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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

Analysis of KNAT2p:GUS reporter construct in rpk1-7 background. Shown are wild-type a and monocot rpk1-7b with GUS stain in the SAM (arrow), SAM-less monocot rpk1-7 seedlings c-j with weak GUS expression (c), with variably extended GUS expression in the presumptive SAM position (d-h; arrowheads) and the cotyledon (f-h; short lines) and with an eSM in the cotyledon j. Insets show details as magnifications. C: marks cotyledon in a-c and j. Scale bars: 1 mm
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Fig6: Analysis of KNAT2p:GUS reporter construct in rpk1-7 background. Shown are wild-type a and monocot rpk1-7b with GUS stain in the SAM (arrow), SAM-less monocot rpk1-7 seedlings c-j with weak GUS expression (c), with variably extended GUS expression in the presumptive SAM position (d-h; arrowheads) and the cotyledon (f-h; short lines) and with an eSM in the cotyledon j. Insets show details as magnifications. C: marks cotyledon in a-c and j. Scale bars: 1 mm

Mentions: In order to obtain a larger number of specimen with informative ectopic expression patterns of a SAM-related gene, we analysed Arabidopsis seedlings carrying a KNAT2p:GUS reporter [29]. KNAT2 is a STM-dependent transcription factor whose expression is localised in the SAM [30] (Fig. 6a). The monocot pedigree of a rpk1-7 X KNAT2p:GUS cross contained normal dicot, SAM-bearing monocot and SAM-less monocot seedlings. The former two exhibited GUS stain as expected at the apex next to the base of the cotyledon(s) (Fig. 6a, b). The SAM-less monocots displayed a spectrum of variants with respect to KNAT2 expression. Many seedlings showed very weak (Fig. 6c) to more intensive GUS expression in the central (vascular) tissue in the fused hypocotyl-cotyledon structure. This could extend either in direction towards the cotyledon tip or towards the root tip (Fig. 6d-h). The variability was further increased by some seedlings, which displayed smaller or larger patches of GUS staining in the cotyledon lamina (Fig. 6f-h). Monocot seedlings generating an eSM showed a strong GUS staining in the cotyledon (Fig. 6j). The variable KNAT2 expression in the cotyledon coincided with the results of the foregoing experiments. Thus, all expression data together suggest that SAM-less seedlings display an aberrant SAM gene expression pattern causing the generation of an eSM to be a rare event because it requires the concerted and precise coordination of several SAM genes.Fig. 6


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)

Analysis of KNAT2p:GUS reporter construct in rpk1-7 background. Shown are wild-type a and monocot rpk1-7b with GUS stain in the SAM (arrow), SAM-less monocot rpk1-7 seedlings c-j with weak GUS expression (c), with variably extended GUS expression in the presumptive SAM position (d-h; arrowheads) and the cotyledon (f-h; short lines) and with an eSM in the cotyledon j. Insets show details as magnifications. C: marks cotyledon in a-c and j. Scale bars: 1 mm
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Fig6: Analysis of KNAT2p:GUS reporter construct in rpk1-7 background. Shown are wild-type a and monocot rpk1-7b with GUS stain in the SAM (arrow), SAM-less monocot rpk1-7 seedlings c-j with weak GUS expression (c), with variably extended GUS expression in the presumptive SAM position (d-h; arrowheads) and the cotyledon (f-h; short lines) and with an eSM in the cotyledon j. Insets show details as magnifications. C: marks cotyledon in a-c and j. Scale bars: 1 mm
Mentions: In order to obtain a larger number of specimen with informative ectopic expression patterns of a SAM-related gene, we analysed Arabidopsis seedlings carrying a KNAT2p:GUS reporter [29]. KNAT2 is a STM-dependent transcription factor whose expression is localised in the SAM [30] (Fig. 6a). The monocot pedigree of a rpk1-7 X KNAT2p:GUS cross contained normal dicot, SAM-bearing monocot and SAM-less monocot seedlings. The former two exhibited GUS stain as expected at the apex next to the base of the cotyledon(s) (Fig. 6a, b). The SAM-less monocots displayed a spectrum of variants with respect to KNAT2 expression. Many seedlings showed very weak (Fig. 6c) to more intensive GUS expression in the central (vascular) tissue in the fused hypocotyl-cotyledon structure. This could extend either in direction towards the cotyledon tip or towards the root tip (Fig. 6d-h). The variability was further increased by some seedlings, which displayed smaller or larger patches of GUS staining in the cotyledon lamina (Fig. 6f-h). Monocot seedlings generating an eSM showed a strong GUS staining in the cotyledon (Fig. 6j). The variable KNAT2 expression in the cotyledon coincided with the results of the foregoing experiments. Thus, all expression data together suggest that SAM-less seedlings display an aberrant SAM gene expression pattern causing the generation of an eSM to be a rare event because it requires the concerted and precise coordination of several SAM genes.Fig. 6

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