skip to main content

Title: Photomorphogenesis of the root system in developing sunflower seedlings: a role for sucrose

The domestic sunflower (Helianthus annuusL. cv. ‘Giganteus’) has been used since the 19th century as a model plant for the study of seedling development in darkness and white light (WL) (scoto‐versusphotomorphogenesis). However, most pertinent studies have focused on the developmental patterns of the hypocotyl and cotyledons, whereas the root system has been largely ignored.

In this study, we analysed entire sunflower seedlings (root and shoot) and quantified organ development in the above‐ and belowground parts of the organism under natural (non‐sterile) conditions.

We document that seedlings, raised in moist vermiculite, are covered with methylobacteria, microbes that are known to promote root development inArabidopsis. Quantitative data revealed that during photomorphogenesis inWL, the root system expands by 90%, whereas stem elongation is inhibited, and hook opening/cotyledon expansion occurs. Root morphogenesis may be mediatedviaimported sucrose provided by the green, photosynthetically active cotyledons. This hypothesis is supported by the documented effect of sucrose on the induction of lateral root initials in sunflower cuttings. Under these experimental conditions, phytohormones (auxin, cytokinin, brassinolide) exerted little effect on root and cotyledon expansion, and no hormone‐induced initiation of lateral roots was observed.

It is concluded that sucrose not only acts as an energy source to fuel cell metabolism but is also a shoot‐derived signalling molecule that triggers root morphogenesis.

more » « less
Author(s) / Creator(s):
 ;  ;
Publisher / Repository:
Date Published:
Journal Name:
Plant Biology
Page Range / eLocation ID:
p. 627-633
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Summary

    Diacylglycerol kinase (DGK) phosphorylates diacylglycerol (DAG) to generate phosphatidic acid (PA), and bothDAGandPAare lipid mediators in the cell. Here we show thatDGK1 in rice (Oryza sativa) plays important roles in root growth and development.

    Two independentOsDGK1‐knockout (dgk1) lines exhibited a higher density of lateral roots (LRs) and thinner seminal roots (SRs), whereasOsDGK1‐overexpressing plants displayed a lowerLRdensity and thickerSRsthan wild‐type (WT) plants.

    Overexpression ofOsDGK1led to a decline in theDGKsubstrateDAGwhereas specificPAspecies decreased indgk1roots. Supplementation ofDAGtoOsDGK1‐overexpressing seedlings restored theLRdensity andSRthickness whereas application ofPAtodgk1seedlings restored theLRdensity andSRthickness to those of theWT. In addition, treatment of rice seedlings with theDGKinhibitor R59022 increased the level ofDAGand decreasedPA, which also restored the root phenotype ofOsDGK1‐overexpressing seedlings close to that of theWT.

    Together, these results indicate thatDGK1 and associated lipid mediators modulate rice root architecture;DAGpromotesLRformation and suppressesSRgrowth whereasPAsuppressesLRnumber and promotesSRthickness.

    more » « less
  2. Summary

    Rhamnose is required inArabidopsis thalianafor synthesizing pectic polysaccharides and glycosylating flavonols.RHAMNOSE BIOSYNTHESIS1(RHM1) encodes aUDPl‐rhamnose synthase, andrhm1mutants exhibit many developmental defects, including short root hairs, hyponastic cotyledons, and left‐handed helically twisted petals and roots. It has been proposed that the hyponastic cotyledons observed inrhm1mutants are a consequence of abnormal flavonol glycosylation, while the root hair defect is flavonol‐independent. We have recently shown that the helical twisting ofrhm1petals results from decreased levels of rhamnose‐containing cell wall polymers. In this study, we found that flavonols indirectly modify therhm1helical petal phenotype by altering rhamnose flux to the cell wall. Given this finding, we further investigated the relationship between flavonols and the cell wall inrhm1cotyledons. We show that decreased flavonol rhamnosylation is not responsible for the cotyledon phenotype ofrhm1mutants. Instead, blocking flavonol synthesis or rhamnosylation can suppressrhm1defects by divertingUDPl‐rhamnose to the synthesis of cell wall polysaccharides. Therefore, rhamnose is required in the cell wall for normal expansion of cotyledon epidermal cells. Our findings suggest a broad role for rhamnose‐containing cell wall polysaccharides in the morphogenesis of epidermal cells.

    more » « less
  3. Summary

    Plant lateral organ development is a complex process involving both transcriptional activation and repression mechanisms. TheWOXtranscriptional repressorWOX1/STF, theLEUNIG(LUG) transcriptional corepressor and theANGUSTIFOLIA3 (AN3) transcriptional coactivator play important roles in leaf blade outgrowth and flower development, but how these factors coordinate their activities remains unclear. Here we report physical and genetic interactions among these key regulators of leaf and flower development.

    We developed a novelin plantatranscriptional activation/repression assay and suggest thatLUGcould function as a transcriptional coactivator during leaf blade development.

    MtLUGphysically interacts with MtAN3, and this interaction appears to be required for leaf and flower development. A single amino acid substitution at position 61 in theSNHdomain of MtAN3 protein abolishes its interaction with MtLUG, and its transactivation activity and biological function. Mutations inlugandan3enhanced each other's mutant phenotypes. Both thelugand thean3mutations enhanced thewox1 prsleaf and flower phenotypes inArabidopsis.

    Our findings together suggest that transcriptional repression and activation mediated by theWOX,LUGandAN3 regulators function in concert to promote leaf and flower development, providing novel mechanistic insights into the complex regulation of plant lateral organ development.

    more » « less
  4. Abstract

    Rodents regularly rely on emerged epicotyls to locate and remove cotyledons still containing valuable nutrients. However, the extent to which acorn characteristics influence tolerance to post‐germination predation has received little attention.

    Here, we investigated the impact of cotyledon removal following epicotyl emergence on seedling performance and survival of seven oak (Quercus) species. We imitated cotyledon predation at different stages of seedling establishment and development in order to detect effects on seedling height, leaf number and tissue/component mass.

    Seedling growth and survival were negatively affected by cotyledon loss regardless of oak species. However, these negative effects decreased as the epicotyl length at which cotyledons were removed increased. We also found that there was a threshold epicotyl length above which seedling survival and performance were relatively unaffected in white oak species compared to red oak species.

    Following cotyledon removal, early germinating white oak (sectionQuercus) seedlings survived and/or grew better than the late germinating red oak (sectionLobatae) seedlings. This was likely caused by a difference in dependence on cotyledon reserves, which ultimately affected the ability of seedlings to tolerate cotyledon removal.

    Synthesis.From an evolutionary perspective, this is likely to follow from the early germination in white oaks and the ability of seed consumers to locate young seedlings from the emerging epicotyls. Our study has implications for forest regeneration by suggesting additional opportunities for white oak species to establish following epicotyl emergence. Future studies should consider quantifying the rates of post‐germination cotyledon loss.

    more » « less
  5. Summary

    The shoot stem cell niche, contained within the shoot apical meristem (SAM) is maintained in Arabidopsis by the homeodomain proteinSHOOT MERISTEMLESS(STM).STMis a mobile protein that traffics cell‐to‐cell, presumably through plasmodesmata. In maize, theSTMhomologKNOTTED1 shows clear differences betweenmRNAand protein localization domains in theSAM. However, theSTM mRNAand protein localization domains are not obviously different in Arabidopsis, and the functional relevance ofSTMmobility is unknown. Using a non‐mobile version ofSTM(2xNLSYFPSTM), we show thatSTMmobility is required to suppress axillary meristem formation during embryogenesis, to maintain meristem size, and to precisely specify organ boundaries throughout development.STMand organ boundary genesCUP SHAPED COTYLEDON1(CUC1),CUC2andCUC3regulate each other during embryogenesis to establish the embryonicSAMand to specify cotyledon boundaries, andSTMcontrolsCUCexpression post‐embryonically at organ boundary domains. We show that organ boundary specification by correct spatial expression ofCUCgenes requiresSTMmobility in the meristem. Our data suggest thatSTMmobility is critical for its normal function in shoot stem cell control.

    more » « less