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Vegetative propagation is very important for the survival of species with long juvenile and adult vegetative phases as it is the case for bulbous plants Bulbous plants are ornamental geophytes with a bulb as an underground storage organ Among flower bulbs tulip and lily are the two commercially leading plants in The Netherlands Tulip propagates vegetatively via axillary bud outgrowth while lily propagates via adventitious bulblet formation The vegetative propagation rate in tulip is very low due to the limited amount of axillary buds that will grow successfully Moreover tulip is very recalcitrant to in vitro regeneration On the other hand lily propagates efficiently via adventitious bulblet formation either naturally from the underground portion of the stem of the apical bud or artificially from detached bulb scales This thesis study aimed to understand how axillary bud outgrowth is controlled in tulip bulbs and how regeneration capacity is established in lily bulb scales As a first step towards these goals the state of the art of the molecular control of sexual and vegetative reproduction was reviewed for model species Moreover two approaches bottom-up and top-down to transfer the knowledge from model to non-model species were described Chapter 2 In short the bottom-up approach usually goes from individual genes to systems assuming conservation of molecular pathways and using sequence homology searches to identify candidate genes Top-down methodologies go from systems to genes and are based on large scale transcriptome profiling via e g microarrays or RNA sequencing followed by the identification of associations between phenotypes genes and gene expression patterns and levels Next Chapter 3 two sets of high quality transcriptomes one for tulip and one for lily were generated from a collection of several tissues using the Illumina HiSeq 2000 platform Several assembly filtering parameters were applied to highlight the limitations of stringent but routinely used filtering in de novo transcriptome assembly The final created transcriptomes were made publicly available via a user friendly Transcriptome browser http www bioinformatics nl bulbs db species index and their usefulness was exemplified by a search for all potential transcription factors in lily and tulip with special focus on the TCP transcription factor family One TCP member was of special interest because it has proven to integrate several pathways that control axillary bud outgrowth in a wide range of species It is called TEOSINTE BRANCHED 1 TB1 in monocots and BRNACHED 1 BRC1 in dicots A Tulipa gesneriana TB1 transcript was identified from the generated transcriptome and subsequently tulip axillary bud outgrowth was studied through a bottom-up approach Chapter 4 The degree of axillary bud outgrowth in tulip determines the success of their vegetative propagation However the number of axillary meristems in one bulb is low six on average and not all of them seem to have the same growth capacity The combination of physiological and targeted molecular experiments indicated that the first two inner located buds do not seem to experience dormancy assessed by weight increase and TgTB1expression at any point of the growth cycle while mid-located buds enter dormancy by the end of the growing season Moreover it was shown that TgTB1 expression in tulip bulbs can be modulated by sucrose cytokinin and strigolactone just as it has been reported for other species However the limited growth of mid-located buds even when their TgTB1 expression was naturally or artificially downregulated pointed at other factors probably physical inhibiting their growth Next the remarkable regeneration capacity of lily by initiating de novo shoot meristems from excised bulb scales without the addition of exogenous hormones or growth regulators was studied using a top-down approach Chapter 5 An extensive and comprehensive transcriptome set was generated from lily bulb scales in a time-series using two cultivars and two explant types all differing in regeneration capacity This set up provided first insight in the key molecular process underlying pro-meristem induction and meristem initiation in lily We found that wounding activates a very fast regeneration response probably mediated by APETALA2 ETHYLENE RESPONSIVE FACTORS AP2 ERF such as LoERF115 and WOUND INDUCED DEDIFFERENTIATION 2 LoWIND2 which in turn might mediate polar auxin re-distribution cell proliferation and de-differentiation Moreover the timing and level of induction of shoot meristem regulators such as ENHANCER OF SHOOT REGENERATION 2 LoESR2 and SHOOT MERISTEMLESS LoSTM correlated with the regeneration capacity of the scale Regardless the regeneration capacity of the different explants e g cultivar or position within the scale regeneration occurs at the proximal-adaxial side of the bulb scale right on top of the excision line Thus the possible cellular and physiological factors granting lily bulb scales their competence to regenerate was investigated Chapter 6 We found that the adaxial parenchyma tissue seems to be more competent than the abaxial tissue partially because of higher number of secondary veins and larger cell population than the abaxial parenchyma region It was proposed that upon explant excision the polar auxin transport is disrupted creating an auxin maximum at the excision line which might create a gradient of cell divisions favouring the adaxial parenchyma tissue The direction of this cell division gradient proved to be negatively affected by the absence of the adaxial epidermis Moreover explants without epidermis reduced dramatically their regeneration capacity and lost the typical proximal-adaxial orientation of regeneration Thus a better understanding of the composition and physiology of the epidermis in lily bulb scales is essential to identify the regeneration stimulating signals originating from this tissue layer in Lilium sp Finally in Chapter 7 integration of all the results was done and I addressed how this may contributes to the fundamental and applied understanding of vegetative propagation in bulbous plants Also some challenges are discussed for example the complexity in the architecture of tulip bulbs and how this influences ways for improving its rate of axillary bud outgrowth The challenge to prove the findings of this thesis through functional analysis is also discussed and the possibility of using transient virus-induced gene silencing is highlighted Moreover the potential of lily bulb scales as a model system to study some aspects of de novo regeneration as well as to study the recalcitrance of in vitro propagation is highlighted supporting the idea that more omics data and biotechnological tools for bulbous plant research are necessary

Mechanisms of vegetative propagation in bulbs : a molecular approach

Phytophthora infestans : bescherming van het zich ontwikkelende groeipunt van aardappelplanten door fungiciden

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Mechanisms of vegetative propagation in bulbs : a molecular approach

Phytophthora infestans : bescherming van het zich ontwikkelende groeipunt van aardappelplanten door fungiciden

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with scheuttoppen as keyword