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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

SAM-less rpk1 seedlings produce ectopic shoot meristems on cotyledons. a Monocot rpk1-7 seedling with an adaxial ectopic shoot meristem (eSM) carrying several leaves (inset: scheme for clarification). b The same on a rpk1-7 monocot seedling from long-term cultivation. The cotyledon has lost its greening. Arrowheads point to the root. c-d Monocot rpk1-7 seedlings in GL1 background with a normally positioned SAM c and with an eSM d respectively. Note the trichomes on the normal and ectopic primary leaves. A characteristic tissue outgrowth carries the eSM (arrow). e1-e5 Growth of an eSM (black arrow) on a cotyledon from a rpk1-7 seedling during the first two weeks. e6 The same after one month. f Progeny from the eSM rpk1-7 plant shown in e1-e6. g A rpk1-6 monocot seedling carrying two leaf outgrowths (arrows) on top of an abnormally thickened cotyledon. The arrowhead points to a trichome. Cotyledons (c), normal (lf) and ectopic leaves (elf) are indicated. Scale bars: 1 mm except in e6 e6: 1 cm
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Fig2: SAM-less rpk1 seedlings produce ectopic shoot meristems on cotyledons. a Monocot rpk1-7 seedling with an adaxial ectopic shoot meristem (eSM) carrying several leaves (inset: scheme for clarification). b The same on a rpk1-7 monocot seedling from long-term cultivation. The cotyledon has lost its greening. Arrowheads point to the root. c-d Monocot rpk1-7 seedlings in GL1 background with a normally positioned SAM c and with an eSM d respectively. Note the trichomes on the normal and ectopic primary leaves. A characteristic tissue outgrowth carries the eSM (arrow). e1-e5 Growth of an eSM (black arrow) on a cotyledon from a rpk1-7 seedling during the first two weeks. e6 The same after one month. f Progeny from the eSM rpk1-7 plant shown in e1-e6. g A rpk1-6 monocot seedling carrying two leaf outgrowths (arrows) on top of an abnormally thickened cotyledon. The arrowhead points to a trichome. Cotyledons (c), normal (lf) and ectopic leaves (elf) are indicated. Scale bars: 1 mm except in e6 e6: 1 cm

Mentions: During the analyses of rpk1-7 monocots we repeatedly found SAM-less seedlings, which could enter another rare developmental route by developing an eSM on the adaxial surface of the cotyledon (Fig. 2). The eSMs did not develop on any other SAM-bearing dicot or monocot rpk1 seedling and displayed some specific characteristics. Firstly, the eSM was positioned on the recognizable adaxial not on the abaxial site of the cotyledon (Fig. 2a-e). Secondly, the eSM appeared in median position on the cotyledon i. e. near the mid-rip (Fig. 2a, b, e1-e5). Thirdly, the eSM generated primary leaves with irregular phyllotactic patterns not additional cotyledons (Fig. 2a, b). Primary leaves of the original line carrying the glabra1 mutation did not form the trichomes. However, back-crossing to GLABRA1 background (Table 1) demonstrated that these developed the leaf specific trichomes (Fig. 2c, d). The eSMs generated single leaf organs or (in the other extreme) even rosettes with fertile shoots (Fig. 2e6). The resulting pedigree exhibited a similar range of cotyledon defects (Fig. 2f, Additional file 1: Figure S1). A search in rpk1-6 for a similar ectopic outgrowth revealed not more than one case among 737 seedlings (Fig. 2g) showing that this special structure is significantly rare. In order to assess the frequency of eSMs systematically, we grew large numbers (>10.000) of rpk1-7 seedlings in another genetic background (Table 2). The average amount of SAM-bearing and SAM-less monocots remained in the known range. However, the occurrence of eSMs was rare, had no predictable frequency in different pedigrees and was always linked to SAM-less monocots. Together, our observations showed that SAM-less monocot seedlings result from different mutations in RPK1. Therefore, in the following we concentrated on the analysis of the rpk1-7 alone.Fig. 2


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)

SAM-less rpk1 seedlings produce ectopic shoot meristems on cotyledons. a Monocot rpk1-7 seedling with an adaxial ectopic shoot meristem (eSM) carrying several leaves (inset: scheme for clarification). b The same on a rpk1-7 monocot seedling from long-term cultivation. The cotyledon has lost its greening. Arrowheads point to the root. c-d Monocot rpk1-7 seedlings in GL1 background with a normally positioned SAM c and with an eSM d respectively. Note the trichomes on the normal and ectopic primary leaves. A characteristic tissue outgrowth carries the eSM (arrow). e1-e5 Growth of an eSM (black arrow) on a cotyledon from a rpk1-7 seedling during the first two weeks. e6 The same after one month. f Progeny from the eSM rpk1-7 plant shown in e1-e6. g A rpk1-6 monocot seedling carrying two leaf outgrowths (arrows) on top of an abnormally thickened cotyledon. The arrowhead points to a trichome. Cotyledons (c), normal (lf) and ectopic leaves (elf) are indicated. Scale bars: 1 mm except in e6 e6: 1 cm
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4492102&req=5

Fig2: SAM-less rpk1 seedlings produce ectopic shoot meristems on cotyledons. a Monocot rpk1-7 seedling with an adaxial ectopic shoot meristem (eSM) carrying several leaves (inset: scheme for clarification). b The same on a rpk1-7 monocot seedling from long-term cultivation. The cotyledon has lost its greening. Arrowheads point to the root. c-d Monocot rpk1-7 seedlings in GL1 background with a normally positioned SAM c and with an eSM d respectively. Note the trichomes on the normal and ectopic primary leaves. A characteristic tissue outgrowth carries the eSM (arrow). e1-e5 Growth of an eSM (black arrow) on a cotyledon from a rpk1-7 seedling during the first two weeks. e6 The same after one month. f Progeny from the eSM rpk1-7 plant shown in e1-e6. g A rpk1-6 monocot seedling carrying two leaf outgrowths (arrows) on top of an abnormally thickened cotyledon. The arrowhead points to a trichome. Cotyledons (c), normal (lf) and ectopic leaves (elf) are indicated. Scale bars: 1 mm except in e6 e6: 1 cm
Mentions: During the analyses of rpk1-7 monocots we repeatedly found SAM-less seedlings, which could enter another rare developmental route by developing an eSM on the adaxial surface of the cotyledon (Fig. 2). The eSMs did not develop on any other SAM-bearing dicot or monocot rpk1 seedling and displayed some specific characteristics. Firstly, the eSM was positioned on the recognizable adaxial not on the abaxial site of the cotyledon (Fig. 2a-e). Secondly, the eSM appeared in median position on the cotyledon i. e. near the mid-rip (Fig. 2a, b, e1-e5). Thirdly, the eSM generated primary leaves with irregular phyllotactic patterns not additional cotyledons (Fig. 2a, b). Primary leaves of the original line carrying the glabra1 mutation did not form the trichomes. However, back-crossing to GLABRA1 background (Table 1) demonstrated that these developed the leaf specific trichomes (Fig. 2c, d). The eSMs generated single leaf organs or (in the other extreme) even rosettes with fertile shoots (Fig. 2e6). The resulting pedigree exhibited a similar range of cotyledon defects (Fig. 2f, Additional file 1: Figure S1). A search in rpk1-6 for a similar ectopic outgrowth revealed not more than one case among 737 seedlings (Fig. 2g) showing that this special structure is significantly rare. In order to assess the frequency of eSMs systematically, we grew large numbers (>10.000) of rpk1-7 seedlings in another genetic background (Table 2). The average amount of SAM-bearing and SAM-less monocots remained in the known range. However, the occurrence of eSMs was rare, had no predictable frequency in different pedigrees and was always linked to SAM-less monocots. Together, our observations showed that SAM-less monocot seedlings result from different mutations in RPK1. Therefore, in the following we concentrated on the analysis of the rpk1-7 alone.Fig. 2

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