Le contrôle moléculaire du patterning au cours des processus développementaux est aujourd’hui bien décrit chez les organismes multicellulaires. A l’inverse, la contribution de la croissance dans l’émergence des patterns reste peu explorée, et est souvent réduite à un rôle passif. Au cours de cette thèse, j’ai étudié cette question en utilisant le méristème apical caulinaire (MAC) d’Arabidopsis comme modèle. Le méristème est un groupe de cellules en divisions situé à l’extrémité de toutes les tiges et les branches et qui génère tous les organes aériens de la plante selon un patron stéréotypé, aussi appelé phyllotaxie. Dans une première partie, j’ai étudié comment la croissance de la tige pouvait influencer le patron phyllotactique. Plus précisément, en découplant dépôt de la cellulose dans la paroi et l’orientation des microtubules, j’ai montré que le patron de phyllotaxie devenait bimodal en raison de l’induction d’une torsion lors de la croissance de la tige. Dans une seconde partie, j’ai analysé le lien entre forme du MAC et expression génétique. En particulier, j’ai pu corréler l’expression d’un gène maître : SHOOTMERISTEM LESS (STM) au degré de courbure dans le MAC. De plus, en utilisant des approches de micromécaniques, j’ai aussi pu montrer que l’expression de STM pouvait être induite par le patron de contraintes localement généré par la courbure. Pour finir, j’ai aussi étudié comment la taille du méristème influence la robustesse du pattern de phyllotaxie sur la tige en modulant la fréquence d’initiation des organes. L’ensemble de ce travail met ainsi en avant le rôle de la croissance dans le patterning, notamment via des mécanismes de rétrocontrôles géométriques et mécaniques. / The molecular mechanisms behind the emergence of patterns during developmental processes have been well described in multicellular organisms. However, the contribution of growth in patterning is still poorly understood; growth is often seen as a passive output of the activity of the patterning signals. In this PhD, I have studied the relation between growth and patterning using the shoot apical meristem of Arabidopsis as a model system. The meristem is a group of dividing cells located at the tip of every stems and branches that generates all the aerial organs of the plant following a typical spatio-temporal pattern also called phyllotaxis. In a first part, the influence of post-meristematic growth on phyllotaxis was assessed. More precisely, by uncoupling cellulose deposition from the orientation of the microtubule array, I showed that the resulting stem torsion induces the emergence of a new and robust bimodal phyllotactic pattern. In a second part, the relation between meristem shape and gene expression was analyzed. More precisely, I correlated the expression of a master regulatory gene: SHOOT MERISTEMLESS (STM) to tissue curvature in the boundary domain that separates the emerging organ from the meristem. Furthermore, I showed that STM expression can be induced by micromechanical perturbations thus suggesting that shape-derived mechanical stresses in the meristem boundary contribute to STM expression. Finally, I also studied how meristem size can influence the robustness of the pattern of phyllotaxis along the stem through a modulation of the frequency of organ initiation. Altogether, this work highlights the important contribution of growth in patterning, notably thanks to the existence of geometrical and mechanical feedbacks.

No description available.

Among the major phytohormones, the cytokinin exhibits unique features for its ability to positively affect the developmental status of plastids. Even early on in its research, cytokinins were known to promote plastid differentiation and to reduce the loss of chlorophyll in detached leaves. Since the discovery of the components of cytokinin perception and primary signaling, the genes involved in photosynthesis and plastid differentiation have been identified as those directly targeted by type-B response regulators. Furthermore, cytokinins are known to modulate versatile cellular processes such as promoting the division and differentiation of cells and, in concert with auxin, initiating the de novo formation of shoot apical meristem (SAM) in tissue cultures. Yet how cytokinins precisely participate in such diverse cellular phenomena, and how the associated cellular processes are coordinated as a whole, remains unclear. A plausible presumption that would account for the coordinated gene expression is the tight and reciprocal communication between the nucleus and plastid. The fact that cytokinins affect plastid developmental status via gene expression in both the nucleus and plastid is interpreted here to suggest that cytokinin functions as an initiator of anterograde (nucleus-to-plastid) signaling. Based on this viewpoint, we first summarize the physiological relevance of cytokinins to the coordination of plastid differentiation with de novo shoot organogenesis in tissue culture systems. Next, the role of endogenous cytokinins in influencing plastid differentiation within the SAM of intact plants is discussed. Finally, a presumed plastid-derived signal in response to cytokinins for coupled nuclear gene expression is proposed.

No description available.

[ES] En los cítricos, la baja temperatura promueve la inducción floral en otoño-invierno aumentando la expresión del gen promotor CiFT3 (homólogo en los cítricos del gen FLOWERING LOCUS T). La presencia de un gran número de frutos en el árbol durante ese momento inhibe la expresión de CiFT3 y la floración, pero se desconoce la señal inhibitoria que genera el fruto. Las hipótesis mayormente aceptadas proponen que la señal puede ser hormonal o nutricional. En el primer caso, el efecto inhibidor se atribuye a las hormonas que el fruto produce y exporta durante su desarrollo. En el segundo caso, el efecto inhibidor se atribuye a la alta demanda y consumo de carbohidratos por los frutos en desarrollo. Ambas hipótesis son complementarias y no excluyentes entre sí. Además, se ha demostrado que el fruto promueve la activación epigenética del represor de la floración CcMADS19 (homólogo en los cítricos del gen FLOWERING LOCUS C), que inhibe la expresión del gen CiFT3. Con el objetivo de determinar qué señal produce el fruto para inhibir la floración, en esta Tesis se propone la siguiente hipótesis: El fruto inhibe la floración a través de la síntesis y exportación de auxinas que activa la síntesis de giberelinas y, a su vez, la expresión de CcMADS19. Mediante experimentos con tratamientos exógenos de auxinas, giberelinas, y sus antagonistas, aclareo de frutos, y la interrupción del transporte por el floema entre el fruto y las yemas, los resultados indican que ni las giberelinas ni las auxinas se relacionan de forma consistente con la activación de la expresión de CcMADS19 en las hojas. En las yemas, las giberelinas se relacionan con la activación del gen inhibidor CENTRORRADIALIS (CEN), cuando hay fruto por aumento de la síntesis de GA4, y cuando no hay fruto por su aplicación exógena. La presencia del fruto aumenta la concentración de auxinas en el tallo y la yema en el momento de la inducción, y reprime su síntesis y trasporte. Pero esto no impide que, en la yema, el gen CcMADS19 esté epigenéticamente silenciado y que el silenciamiento se transmita a los nuevos brotes vegetativos. Estos brotes florecen en el siguiente ciclo, y, en sus yemas, la diferenciación floral se relaciona con un aumento de la síntesis y trasporte de auxinas y una reducción de la síntesis de giberelinas. / [CA] Als cítrics, les baixes temperatures promouen la inducció floral a la tardor i l'hivern augmentant l'expressió del gen promotor CiFT3 (homòleg en els cítrics del gen FLOWERING LOCUS T). La presència d'un gran nombre de fruita a l'arbre en aquest moment inhibeix l'expressió de CiFT3 i la floració, però es desconeix la senyal inhibidora que genera la fruita. Les hipòtesis majoritàriament acceptades proposen que la senyal pot ser hormonal o nutricional. En el primer cas, l'efecte inhibidor s'atribueix a les hormones que la fruita produeix i exporta durant el seu desenvolupament. En el segon cas, l'efecte inhibidor s'atribueix a la alta demanda i consum de carbohidrats per part de la fruita en desenvolupament. Ambdues hipòtesis són complementàries i no es descarten mútuament. A més, s'ha demostrat que la fruita promou l'activació epigenètica del repressor de la floració CcMADS19 (homòleg en els cítrics del gen FLOWERING LOCUS C), que inhibeix l'expressió del gen CiFT3. Amb l'objectiu de determinar quina senyal produeix la fruita per inhibir la floració, en aquesta Tesi es proposa la següent hipòtesi: La fruita inhibeix la floració mitjançant la síntesi i exportació d'auxines que activa la síntesi de giberelines i, al seu torn, l'expressió de CcMADS19. Mitjançant experiments amb tractaments exògens d'auxines, giberelines i els seus antagonistes, aclarida de fruita i la interrupció del transport pel floema entre la fruita i les brots, els resultats indiquen que ni les giberelines ni les auxines es relacionen de manera consistent amb l'activació de l'expressió de CcMADS19 a les fulles. A les gemmes, les giberelines es relacionen amb l'activació del gen inhibidor CENTRORRADIALIS (CEN) quan hi ha fruita per l'augment de la síntesi de GA4 i quan no hi ha fruita per la seua aplicació exògena. La presència de la fruita augmenta la concentració d'auxines a la tija i la gemma en el moment de la inducció i reprimeix la seua síntesi i transport. Però això no impedeix que, a la gemma, el gen CcMADS19 estigui epigenèticament silenciat i que el silenciament es transmeti als nous brots vegetatius. Aquests brots floreixen al següent cicle i, a les seues gemmes, la diferenciació floral es relaciona amb un augment de la síntesi i transport d'auxines i una reducció de la síntesi de giberelines. / [EN] In citrus, low temperature promotes flower induction in autumn-winter by increasing the expression of the CiFT3 promoter gene (citrus homologue of the FLOWERING LOCUS T gene). The presence of large numbers of fruits on the tree at this time inhibits CiFT3 expression and flowering, but the inhibitory signal produced by the fruits is unknown. The most widely accepted hypotheses are that the signal is hormonal or nutritional. In the first case, the inhibitory effect is attributed to hormones produced and exported by the fruit during development. In the second case, the inhibitory effect is attributed to the high demand and consumption of carbohydrates by the developing fruit. The two hypotheses are complementary and not mutually exclusive. In addition, it has been shown that the fruit promotes the epigenetic activation of the flowering repressor CcMADS19 (citrus homolog of the FLOWERING LOCUS C gene), which inhibits the expression of the CiFT3 gene. To determine which signal is produced by the fruit to inhibit flowering, the following hypothesis is proposed in this thesis: The fruit inhibits flowering through the synthesis and export of auxins, which activates the synthesis of gibberellins and, in turn, the expression of CcMADS19. Experiments with exogenous treatments of auxins, gibberellins and their antagonists, fruit thinning, and disruption of phloem transport between fruit and buds indicate that neither gibberellins nor auxins are consistently associated with the activation of CcMADS19 expression in leaves. In buds, gibberellins are associated with the activation of the flowering inhibitor CENTRORADIALIS (CEN), in the presence of fruit by increasing GA4 synthesis, and in the absence of fruit by its exogenous application. The presence of fruit increases the concentration of auxin in the stem and bud at the time of induction and suppresses its synthesis and transport. However, this does not prevent the epigenetic silencing of the CcMADS19 gene in the bud, which is transmitted to the leaves of the new vegetative shoots. These shoots flower in the following cycle, where floral differentiation is associated with an increase in auxin synthesis and transport and a decrease in gibberellin synthesis in the bud. / Marzal Blay, A. (2024). Study of the Fruit Inhibitory Mechanism on Citrus flowering. Nutritional, Hormonal and Genetic Factors [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/203155

Au vu de l’impact croissant du changement climatique global et en particulier de la sécheresse sur les forêts, il est nécessaire de comprendre les mécanismes de réponse des arbres face à des variations de disponibilité en eau. Ces dernières années, des études ont montré un contrôle épigénétique et notamment par la méthylation de l’ADN de la plasticité phénotypique des plantes en réponse aux variations environnementales. Dans ce contexte, cette thèse visait à évaluer le rôle de la méthylation de l’ADN des cellules du méristème apical caulinaire dans la plasticité développementale de la tige feuillée en réponse à des variations de disponibilité en eau chez le peuplier, un arbre modèle. A cette fin, le méthylome de la chromatine non condensée dans le méristème apical caulinaire de Populus trichocarpa a été caractérisé. Ensuite, l’impact de variations de disponibilité en eau sur la méthylation de l’ADN a été étudié dans l’apex caulinaire de différents hybrides (P. × euramericana). Les loci et les réseaux de gènes affectés pour leur expression et leur méthylation ont ainsi été identifiés. Ces travaux ont montré que dans le méristème apical caulinaire, la majorité des gènes étaient dans un état non condensé de la chromatine et méthylés dans leur corps. Ils ont également mis en évidence une forte variation de la méthylation globale de l’ADN selon les génotypes et en réponse à des variations de disponibilité en eau. De plus, des corrélations ont été établies entre les niveaux de croissance des arbres et de méthylation globale de l’ADN dans l’apex caulinaire. Enfin, les variations de la méthylation de l’ADN en réponse aux variations de la disponibilité en eau s’accompagnent de variations d’expression et ont ciblé particulièrement des gènes impliqués dans la signalisation par les phytohormones ou la morphogenèse. Ainsi, les travaux effectués lors de cette thèse suggèrent un rôle de la méthylation de l’ADN dans la plasticité phénotypique en réponse à des variations de disponibilité en eau chez le peuplier via le contrôle de l’expression de réseaux de gènes dans le méristème apical caulinaire. / Predicted climate changes and particularly drought represent a major threat to forest health. Therefore, understanding mechanisms that control trees response to variations in water availability is of great interest. These last years, epigenetic marks such as DNA methylation have been involved in plant phenotypic plasticity in response to environmental stresses. In this context, this work aimed at assessing the role of shoot apical meristem cells DNA methylation in the shoot developmental plasticity towards variations in water availability in poplar, a model tree. For this purpose, the methylome of non condensed chromatin in Populus trichocarpa shoot apical meristem was characterized. Then, the impact of variations in water availability on shoot apex DNA methylation in different hybrids (P. × euramericana) was studied. Loci and gene networks affected by DNA methylation and expression changes were thus identified. This work showed that in shoot apical meristem, most of the genes was in non condensed chromatin state with DNA methylation in their body. A strong variation in DNA methylation depending on genotypes and water availability was highlighted. Moreover, correlations between trees growth and shoot apex DNA methylation levels were established. Lastly, DNA methylation changes in response to variations in water availability correlated to expression variations were identified for genomic loci and gene networks. Thus, the work performed during this thesis suggests a role for DNA methylation in poplar phenotypic plasticity in response to variations in water availability through the control of gene networks transcription in the shoot apical meristem.

Much is known about how organs acquire their identity, yet we are only beginning to learn how their shape is regulated. Recent work has elucidated the role of coordinated cell division & expansion in determining plant organ shape. For instance, in Antirrhinum, leaf shape is affected in the cincinnata (cin) mutant because of an alteration in the cell division pattern. CIN codes for a TCP transcription factor and controls cell proliferation. It is unclear how exactly CIN-like genes regulate leaf morphogenesis. We have taken biochemical and genetic approach to understand the TCP function in general and the role of CIN-like genes in leaf morphogenesis in Antirrhinum and Arabidopsis. Targets of CINCINNATA To understand how CIN controls Antirrhinum leaf shape, we first determined the consensus target site of CIN as GTGGTCCC by carrying out RBSS assay. Mutating each of this target sequence, we determined the core binding sequence as TGGNCC. Hence, all potential direct targets of CIN are expected to contain a TGGNCC sequence. Earlier studies suggested that CIN activates certain target genes that in turn repress cell proliferation. To identify these targets, we compared global transcripts of WT and cin leaves by differential display PCR and have identified 18 unique, differentially expressed transcripts. To screen the entire repertoire of differentially expressed transcripts, we have carried out extensive micro-array analysis using 44K Arabidopsis chips as well as 13K custom-made Antirrhinum chips. Combining the RBSS data with the results obtained from the micro-array experiments, we identified several targets of CIN. In short, CIN controls expression of the differentiation-specific genes from tip to base in a gradient manner. In cin, such gradient is delayed, thereby delaying differentiation. We also find that gibberellic acid, cytokinin and auxin play important role in controlling leaf growth. Genetic characterization of CIN-homologues in Arabidopsis Arabidopsis has 24 TCP genes. Our work and reports from other groups have shown that TCP2, 4 and 10 are likely to be involved in leaf morphogenesis. These genes are controlled by a micro RNA miR319. To study the role of TCP4, the likely orthologue of CIN, we generated both stable and inducible RNAi lines. Down-regulation of TCP4 transcript resulted in crinkly leaves, establishing the role of TCP4 in leaf shape. To study the function of TCP2, 4 & 10 in more detail, we isolated insertion mutants in these loci. The strongest allele of TCP4 showed embryonic lethal phenotype, indicating a role for TCP4 in embryo growth. All other mutants showed mild effect on leaf shape, suggesting their redundant role. Therefore, we generated and studied various combinations of double and triple mutants to learn the concerted role of these genes on leaf morphogenesis. To further study the role of TCP4 in leaf development, we generated inducible RNAi and miRNA-resistant TCP4 transgenic lines and carried out studies with transient down-regulation and up-regulation of TCP4 function. Upon induction, leaf size increased in RNAi transgenic plants whereas reduced drastically in miR319 resistant lines, suggesting that both temporal & spatial regulation of TCP4 is required for leaf development. Biochemical characterization of TCP domain To study the DNA-binding properties of TCP4, random binding site selection assay (RBSS) was carried out and it was found that TCP4 binds to a consensus sequence of GTGGTCCC. By patmatch search and RT-PCR analysis, we have shown that one among 74 putative targets, EEL (a gene involved in embryo development), was down regulated in the RNAi lines of TCP4. This suggests that EEL could be the direct target of TCP4. We have tested this possibility in planta by generating transgenic lines in which GUS reporter gene is driven by EEL upstream region with either wild type or mutated TCP4 binding site. GUS analysis of embryos shows that transgenic with mutated upstream region had significantly reduced reporter activity in comparison to wild type, suggesting that EEL is a direct target of TCP4. We have further shown that TCP4 also binds to the upstream region of LOX2, a gene involved in Jasmonic acid (JA) biosynthesis (in collaboration with D. Weigel, MPI, Tubingen, Germany). TCP domain has a stretch of basic residues followed by a predicted helix-loop-helix region (bHLH), although it has little sequence homology with canonical bHLH proteins. This suggests that TCP is a novel and uncharacterized bHLH domain. We have characterized DNA-binding specificities of TCP4 domain. We show that TCP domain binds to the major groove of DNA with binding specificity comparable to that of bHLH proteins. We also show that helical structure is induced in the basic region upon DNA binding. To determine the amino acid residues important for DNA binding, we have generated point mutants of TCP domain that bind to the DNA with varied strength. Our analysis shows that the basic region is important for DNA binding whereas the helix-loop-helix region is involved in dimerization. Based on these results, we have generated a molecular model for TCP domain bound to DNA (in Collaboration with Prof. N. Srinivasan, IISc, Bangalore). This model was validated by further site-directed mutagenesis of key residues and in vitro assay. Functional analysis of TCP4 in budding yeast To assess TCP4 function in regulation of eukaryotic cell division, we have introduced TCP4 in S. cerevisiae under the GAL inducible promoter. TCP4 induction in yeast cells always slowed down its growth, indicative of its detrimental effect on yeast cell division. Flow cytometry analysis of synchronized cells revealed that TCP4 arrests yeast cell division specifically at G1→S boundary. Moreover, induced cells showed distorted cell morphology resembling shmoo phenotype. Shmooing is a developmental process which usually happened when the haploid cells get exposed to the cells of opposite mating type and get arrested at late G1 phase due to the inhibition of cdc28-cln2 complex. This suggested that TCP4-induced yeast cells are arrested at late G1 phase probably by the inhibition of cdc28-cln2 complex. To further investigate how TCP4 induce G1→S arrest, we carried out microarray analysis and found expression of several cell cycle markers significantly altered in TCP4-induced yeast cells. Studies on crinkly1, a novel leaf mutant in Arabidopsis To identify new genes involved in leaf morphogenesis, we have identified crinkly1 (crk1), a mutant where leaf shape and size are altered. We observed that crk1 also makes more number of leaves compared to wild type. Phenotypic analysis showed that crk1 leaf size is ~5 times smaller than that of wild type. Scanning electron microscopy (SEM) showed that both cell size and number are reduced in the mutant leaf, which explains its smaller size. We have mapped CRK1 within 3 cM on IV chromosome.

Plant form is a product of emergent processes of cell division, patterning and morphogenesis. These fundamental processes remain poorly characterised in plants. However, engineering approaches can provide new tools and frameworks for the study and manipulation of plant development. This dissertation describes the development of engineering frameworks for reprogramming of the early diverging land plant Marchantia polymorpha (Marchantia). I describe the generation of genomic and transcriptomic datasets for Marchantia, which has provided the basis for the compilation of a gene-centric registry of DNA parts for engineering (MarpoDB). I describe the development of Loop assembly, an efficient and standardised DNA assembly system based on Type IIS restriction enzymes for recursive fabrication of DNA circuits with high efficiency. MarpoDB was used to mine new DNA parts compatible with Loop assembly which were used to generate plant transformation vectors for labelling of cellular features to study aspects of growth and development. I performed image analysis of genetic markers for segmentation and quantification of cellular properties in germinating gemmae. I implemented high-efficiency Cas9-mediated mutagenesis in Marchantia for use in functional molecular genetics studies. Furthermore, I produced inducible systems for expression of heterologous elements by transactivation which showed negligible levels of basal activity. It was possible to use this system for induction of gene expression in single cells. Finally, these new frameworks were applied to study the gametophytic meristem in Marchantia gemmae. I mapped the expression of several putative candidate homologues for higher plant meristem regulators, performed overexpression and loss-of-function studies for homologues of WUSCHEL, CLAVATA3 and SHOOT MERISTEMLESS. A strategy for misregulation of endogenous genes was developed using inducible transactivation, and was used with cellular markers for WUSCHEL and CLAVATA3 homologues in Marchantia.

La morphogenèse des plantes repose sur deux mécanismes cellulaires : la division et l'élongation. Par ailleurs, la croissance est source de contraintes mécaniques qui affectent les cellules et guident la morphogenèse. Si les contraintes mécaniques influencent l'orientation du plan de division dans les cellules animales, rien n'est prouvé pour les cellules végétales. À l'heure actuelle, la forme de la cellule est proposée comme le facteur principal gouvernant l'orientation du plan dans les divisions symétriques : les cellules se divisent selon un des plans les plus courts. Cette règle géométrique a été validée dans des tissus à croissance ou courbure isotropes, mais les mécanismes moléculaires sous-jacents demeurent inconnus. Dans cette thèse, un pipeline a été mis au point pour analyser les divisions cellulaires dans les différents domaines du méristème apical caulinaire d'Arabidopsis thaliana et questionner l'application de la règle géométrique dans ce tissu. La zone frontière du méristème présente une proportion anormalement basse de plans de division très courts. Des simulations de tissus en croissance, dans lesquelles une règle de division mécanique a été implémentée, ont montrées le même biais sur les orientation des plans, comparé à la règle géométrique. Des ablations laser de quelques cellules de l'épiderme ont également été effectuées afin de perturber localement le patron de contraintes mécaniques. Les résultats montrent que l'orientation du plan des divisions postérieures à cette perturbation suit le nouveau patron de contraintes. Enfin, une nouvelle méthode quantitative, basée sur l'utilisation d'un micro-indenteur, a été mise au point pour quantifier la réponse du cytosquelette, et en particulier des microtubules, aux contraintes mécaniques. Le protocole de compression a été testé et validé sur les mutants katanin et spiral2, dans lesquels la réponse aux contraintes est respectivement faible ou amplifiée. / Morphogenesis during primary plant growth is driven by cell division and elongation. In turn, growth generates mechanical stress, which impacts cellular events and channels morphogenesis. Mechanical stress impacts the orientation of division plane in single animal cells; this remains to be fully demonstrated in plants. Currently, cell geometry is proposed to be the main factor determining plane orientation in symmetric divisions: cell divide along one the shortest paths. This geometrical rule was tested on tissues with rather isotropic shapes or growth and the corresponding molecular mechanism remains unknown, although it could involve tension within the cytoskeleton. To address these shortcomings, we developed a pipeline to analyze cell divisions in the different domains of the shoot apical meristem of Arabidopsis thaliana. We computed the probability of each possible planes according to cell geometry and compared the output to observed orientations. A quarter of the cells did not follow the geometrical rule. Boundary domain was enriched in long planes aligned with supracellular maximal tension lines. Computer simulations of a growing tissue following a division rule that relies on tension gave the most realistic outputs. Mechanical perturbations of local stress pattern, by laser ablations, further confirmed the importance of mechanical stress in cell division. To explore the role of microtubules in this process, we developed a microindenter-based protocol to quantify the cytoskeletal response to mechanical stress. This protocol was tested and validated in the katanin and spiral2 mutants, in which the response to stress is delayed or promoted respectively.

In angiosperms, specialized reproductive structures are borne in flowers to ensure their reproductive success. After the vegetative growth, plants undergo reproductive phase change to produce flowers. Floral meristems (FMs) are generated on the flanks of inflorescence and groups of specialized stem cells in the FM differentiate into four whorls of organs of a flower. In dicots, floral meristem successively gives rise to sepals, petals, stamens and carpels; after which it terminates. The fate of organs formed on FM is under the control of genetic regulators, key among which are members of MADS box transcription factor family. Their individual and combined act confers distinct identities to floral organs. Grass flowers are highly modified in structure. Rice flower, a model for grasses, is borne on a short branch called spikelet and they together from the basic structural units of the rice infloresences known as panicle. The outer whorl organs of a grass floret are bract-like structures known as lemma and palea to dicot sepals is highly dibated (see Chapter 1). In grass florets, petal homologs are a pair of highly reduced, fleshy bracts known as lodicules, while stamen and carpel homologs occupy the same position and share the same functions as their dicot counterparts. Aside from these distinct outer whorl organs, the florets are subtended by two pairs of bracts known as empty glumes and rudimentary glumes. The genetic regulators that control their unique identities and those that perform conserved functions are very intriguing and central questions in plant developmental biology. Using various contemporary and complementary technologies, we have analysed the molecular functions and downstream pathways of a MADS box transcription factor, OsMADSI during the rice floret meristem specification and organ development. Further by reverse genetics and overexpression studies, we have also functionally characterized two target genes of OsMADSI, OsETTINI and OsETTINI2 to understand their roles downstream to OsMADSI during the rice floret development.