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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

In situ hybridization analysis of monocot rpk1-7 seedlings. Shown are (serial) sections of torpedo embryos (dicot and monocot embryos indicated). a–e6 In situ hybridization with the STM probe. f-g4 In situ hybridization with the ENP probe for comparison. a-c8 and f-g4 show longitudinal sections and d1-6; e1-6 show cross-sections respectively. Brackets in a and d2-5, e2-5 indicate the distance of 15-20 μm (cross sections have 3,5 μm thickness). The stippled line in b2 and b3 indicates the extension of the STM signal along the SAM region and the hypocotyl. Arrows point to the localized SAM signals of STM and (late) ENP respectively. Arrowheads indicate the additional ENP signal in the cotyledon epidermis. Scale bars: 20 μm
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Fig5: In situ hybridization analysis of monocot rpk1-7 seedlings. Shown are (serial) sections of torpedo embryos (dicot and monocot embryos indicated). a–e6 In situ hybridization with the STM probe. f-g4 In situ hybridization with the ENP probe for comparison. a-c8 and f-g4 show longitudinal sections and d1-6; e1-6 show cross-sections respectively. Brackets in a and d2-5, e2-5 indicate the distance of 15-20 μm (cross sections have 3,5 μm thickness). The stippled line in b2 and b3 indicates the extension of the STM signal along the SAM region and the hypocotyl. Arrows point to the localized SAM signals of STM and (late) ENP respectively. Arrowheads indicate the additional ENP signal in the cotyledon epidermis. Scale bars: 20 μm

Mentions: Late monocot rpk1 embryos displayed a “banana“-like appearance with a more or less recognizable notch harbouring the presumptive SAM region. As expected, we mostly detected correct expression patterns. STM showed a larger while CLV3 exhibited a small expression domain as known (Fig. 5a-e6, Additional file 1: Figures S3 and S4 for comparison). Similarly, ENP and PID showed normal late expression in cotyledons and the SAM (Fig. 5f, g1-g4; Additional file 1: Figures S5 and S6 for comparison). Although any of these probes could have potentially detected an abnormal expression pattern, we found only one among 30 monocots (out of 328 rpk1-7 torpedo embryos). Considering the 10 % frequency of SAM-less seedlings among monocot rpk1-7 seedlings, this is in the same range. Surprisingly, in the identified monocot embryo the hybridization with the STM probe extended almost along the complete embryonic hypocotyl but not into the cotyledon tissue, with the strongest concentration being at the normal SAM position (Fig. 5b1-b5; stippled line in B2 and B3). The size of the domain expressing STM in this specimen clearly exceeded 15-20 μm in apical-basal axis, which is the size displayed in dicot and monocot SAM-bearing rpk1-7 torpedo embryos (Fig. 5a, d1-d6, e1-e6; brackets). This result coincides with one of the subsequently observed KNAT2p:GUS expression pattern variants in SAM-less monocot rpk1-7 seedlings (see below).Fig. 5


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)

In situ hybridization analysis of monocot rpk1-7 seedlings. Shown are (serial) sections of torpedo embryos (dicot and monocot embryos indicated). a–e6 In situ hybridization with the STM probe. f-g4 In situ hybridization with the ENP probe for comparison. a-c8 and f-g4 show longitudinal sections and d1-6; e1-6 show cross-sections respectively. Brackets in a and d2-5, e2-5 indicate the distance of 15-20 μm (cross sections have 3,5 μm thickness). The stippled line in b2 and b3 indicates the extension of the STM signal along the SAM region and the hypocotyl. Arrows point to the localized SAM signals of STM and (late) ENP respectively. Arrowheads indicate the additional ENP signal in the cotyledon epidermis. Scale bars: 20 μm
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Fig5: In situ hybridization analysis of monocot rpk1-7 seedlings. Shown are (serial) sections of torpedo embryos (dicot and monocot embryos indicated). a–e6 In situ hybridization with the STM probe. f-g4 In situ hybridization with the ENP probe for comparison. a-c8 and f-g4 show longitudinal sections and d1-6; e1-6 show cross-sections respectively. Brackets in a and d2-5, e2-5 indicate the distance of 15-20 μm (cross sections have 3,5 μm thickness). The stippled line in b2 and b3 indicates the extension of the STM signal along the SAM region and the hypocotyl. Arrows point to the localized SAM signals of STM and (late) ENP respectively. Arrowheads indicate the additional ENP signal in the cotyledon epidermis. Scale bars: 20 μm
Mentions: Late monocot rpk1 embryos displayed a “banana“-like appearance with a more or less recognizable notch harbouring the presumptive SAM region. As expected, we mostly detected correct expression patterns. STM showed a larger while CLV3 exhibited a small expression domain as known (Fig. 5a-e6, Additional file 1: Figures S3 and S4 for comparison). Similarly, ENP and PID showed normal late expression in cotyledons and the SAM (Fig. 5f, g1-g4; Additional file 1: Figures S5 and S6 for comparison). Although any of these probes could have potentially detected an abnormal expression pattern, we found only one among 30 monocots (out of 328 rpk1-7 torpedo embryos). Considering the 10 % frequency of SAM-less seedlings among monocot rpk1-7 seedlings, this is in the same range. Surprisingly, in the identified monocot embryo the hybridization with the STM probe extended almost along the complete embryonic hypocotyl but not into the cotyledon tissue, with the strongest concentration being at the normal SAM position (Fig. 5b1-b5; stippled line in B2 and B3). The size of the domain expressing STM in this specimen clearly exceeded 15-20 μm in apical-basal axis, which is the size displayed in dicot and monocot SAM-bearing rpk1-7 torpedo embryos (Fig. 5a, d1-d6, e1-e6; brackets). This result coincides with one of the subsequently observed KNAT2p:GUS expression pattern variants in SAM-less monocot rpk1-7 seedlings (see below).Fig. 5

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