plants Review Coordination of Leaf Development Across Developmental Axes James W. Satterlee and Michael J. Scanlon * School of Integrative Plant Science, Cornell University, Ithaca, NY 14853, USA; [email protected] * Correspondence: [email protected]; Tel.: +01-607-254-1156 Received: 10 September 2019; Accepted: 18 October 2019; Published: 22 October 2019 Abstract: Leaves are initiated as lateral outgrowths from shoot apical meristems throughout the vegetative life of the plant. To achieve proper developmental patterning, cell-type specification and growth must occur in an organized fashion along the proximodistal (base-to-tip), mediolateral (central-to-edge), and adaxial–abaxial (top-bottom) axes of the developing leaf. Early studies of mutants with defects in patterning along multiple leaf axes suggested that patterning must be coordinated across developmental axes. Decades later, we now recognize that a highly complex and interconnected transcriptional network of patterning genes and hormones underlies leaf development. Here, we review the molecular genetic mechanisms by which leaf development is coordinated across leaf axes. Such coordination likely plays an important role in ensuring the reproducible phenotypic outcomes of leaf morphogenesis. Keywords: leaf; developmental patterning; transcription factors; plant hormones; differentiation 1. Introduction The morphology of angiosperm leaves is extraordinarily diverse, and is a key component of natural variation in plant architecture. Nonetheless, all leaves share two ontogenetic features in common; they are (1) derived from the shoot apical meristem (SAM) and (2) asymmetrical from their inception [1]. The SAM is a stem cell reservoir at the growing tip(s) of the plant that ultimately supplies cells for all the above-ground organs. In this way, the SAM accounts for the continuous development of vegetative structures far beyond embryogenesis, which comprises a critical, strategic difference between plant and animal development. A leaf transitions from its origins as a small primordium to its mature form through the establishment and maintenance of three developmental axes upon which growth and differentiation proceed (Figure1A,B) [ 2,3]. The leaf proximodistal axis (i.e., base-to-tip) is defined by the polarized growth of leaf initials away from the shoot, and later becomes elaborated by the establishment of proximal and distal cell and tissue types. For example, in many eudicot leaves, such as those of Arabidopsis, the leaf is subdivided into the proximal petiole and distal lamina. Leaf development also involves specialization of the upper and lower leaf surfaces, defining an adaxial–abaxial (top-bottom) axis of asymmetry. At its inception, the leaf primordium possesses inherent asymmetry along this axis due to the proximity of the adaxial leaf surface to the SAM relative to the abaxial leaf surface [1,4]. This early asymmetry drives the differentiation of diverging cell and tissue fates in the adaxial and abaxial domains of the leaf. Common anatomical differences between these domains include the formation of adaxial xylem and abaxial phloem, as well as differences in epidermal and mesophyll cell morphology. Finally, leaves grow to form a widened, flattened lamina from a marginal domain at the juxtaposition of the adaxial and abaxial leaf faces, thereby defining the mediolateral axis of the leaf. Development along the mediolateral axis is associated with the positioning of the medial leaf midvein and the proliferative transverse outgrowth of cells to form the leaf lamina, or blade. Plants 2019, 8, 433; doi:10.3390/plants8100433 www.mdpi.com/journal/plants Plants 2019, 8, 433 2 of 19 Plants 2019, 8, x FOR PEER REVIEW 2 of 19 Figure 1. LeafLeaf anatomy anatomy and and growth axes. ( (AA)) Leaves Leaves from from maize, maize, a a monocot, monocot, and and the the two two closely related eudicots,eudicots,Cardamine Cardamine hirsuta hirsutaand andArabidopsis Arabidopsis thaliana thaliana, highlighting, highlighting major featuresmajor features of leaf anatomy. of leaf anatomy.(B) Cross-section (B) Cross-section of a mature of eudicota mature leaf eudicot illustrating leaf illustrating the characteristic the characteristic features of thefeatures adaxial of andthe adaxialabaxial leafand domains.abaxial leaf domains. A host of transcriptional regulators underlie the establishment and maintenance of the leaf developmental axes, axes, often often making making use use of of inhibitory inhibitory interactions interactions to tospecify specify opposing opposing cell cell fates. fates. A classicA classic example example of ofthis this type type of of regulatory regulatory logic logic is is seen seen in in the the context context of of the the conserved antagonistic relationship between Class I KNOTTED-LIKE HOMEOBOX (KNOX), (KNOX), ASYMMETRIC ASYMMETRIC LEAVES1 LEAVES1 (AS1), and ASYMMETRIC LEAVES2 (AS2)(AS2) transcriptiontranscription factorsfactors [[5–14].5–14]. ClassClass I I KNOX genes are expressed in meristematic tissue and promote in indeterminatedeterminate growth, while while AS1 and AS2 repress KNOX, an important first first step in the transition from indeterminate to determinate cell identity along the proximodistal axis. Inhibitory Inhibitory interactions interactions be betweentween adaxial adaxial and and abaxial abaxial patterning patterning regulators regulators are are also well-described. Alon Alongg this this axis, axis, the the adaxial adaxial cell cell fate-pro fate-promotingmoting HD-ZIP HD-ZIP Class Class III III (HD-ZIP (HD-ZIP III) III) transcription factorsfactors are are repressed repressed by by abaxial abaxial cell cell fate-promoting fate-promoting KANADI KANADI (KAN) (KAN) transcription transcription factors factorsand the HD-ZIPIII-targetingand the HD-ZIPIII-targeting miR165/166 miR165/166 microRNAs microRNAs [15–20]. In a parallel[15–20]. pathway,In a parallel transcripts pathway, of the transcriptsabaxial cell of fate-promoting the abaxial cell ETTIN fate-promoting/AUXIN RESPONSE ETTIN/AUXIN FACTOR3 RESPONSE (ETT/ARF3) FACTOR3 and ARF4 (ETT/ARF3) transcription and ARF4factors transcription are targeted byfactors small-interfering are targeted RNAsby small-interfering (ta-siARFs) expressed RNAs in(ta-siARFs) the adaxial expressed domain [21in– the23]. adaxialTogether, domain these mutually [21–23]. inhibitory Together, interactions these mutually precisely inhibitory specify and interactions maintain theprecisely boundaries specify between and maintainthe adaxial the and boundaries abaxial faces between of the the leaf. adaxial and abaxial faces of the leaf. The reproducible phenotypic outcomes of leaf dev developmentelopment may in part be ensured by multiple layers of redundancy built into thethe underlyingunderlying geneticgenetic patterningpatterning network.network. Not Not only only do multiple pathways actact in in parallel parallel to regulateto regulate the samethe same patterning patterning processes, processes, but gene but duplication gene duplication has expanded has expandedthe number the of number redundantly of redundantly acting pattering acting genes, patteri potentiallyng genes, potentially allowing for allowing patterning for robustnesspatterning robustnessin the face ofin networkthe face of perturbation network perturba or noisetion [24 or– 27noise]. For [24–27 example,]. For example, the Arabidopsis the Arabidopsisgenome containsgenome containsfour copies four of thecopies abaxially of the expressed abaxially KANexpressedtranscription-factor-encoding KAN transcription-factor-encoding genes, which genes, redundantly which redundantlypromote abaxial promote cell fate abaxial [28,29 cell]. Whilefate [28,29]. genetic While redundancy genetic redundancy may allow formay stable allow and for predictablestable and predictabledevelopmental developmental outcomes along outcomes a given along leaf axis,a given leaves leaf develop axis, leaves simultaneously develop simultaneously along multiple along axes multiplesuch that mechanismsaxes such controllingthat mechanisms three-dimensional controlling development three-dimensional must be synchronized development to some must extent. be synchronizedIn the context ofto vertebratesome extent. limb In bud the development, context of vertebrate evidence haslimb emerged bud development, for molecular evidence coordination has emergedacross organ for axes.molecular For example, coordination the transcription across orga factorsn axes. HAND2 For example, and GLI3R, the which transcription promote anteriorfactors HAND2 and GLI3R, which promote anterior and posterior cell fate, respectively, have been shown to directly cross-regulate the expression of genes important for proximodistal patterning [30–32]. In Plants 2019, 8, 433 3 of 19 and posterior cell fate, respectively, have been shown to directly cross-regulate the expression of genes important for proximodistal patterning [30–32]. In plants, a model wherein adaxial–abaxial patterning establishes outgrowth of both the mediolateral and proximodistal leaf axes was proposed almost 25 years ago [33]. Here, we review several decades of developmental genetics research that test and extend this classical model of the mechanisms whereby leaves initiate from the SAM and undergo coordinated development along three axes. In general, coordinating factors sit at the top of complex gene regulatory networks, and work to organize and control the expression of well-characterized patterning modules. Their context-specific functions are frequently mediated by physical