hirsuta and tomato, downregulation of KNOXI genes result in compound leaves with reduced leaflets. hirsuta, KNOXI genes are reactivated in leaf primordia after initial downregulation to initiate leaf development. In species with simple leaves, downregulation of KNOXI genes is permanent, whereas in most compound-leafed eudicot species, including tomato and C. Leaf initiation requires downregulation of KNOXI genes at the incipient sites of leaf primordia (P0, P for plastochron). Studies have shown that class I knotted-like homeodomain transcription factors, KNOXI proteins, are required for the maintenance of the meristematic activity of the SAM. Plant leaves, either simple or compound, initiate as peg-like structures from the flanks of the shoot apical meristem (SAM), a pluripotent structure capable of self-renewal. These provide opportunities to compare the regulatory mechanisms for compound- and simple-leaf development and provide new insights on leaf form evolution. In the last few decades, many genetic loci responsible for some of the classic leaf form mutants have been cloned, unveiling the nature of genes and the associated regulatory mechanisms underlying leaf development in diverse species. thaliana), tobacco ( Nicotiana tabacum), pea ( Pisum sativum), tomato ( Solanum lycopersicum), C. In earlier studies, numerous leaf form mutants have been isolated and characterized in both simple- and compound-leafed species, such as maize ( Zea may), snapdragon ( Antirrhinum majus), Arabidopsis ( A. Consistent with this, both conserved and distinct molecular mechanisms underlying the development of different leaf forms have been uncovered in different lineages. Studies of leaf evolution suggest that compound leaves evolved independently several times during plant evolution. The diverse leaf shape and form variation seen in extant plants may reflect natural selection on the function of leaves as the primary photosynthetic organ critical to plant growth and survival. lanceolata) and Medicago truncatula, in which leaf forms change from simple to compound along the shoot axis. In addition, different leaf forms can be found in the same plant, such as green ash ( Fraxinus pennsylvanica var. Different leaf forms can be found in closely-related plants, such as Arabidopsis thaliana, with simple leaves, and Cardamine hirsuta, with compound leaves. Many plants exhibit consistent leaf forms, although variations in the leaf morphology, called heteroblasty, often occur during the course of plant growth. Both developmental and environmental factors influence leaf morphogenesis. A simple leaf consists of a single undivided blade with an entire, serrated or lobed margin, and a compound leaf consists of multiple blades, known as leaflets, organized as pinnate, palmate or higher-ordered structures. Conceptually, leaf forms can be broadly grouped into two categories: simple and compound or dissected. From ferns to angiosperms, enormous variations in leaf forms have been evolved. The photosynthetic structures of the earliest vascular plants are believed to be branched axes, which evolved into three-dimensional lateral branch systems identified as the earliest leaves in the fossil record. Together, photosynthetic organisms, including plants, algae and some bacteria, capture approximately 100 terawatts (10 12 watts) of solar energy and convert about 100–150 petagrams (10 15 grams) of carbon into biomass per year. As such, it provides the fixed carbon for plant growth and atmospheric oxygen essential for aerobic lives. Plant leaves are the primary photosynthetic organs responsible for the conversion of carbon dioxide (CO 2) and water (H 2O) to carbohydrate (C 6H 12O 6) and oxygen (O 2) using solar energy.
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