central role in zygotic and somatic embryogenesis (SE). LEC2 overexpression induced in planta leads to spontaneous somatic embryo formation, but impairs the embryogenic response of explants cultured in vitro under auxin treatment. The auxin-related functions of LEC2 appear during SE induction, and the aim of the present study was to gain further insights into this phenomenon. To this end, the effect of LEC2 overexpression on the morphogenic responses of Arabidopsis explants cultured in vitro under different auxin treatments was evaluated. The expression profiles of the auxin biosynthesis genes were analysed in embryogenic cultures with respect to LEC2 activity. The results showed that LEC2 overexpression severely modifies the requirement of cultured explants for an exogenous auxin concentration at a level that is effective in SE induction and suggested an increase in the auxin content in 35S::LEC2-GR transgenic explants. The assumption of an LEC2 promoted increase in endogenous auxin in cultured explants was further supported by the expression profiling of the genes involved in auxin biosynthesis. The analysis indicated that YUCCAs and TAA1, working in the IPA-YUC auxin biosynthesis pathway, are associated with SE induction, and that the expression of three YUCCA genes (YUC1, YUC4 and YUC10) is associated with LEC2 activity. The results also suggest that the IAOx-mediated auxin biosynthesis pathway involving ATR1/MYB34 and CYP79B2 does not seem to be involved in SE induction. We conclude that de novo auxin production via the tryptophan-dependent IPA-YUC auxin biosynthesis pathway is implicated in SE induction, and that LEC2 plays a key role in this mechanism.Electronic supplementary materialThe online version of this article (doi:10.1007/s00425-013-1892-2) contains supplementary material, which is available to authorized users.

A transcriptional view on somatic embryogenesis

“…LEC2 activates expression of TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS 1 ( TAA1 ) and the YUCCA2 and 4 ( YUC ) genes, which encode key enzymes in the auxin biosynthesis pathway (Zhao, 2014). LEC2 overexpression can compensate for a suboptimal amount of 2,4‐D or for auxins that are less efficient in somatic embryo induction, such as IAA or NAA (Wojcikowska et al., 2013). Conversely, ectopic overexpression of LEC2 in combination with a normal 2,4‐D concentration is detrimental for somatic embryo production, with callus being produced instead of somatic embryos (Ledwon & Gaj, 2009; Wojcikowska et al., 2013).…”

“…LEC2 overexpression can compensate for a suboptimal amount of 2,4‐D or for auxins that are less efficient in somatic embryo induction, such as IAA or NAA (Wojcikowska et al., 2013). Conversely, ectopic overexpression of LEC2 in combination with a normal 2,4‐D concentration is detrimental for somatic embryo production, with callus being produced instead of somatic embryos (Ledwon & Gaj, 2009; Wojcikowska et al., 2013). LEC1 also binds to and activates expression of YUC10 in seedlings, but only when they are treated with ABA (see above) (Junker et al., 2012).…”

“…LEC1 also binds to and activates expression of YUC10 in seedlings, but only when they are treated with ABA (see above) (Junker et al., 2012). The importance of endogenous auxin production for 2,4‐D‐induced SE was demonstrated using yuc knockout mutants, which show a reduced response in 2,4‐D‐treated secondary somatic embryo cultures (Bai, Su, Yuan, & Zhang, 2013; Wojcikowska et al., 2013), but its significance for LEC‐induced SE is not known. YUC overexpression in seedlings is not sufficient to trigger spontaneous SE in seedlings, but rather induces epinastic cotyledon growth (Cheng, Dai, & Zhao, 2006; Kim et al., 2007; Woodward et al., 2005), a typical auxin overproduction phenotype.…”

Azacitidine (5-AzaC)-treatment and mutations in DNA methylase genes affect embryogenic response and expression of the genes that are involved in somatic embryogenesis in Arabidopsis

“…LEC1 and LEC2, which are the master regulators of zygotic embryogenesis in Arabidopsis (Braybrook and Harada 2008), were found to be essential for SE induction (Gaj et al 2005). LEC1 and LEC2 contribute to SE induction via the regulation of the auxin response genes (Braybrook et al 2006) and LEC2 was indicated as activating auxin biosynthesis during SE (Wójcikowska et al 2013). Recently, the LEC1, LEC2 and BBM genes were indicated as functioning in the same molecular pathway in which BBM transcriptionally regulates LEC1 and LEC2 to induce SE (Horstman et al 2017).…”

LEAFY COTYLEDON2-mediated control of the endogenous hormone content: implications for the induction of somatic embryogenesis in Arabidopsis

“…The LEC2 gene, which is a master regulator of zygotic embryogenesis in Arabidopsis, has been suggested as being in control of embryogenic development in somatic cells and the SE-promoting effect of LEC2 overexpression in plants and explants cultured under auxin-free conditions was reported (Stone et al 2001;Ledwoń and Gaj 2009 (Wójcikowska et al 2013). …”