Arabidopsis LEAFY COTYLEDON2 induces maturation traits and auxin activity: Implications for somatic embryogenesis

Abstract

LEAFY COTYLEDON2 (LEC2) is a central regulator of embryogenesis sufficient to induce somatic cells to form embryos when expressedectopically. Here, we analyze the cellular processes induced by LEC2, a B3 domain transcription factor, that may underlie its ability to promote somatic embryogenesis. We show auxin-responsive genes are induced after LEC2 activation in seedlings. Genes encoding enzymes involved in auxin biosynthesis, YUC2 and YUC4, are activated within 1 h after induction of LEC2 activity, and YUC4 appears to be a direct transcriptional target of LEC2. We also show ectopic LEC2 expression induces accumulation of seed storage protein and oil bodies in vegetative and reproductive organs, events that normally occur during the maturation phase of embryogenesis. Furthermore, LEC2 activates seed protein genes before an increase in RNAs encoding LEC1 or FUS3 is observed. Thus, LEC2 causes rapid changes in auxin responses and induces cellular differentiation characteristic of the maturation phase. The relevance of these changes to the ability of LEC2 to promote somatic embryogenesis is discussed.seed development ͉ totipotency A n outstanding characteristic of plants is their totipotency. That is, a variety of cells can be induced to regenerate the adult organism, and several cell types do so by undergoing embryogenesis. For example, the fertilized egg cell undergoes zygotic embryogenesis, a number of differentiated cells of the sporophyte can be induced to undergo somatic embryogenesis, microspores can be diverted from their development into pollen grains to enter microspore embryogenesis, and a number of ovule cell types undergo asexual embryo development in a suite of processes known collectively as apomixis (1-4). Evidence suggests the morphological pathways of development used by these different types of embryos are similar to those of zygotic embryos (5, 6). However, the processes that induce a cell to change its fate and enter an embryonic program of development are not known, nor is it known whether a common pathway is used to initiate embryonic development in all of these diverse cell types.To gain insight into these questions, we focused on Arabidopsis LEAFY COTYLEDON2 (LEC2). LEC2 regulates many distinct aspects of embryogenesis (7,8). For example, during the early morphogenesis phase of embryogenesis in which the basic body plan of the embryo is established, loss-of-function mutations in LEC2 affect the maintenance of embryonic cell fate and specification of cotyledon identity. Later in embryogenesis, lec2 mutants have cotyledon tips that do not accumulate storage reserves nor acquire desiccation tolerance, indicating defects in the initiation and/or maintenance of the maturation phase. Consistent with the pleiotropic effects of the lec2 mutation, LEC2 encodes a transcription factor with a B3 domain, a DNA binding region found thus far only in plant proteins (8-10). Two transcription factors most closely related to LEC2, ABA INSENSITIVE3 (ABI3) and another LEC protein, FUSCA3 (FUS3), also play criti...

Truncation of LEAFY COTYLEDON1 Protein Is Required for Asexual Reproduction in Kalanchoë daigremontiana

“…3D). These results are consistent with other studies that have shown that ectopic-expressing LEC genes promote somatic embryo development and that, particularly for 35S::LEC1, seedlings arrest as embryo-like seedlings and fail to develop further (Lotan et al, 1998;Stone et al, 2001Stone et al, , 2008Santos-Mendoza et al, 2005). Here, we show that contrary to nontransformed leaf embryos, functional KdLEC1 expression under the control of LEC1 promoter and terminator in K. daigremontiana resulted in dormant seed-like leaf embryos.…”

“…Together, these results show that the functional KdLEC1 transgene is expressed in all the transgenic plant leaf margins analyzed and indicate that the phenotypes observed on plantlet development of the transgenic plants are due to the expression of the functional KdLEC1 gene under the regulation of LEC1 promoter and terminator. These results are consistent with other analysis showing that ectopic expression of LEC1, FUSCA3 (FUS3), and LEC2 genes induces the accumulation of lipid and protein reserves characteristic of developing seeds in cells of vegetative and reproductive tissues (Lotan et al, 1998;Stone et al, 2001Stone et al, , 2008Santos-Mendoza et al, 2005). Moreover, seed-specific overexpression of maize (Zea mays) LEC1 (ZmLEC1) resulted in an average 35% increase in seed oil but reduced germination and vegetative growth in the field (Shen et al, 2010) In Arabidopsis, the seed maturation program is controlled by a network of transcription factors that includes the B3 domain transcription factors (ABA-INSENSITIVE3 [ABI3], FUS3, and LEC2; Giraudat et al, 1992;Luerssen et al, 1998;Stone et al, 2001) and two LEC1-type HAP3 family CCAAT-binding factors, LEC1 and L1L (Lotan et al, 1998;Kwong et al, 2003).…”

BABY BOOM target genes provide diverse entry points into cell proliferation and cell growth pathways

“…Target genes activated by LEC2 have been described (Braybrook et al 2006). LEC2 expression activates genes involved in auxin response and seed maturation, and arguments have been made to link these two pathways to the initiation of somatic embryo development (Stone et al 2008). We did not identify any of the LEC2 target genes in our experiments, suggesting that BBM and LEC activate different sets of primary target genes, and perhaps different initial signaling pathways.…”

Overexpression of PGA37/MYB118 and MYB115 promotes vegetative-to-embryonic transition in Arabidopsis

“…Subsequently, the de-differentiated cells or calli re-differentiate and develop as somatic embryos by an unknown mechanism. Whereas phytohormones are generally believed to activate a signaling pathway for the induction of embryogenic competence, a number of genetic loci have also been shown to play critical roles during somatic embryo development [2][3][4]. Mutations in the Arabidopsis LEAFY COTYLEDON1 (LEC1) and LEC2 genes cause similar defects in embryo maturation and partially transform cotyledons into leaves [5,6].…”

“…LEC1 and LEC2 encode a CCAAT-binding factortype transcription factor and a plant-specific B3 transcription factor, respectively [7,8]. Ectopic expression of these two embryo-specific genes causes the formation of somatic embryos on vegetative tissues [4,7,8]. A recent study shows that the auxin biosynthetic genes YUC2 and YUC4 are rapidly induced by LEC2, and YUC4 is likely a direct target of LEC2 [4].…”