The final morphology of a plant is determined by the activity of two poles of growth that form early during embryogenesis, the root meristem (RAM) and the shoot meristem (SAM). The determination of all the aerial organs, and ultimately of the whole plant architecture, depends on the SAM. The shoot apical meristem is a small group of cells at the shoot apex characterized by a central zone with pluripotent slow dividing cells that is important for meristem maintenance (stem cells), and a peripheral zone where cells rapidly divide to give rise to lateral organs. The type of organ produced by the apical meristem depends on the developmental stage of the plant: during the vegetative phase, leaves and lateral vegetative meristems will form whereas lateral buds and inflorescence stems will develop in the adult phase and proper flowers only during the reproductive phase.
Hence, all of the aerial organ formation and phase-change from vegetative to reproductive stage will result from the equilibrium between cell division and cell differentiation in the SAM as well as from the SAM ability to respond to internal and external cues. These processes are under a strict genetic (transcription factors, chromatin structure and methylation state regulators) and hormonal (cytokinins, auxin, gibberellin) control.
KNOX genes encode for a small family of homeobox transcription factors that play a relevant role in meristem formation, maintenance of indeterminate cell fate and leaf primordia determination. The exclusion of class I KNOX expression in peripheral regions of the SAM is believed to be instrumental in the acquisition of leaf fate.
Our research group in Rome is interested in studying the role of KNOX transcription factors in leaf morphology and in the whole plant architecture determination in different plant species (lettuce, peach, forage legume and Arabidopsis). As a matter of fact, the modulation of KNOX gene expression strongly affects the temporal-spatial development of aerial organs as well as the vegetative-to-reproductive phase change. These effects are likely to be mediated by the alteration of hormone homeostasis. Indeed, we have shown that the overexpression of the Arabidopsis gene KNAT1 in lettuce strongly affects leaf determination and morphology, that associated to an overproduction of IP-type active cytokinins. That allowed us to hypothesize that cytokinins may play a relevant role in the determination of leaf shape by acting on the vascular network formation that underlies leaf development.
The modulation of KNOX gene expression also alters other aspects of plant growth like vegetative phase length, apical dominance, cell fate determination, the competence to shoot regeneration and senescence processes. Our research activity aims to determine the function of the different members of KNOX family in plant developmental processes, hormone homeostasis and plant architecture control, to understand which exogenous and endogenous signals may regulate or modulate their activity as well as to identify the target genes that KNOX factors regulate to exert their function. Ultimately, we would like to uncover the role played by this gene family in the development of inter and intra-specific morphological diversity and to test whether and how the modulation of KNOX gene expression can be finalized to the improvement of agronomical traits in crop species.
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