ZmPIN Genes And Polar Auxin Transport In Maize: Roles on Kernel Development & Plant Architecture Determination

Due to the unquestionable importance of the polar auxin transport in controlling a multitude of developmental processes in plants, several research projects are ongoing towards a better understanding of the genetic regulation and physiology of auxin mediated?morphogenesis. Several components of the auxin transport machinery, and their regulatory networks, have been identified in the model specie Arabidopsis thaliana, shedding light on the molecular and cytological mechanism underlying this unique phenomenon for transmitting spatial and temporal signals in plant development. Evolutionary analysis revealed sequences and functions conservation of the auxin transport components in the entire plant kingdom. Neverthless, very few information are available on the role of auxin polar transport on plant patterning outside the Arabidopsis world. Auxin transport inhibition and analysis of mutants putatively impaired in PAT are associated with several developmental defects in tomato, rice, maize and Arabidopsis as well, suggesting that this phytohormone plays a fundamental role in plant development both in di? and mono?cotyledon. The general purpose of my research was to investigate the role of auxin, and in particular of its PIN?mediated polar transport, during Zea mays development. I investigated the behaviour of PIN genes and proteins during maize embryonic, vegetative and reproductive development by several different approaches. The final gol of this analysis was the confirmation of the primary importance of PAT in this monocotyledonous species, as highlighted in Arabidopsis thaliana. Initially I focused the attention to the identification of PIN orthologous genes in maize and to the analysis of their expression patterns during embryonic and post?embryonic development. Subsequently, the orthologs of Arabidopsis PIN1, PIN2, PIN3, PIN4 and PIN5 were identified in maize, and their transcripts were showed to have differential expression pattern during maize development. Furthermore, the availability of an anti?AtPIN1 monoclonal antibody already tested in maize, led us to analyze ZmPIN1 proteins localization in different maize tissues, revealing maize auxin efflux carriers polarization during tissues and organ differentiation. Based on ZmPIN1 protein polarization, auxin fluxes were inferred and these were then compared with auxin gradients visualized in embryos, endosperm, meristems and inflorescences, utilizing an anti?IAA antibody. The parallel analysis of transcript expression domains, protein localization and polarization and IAA accumulation allowed the prediction of a model for the role of PAT in maize meristem functioning and primordia differentiation. A second model was formulated for the ZmPIN1?mediated transport of auxin and for the related auxin fluxes during maize embryogenesis and endosperm development. These two models were implemented also with data obtained from mutant analysis and from PAT inhibition studies. A further aim of my research was the study of the mechanisms underlying plasmamembrane insertion of ZmPIN1 proteins in maize. Different ZmPIN1 protein localization patterns in different tissues were observed during our immunolocalization assays. To assess if these different patterns are the results of tissue?specific signals controlling proteins localization or if the three ZmPIN1 proteins have sequence?specific localizations, we analyzed the cell membrane targeting ZmPIN1::GFP fusion constructs in homologous and heterologous systems.

Il mais (Zea mays) è, a livello mondiale, una delle specie vegetali più importanti dal punto di vista industriale ed alimentare; per questo conoscere i meccanismi genetici che regolano lo sviluppo della pianta, dall’apparato radicale fino alle sue infiorescenze, passando attraverso le varie fasi dello sviluppo della cariosside e dell’embrione, permetterebbe di intervenire con la selezione per migliorarne i caratteri produttivi desiderati. In questo contesto, lo studio dell’espressione dei geni che regolano lo sviluppo, ha permesso di chiarire solo parzialmente i meccanismi alla base del differenziamento delle radici, delle foglie, delle infiorescenze, dei fiori e quindi delle cariossidi. Numerosi geni responsabili dell’identità meristematica, ad esempio KN1 e geni responsabili del differenziamento delle strutture riproduttive, quali bif1, bif2, ba1, ramosa1, ramosa2 e ramosa3, interagiscono con modalità che non sono ancora completamente note. Inoltre, non è stato ancora spiegato esaustivamente il ruolo che sostanze ormonali quali l’auxina e le citochinine svolgono nella regolazione della fillotassi e nella formazione delle strutture riproduttive dei vegetali superiori. L'obiettivo generale di questo progetto è chiarire il ruolo svolto dall'auxina e in particolare dal suo trasporto polare durante lo sviluppo della pianta di mais. A questo scopo si stanno analizzando i pattern di espressione dei geni e delle proteine coinvolte nel trasporto auxinico a partire dall’embriogenesi e la formazione delle cariossidi, fino alla differenziazione dei primordi nel SAM e dei meristemi secondari nelle infiorescenze.