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Ectopic shoot meristem generation in monocotyledonous rpk1 mutants is linked to SAM loss and altered seedling morphology.

Fiesselmann BS, Luichtl M, Yang X, Matthes M, Peis O, Torres-Ruiz RA - BMC Plant Biol. (2015)

Bottom Line: In strong rpk1 alleles, about 10 % of these (i. e. 1 % of all homozygotes) did not develop a SAM.The results highlight the developmental autonomy of the SAM vs. cotyledons and suggest that the primary rpk1 defect does not lie in the seedling's ability to express SAM genes or to develop a shoot meristem.The specific cotyledon defect in rpk1 mutants thus sheds light upon the developmental implications of the transition from two cotyledons to one.

View Article: PubMed Central - PubMed

Affiliation: Lehrstuhl für Genetik, Technische Universität München, Wissenschaftszentrum Weihenstephan, Emil-Ramann-Str. 8, D-85354, Freising, Germany. birgit.fiesselmann@gmail.com.

ABSTRACT

Background: In dicot Arabidopsis thaliana embryos two cotyledons develop largely autonomously from the shoot apical meristem (SAM). Recessive mutations in the Arabidopsis receptor-like kinase RPK1 lead to monocotyledonous seedlings, with low (10 %) penetrance due to complex functional redundancy. In strong rpk1 alleles, about 10 % of these (i. e. 1 % of all homozygotes) did not develop a SAM. We wondered whether RPK1 might also control SAM gene expression and SAM generation in addition to its known stochastic impact on cell division and PINFORMED1 (PIN1) polarity in the epidermis.

Results: SAM-less seedlings developed a simple morphology with a straight and continuous hypocotyl-cotyledon structure lacking a recognizable epicotyl. According to rpk1's auxin-related PIN1 defect, the seedlings displayed defects in the vascular tissue. Surprisingly, SAM-less seedlings variably expressed essential SAM specific genes along the hypocotyl-cotyledon structure up into the cotyledon lamina. Few were even capable of developing an ectopic shoot meristem (eSM) on top of the cotyledon.

Conclusions: The results highlight the developmental autonomy of the SAM vs. cotyledons and suggest that the primary rpk1 defect does not lie in the seedling's ability to express SAM genes or to develop a shoot meristem. Rather, rpk1's known defects in cell division and auxin homeostasis, by disturbed PIN1 polarity, impact on SAM and organ generation. In early embryo stages this failure generates a simplified monocotyledonous morphology. Once generated, this likely entails a loss of positional information that in turn affects the spatiotemporal development of the SAM. SAM-bearing and SAM-less monocotyledonous phenotypes show morphological similarities either to real monocots or to dicot species, which only develop one cotyledon. The specific cotyledon defect in rpk1 mutants thus sheds light upon the developmental implications of the transition from two cotyledons to one.

No MeSH data available.


Related in: MedlinePlus

Morphology in SAM-less rpk1 monocot seedlings. a Magnifications of parts of monocot rpk1-7 seedlings (gl1/gl1 background) with SAM and primary leaves (top) and without SAM (bottom). b and c Whole plants with long roots (indicated by arrowheads) illustrate the continuous root growth. d A shoot-less monocot seedling from long-term cultivation shows a necrotic cotyledon while the root has continued growth and turned green. e A SAM-less monocot seedling with a homozygous rpk1-7 GL1 background (carrying a PIN1:GFP reporter). f Seedlings cleared with Hoyers mount visualize the vascular system in the contiguous hypocotyl-cotyledon structure with interruptions (white arrowheads) and supernumerary and/or blindly terminating vascular elements (small arrows). There is no bend recognizable, which in the wild-type separates apically the SAM/epicotyl from the laterally placed cotyledon. g A SAM-less monocot seedling originating from the rpk1-6 allele. Cotyledons (c), normal leaf (lf) indicated. Scale bars: 1 mm a-e, 0.5 mm g, 100 μM f
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Fig1: Morphology in SAM-less rpk1 monocot seedlings. a Magnifications of parts of monocot rpk1-7 seedlings (gl1/gl1 background) with SAM and primary leaves (top) and without SAM (bottom). b and c Whole plants with long roots (indicated by arrowheads) illustrate the continuous root growth. d A shoot-less monocot seedling from long-term cultivation shows a necrotic cotyledon while the root has continued growth and turned green. e A SAM-less monocot seedling with a homozygous rpk1-7 GL1 background (carrying a PIN1:GFP reporter). f Seedlings cleared with Hoyers mount visualize the vascular system in the contiguous hypocotyl-cotyledon structure with interruptions (white arrowheads) and supernumerary and/or blindly terminating vascular elements (small arrows). There is no bend recognizable, which in the wild-type separates apically the SAM/epicotyl from the laterally placed cotyledon. g A SAM-less monocot seedling originating from the rpk1-6 allele. Cotyledons (c), normal leaf (lf) indicated. Scale bars: 1 mm a-e, 0.5 mm g, 100 μM f

Mentions: The allele rpk1-7 was induced in a gl1 Columbia background and generates ca. 10 % seedlings with cotyledon abnormalities most of them lacking one cotyledon [22]. We detected that, five days after germination, some of the monocot seedlings did not possess developed SAMs in comparison to their monocot siblings (Fig. 1a-c). The cotyledon of these seedlings varied in shape and size and had a well-developed lamina with recognizable adaxial and abaxial sides (Fig. 1). The SAM-less monocots regularly occurred in the pedigree of crosses with plants of different genetic backgrounds with a frequency ranging between 0.5 % and 1.8 % of all seedlings (Table 1). Upon further growing, part of the SAM-less seedlings developed SAMs at some distance from the cotyledon lamina, suggesting that meristem development lagged behind that of SAM-bearing monocots. We considered that the SAM-less phenotype could be a specific character of the rpk1-7 allele, which is a fast neutron-induced inversion [22]. Therefore, we searched this phenotype in the independently generated rpk1-6 allele, which is a T-DNA insertion in the RPK1 coding region [22] and found SAM-less seedlings with similar frequencies as in rpk1-7 (Table 1). The other SAM-less seedlings did never develop a normal SAM but necrotic cotyledons and green, continuously growing roots as long as cultured in sterile 1/2MS medium (Fig. 1d). Notably, in these seedlings the hypocotyl and cotyledon petiole formed a continuous structure without recognizable separation of a SAM region (Fig. 1c, e and f). This was true for both alleles (compare Fig. 1c, e, g) and showed that cell differentiation in these tissues had been fundamentally altered. Whole mount preparations of rpk1-7 seedlings displayed vascular defects stressing RPK1’s link to PIN1 polarity and auxin transport [22]. In rpk1 monocots, the wild-type diarchic vascular system, which branches into both cotyledons, was variably organized. Either both strands intruded into the remaining cotyledon, or one strand ended in the “hypocotyl“. In other cases supernumerary vascular cell files were formed (Fig. 1f; Additional file 1: Figure S1).Fig. 1


Ectopic shoot meristem generation in monocotyledonous rpk1 mutants is linked to SAM loss and altered seedling morphology.

Fiesselmann BS, Luichtl M, Yang X, Matthes M, Peis O, Torres-Ruiz RA - BMC Plant Biol. (2015)

Morphology in SAM-less rpk1 monocot seedlings. a Magnifications of parts of monocot rpk1-7 seedlings (gl1/gl1 background) with SAM and primary leaves (top) and without SAM (bottom). b and c Whole plants with long roots (indicated by arrowheads) illustrate the continuous root growth. d A shoot-less monocot seedling from long-term cultivation shows a necrotic cotyledon while the root has continued growth and turned green. e A SAM-less monocot seedling with a homozygous rpk1-7 GL1 background (carrying a PIN1:GFP reporter). f Seedlings cleared with Hoyers mount visualize the vascular system in the contiguous hypocotyl-cotyledon structure with interruptions (white arrowheads) and supernumerary and/or blindly terminating vascular elements (small arrows). There is no bend recognizable, which in the wild-type separates apically the SAM/epicotyl from the laterally placed cotyledon. g A SAM-less monocot seedling originating from the rpk1-6 allele. Cotyledons (c), normal leaf (lf) indicated. Scale bars: 1 mm a-e, 0.5 mm g, 100 μM f
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC4492102&req=5

Fig1: Morphology in SAM-less rpk1 monocot seedlings. a Magnifications of parts of monocot rpk1-7 seedlings (gl1/gl1 background) with SAM and primary leaves (top) and without SAM (bottom). b and c Whole plants with long roots (indicated by arrowheads) illustrate the continuous root growth. d A shoot-less monocot seedling from long-term cultivation shows a necrotic cotyledon while the root has continued growth and turned green. e A SAM-less monocot seedling with a homozygous rpk1-7 GL1 background (carrying a PIN1:GFP reporter). f Seedlings cleared with Hoyers mount visualize the vascular system in the contiguous hypocotyl-cotyledon structure with interruptions (white arrowheads) and supernumerary and/or blindly terminating vascular elements (small arrows). There is no bend recognizable, which in the wild-type separates apically the SAM/epicotyl from the laterally placed cotyledon. g A SAM-less monocot seedling originating from the rpk1-6 allele. Cotyledons (c), normal leaf (lf) indicated. Scale bars: 1 mm a-e, 0.5 mm g, 100 μM f
Mentions: The allele rpk1-7 was induced in a gl1 Columbia background and generates ca. 10 % seedlings with cotyledon abnormalities most of them lacking one cotyledon [22]. We detected that, five days after germination, some of the monocot seedlings did not possess developed SAMs in comparison to their monocot siblings (Fig. 1a-c). The cotyledon of these seedlings varied in shape and size and had a well-developed lamina with recognizable adaxial and abaxial sides (Fig. 1). The SAM-less monocots regularly occurred in the pedigree of crosses with plants of different genetic backgrounds with a frequency ranging between 0.5 % and 1.8 % of all seedlings (Table 1). Upon further growing, part of the SAM-less seedlings developed SAMs at some distance from the cotyledon lamina, suggesting that meristem development lagged behind that of SAM-bearing monocots. We considered that the SAM-less phenotype could be a specific character of the rpk1-7 allele, which is a fast neutron-induced inversion [22]. Therefore, we searched this phenotype in the independently generated rpk1-6 allele, which is a T-DNA insertion in the RPK1 coding region [22] and found SAM-less seedlings with similar frequencies as in rpk1-7 (Table 1). The other SAM-less seedlings did never develop a normal SAM but necrotic cotyledons and green, continuously growing roots as long as cultured in sterile 1/2MS medium (Fig. 1d). Notably, in these seedlings the hypocotyl and cotyledon petiole formed a continuous structure without recognizable separation of a SAM region (Fig. 1c, e and f). This was true for both alleles (compare Fig. 1c, e, g) and showed that cell differentiation in these tissues had been fundamentally altered. Whole mount preparations of rpk1-7 seedlings displayed vascular defects stressing RPK1’s link to PIN1 polarity and auxin transport [22]. In rpk1 monocots, the wild-type diarchic vascular system, which branches into both cotyledons, was variably organized. Either both strands intruded into the remaining cotyledon, or one strand ended in the “hypocotyl“. In other cases supernumerary vascular cell files were formed (Fig. 1f; Additional file 1: Figure S1).Fig. 1

Bottom Line: In strong rpk1 alleles, about 10 % of these (i. e. 1 % of all homozygotes) did not develop a SAM.The results highlight the developmental autonomy of the SAM vs. cotyledons and suggest that the primary rpk1 defect does not lie in the seedling's ability to express SAM genes or to develop a shoot meristem.The specific cotyledon defect in rpk1 mutants thus sheds light upon the developmental implications of the transition from two cotyledons to one.

View Article: PubMed Central - PubMed

Affiliation: Lehrstuhl für Genetik, Technische Universität München, Wissenschaftszentrum Weihenstephan, Emil-Ramann-Str. 8, D-85354, Freising, Germany. birgit.fiesselmann@gmail.com.

ABSTRACT

Background: In dicot Arabidopsis thaliana embryos two cotyledons develop largely autonomously from the shoot apical meristem (SAM). Recessive mutations in the Arabidopsis receptor-like kinase RPK1 lead to monocotyledonous seedlings, with low (10 %) penetrance due to complex functional redundancy. In strong rpk1 alleles, about 10 % of these (i. e. 1 % of all homozygotes) did not develop a SAM. We wondered whether RPK1 might also control SAM gene expression and SAM generation in addition to its known stochastic impact on cell division and PINFORMED1 (PIN1) polarity in the epidermis.

Results: SAM-less seedlings developed a simple morphology with a straight and continuous hypocotyl-cotyledon structure lacking a recognizable epicotyl. According to rpk1's auxin-related PIN1 defect, the seedlings displayed defects in the vascular tissue. Surprisingly, SAM-less seedlings variably expressed essential SAM specific genes along the hypocotyl-cotyledon structure up into the cotyledon lamina. Few were even capable of developing an ectopic shoot meristem (eSM) on top of the cotyledon.

Conclusions: The results highlight the developmental autonomy of the SAM vs. cotyledons and suggest that the primary rpk1 defect does not lie in the seedling's ability to express SAM genes or to develop a shoot meristem. Rather, rpk1's known defects in cell division and auxin homeostasis, by disturbed PIN1 polarity, impact on SAM and organ generation. In early embryo stages this failure generates a simplified monocotyledonous morphology. Once generated, this likely entails a loss of positional information that in turn affects the spatiotemporal development of the SAM. SAM-bearing and SAM-less monocotyledonous phenotypes show morphological similarities either to real monocots or to dicot species, which only develop one cotyledon. The specific cotyledon defect in rpk1 mutants thus sheds light upon the developmental implications of the transition from two cotyledons to one.

No MeSH data available.


Related in: MedlinePlus