Molecular and Hormonal Regulation of Leaf Morphogenesis in Arabidopsis

Abstract

Shoot apical meristems (SAM) are tissues that function as a site of continuous organogenesis, which indicates that a small pool of pluripotent stem cells replenishes into lateral organs. The coordination of intercellular and intracellular networks is essential for maintaining SAM structure and size and also leads to patterning and formation of lateral organs. Leaves initiate from the flanks of SAM and then develop into a flattened structure with variable sizes and forms. This process is mainly regulated by the transcriptional regulators and mechanical properties that modulate leaf development. Leaf initiation along with proper orientation is necessary for photosynthesis and thus vital for plant survival. Leaf development is controlled by different components such as hormones, transcription factors, miRNAs, small peptides, and epigenetic marks. Moreover, the adaxial/abaxial cell fate, lamina growth, and shape of margins are determined by certain regulatory mechanisms. The over-expression and repression of various factors responsible for leaf initiation, development, and shape have been previously studied in several mutants. However, in this review, we collectively discuss how these factors modulate leaf development in the context of leaf initiation, polarity establishment, leaf flattening and shape.

Keywords: adaxial/abaxial; leaf development; miRNAs; shoot apical meristem; transcription factors.

Figure 1

Schematic representation of Shoot Apical Meristems (SAM) maintenance by the various interacting genes. (A) The pluripotent stage and a specific number of cells in the SAM are controlled by the (WUS/CLV3) negative-feedback loop. CLAVATA3 (CLV3) is a ligand attached to CLV1 and CLV2 for the restriction of WUS. However, WUS activates CLV3 and works as a stem cell-promoting protein. STM activated the biosynthesis of CK through LONELY GUY (LOG). The STM and KNOX related genes also keep the stem cells undifferentiated by suppressing the expression of AS1 and gibberellic acid (GA) biosynthesis. STM also restricts the expression of the CUC gene due to negative regulators in a specific area. (B) The HAM and WUS-CLV3 loop. The regulatory loop requires CLV3, WUS, and HAM; the CLV3 negatively regulates the WUS expression, and the WUS protein moves from the organizing center to the active central zone (stem cells) to activate CLV3 expression. HAM1/2 is an interacting partner of WUS, and together with WUS protein suppresses the expression of CLV3 in the rib meristems. The expression zone of the CLV3 gene (blue), WUS gene (pink), HAM proteins (green) and the dot marks the WUS protein. (C) Regulatory networks that control leaf initiation. The cells in the SAM are arranged into layers L1, L2, and L3 and further into a distinct group of either tunica or corpus. According to the expression of genes, the SAM architecture is organized with the central zone (CZ), peripheral zone (PZ), organizing zone (OZ), and rib zone (RZ). During leaf initiation, auxin maxima repress the expression of the KNOX1 domains (gene) indeterminate meristem domains. KNOX1 maintains a high level of CK and low levels of GA in the meristem. In the ARP domain that has leaf identity, the leaf primordium separates from SAM by expression of boundary specific genes CUC and BOP regulates the petiole specification and polarity (positive and negative regulations are indicated by pointed and T-shaped arrows).

Figure 2

Regulation of Arabidopsis leaf proliferation and cell expansion transition. (A) the cell cycle arrests the front migration basipetal growth gradient from the apex to the base of the blade petioles, which is a process that is promoted by miR319-TCP modules and repressed by miR393-GRF modules. The gradients of miR319 and miR396 are complementary to TCP and GRF expression. TCP and NGA repressed the blastozone activity. During intercalary growth, the GAs and BRs promote both cell proliferation and expansion (positive and negative regulations are indicated by pointed and T-shaped arrows). Red, genes; blue, hormones; purple, small RNAs. (B) Shortly after its initiation, the young leaf primordium has three regions: adaxial, abaxial, and leaf marginal regions. These regions are determined by the region’s specific transcription factors, such as HD-ZIP III, KANADI, and PRS WOX1, involving multiple negative feedback loop mechanisms. For the sake of clarity, all the interactions are not shown here. Adaxial-abaxial polarity is determined by highly interconnected gene networks. In this network, tasi-ARF limits abaxial determinants and AUXIN RESPONSE FACTORS ARF3/ARF4 to the bottom side. In addition, miR165/166 restricts the expression of adaxial determinants HD-ZIPIII to the top of the leaf (the positive and negative regulations are indicated by pointed and T-shaped arrows, respectively).

Figure 3

Schematic depiction of the factors involved in the modification of leaf margins, the genetic interaction between different genes and hormones cause the formation of leaf serration. The CUC2 gene promotes the auxin maxima via PIN1 efflux protein in the epidermal cell. The high concentrations of auxin at the tips block, the expression of CUC2 and thus, the expression of CUC2 gene is restricted to the sinus region of the leaf; miR164A also suppresses the CUC2 expression. The other regulatory factors shown in the figure is discussed in detail in the text (the positive and negative regulations are indicated by pointed and T-shaped arrows, respectively).

Figure 4

A schematic pathway for miRNA biosynthesis and the degradation of target mRNA. (A) Primary miRNAs are formed by polymerase II that folds back to form a hairpin structure. Splicing and further processing in the nucleus include the interactive role of different proteins and the CAP-binding proteins CBP20 and CBP80. Further processing occurs by DCL1 and forms miRNA-miRNA* duplexes, which are methylated by HEN1 and transported to the cytoplasm. The mature miRNA is unwound to yield a 22-nt single-strand and incorporated into AGO1-containing RISC (RNA-induced silencing complex) and other regulatory proteins that directly inhibit the translation or cleavage of the target mRNA transcript. (B) Leaf development is a complex biological process with multiple regulatory networks regulated by miRNA and their target genes. The leaf development consists of different stages such as establishment, transition, modification, and senescence. The miRNAs play important roles in each stage. The miR160 and miR165/166 control leaf initiation; leaf polarity is determining by miR165/166 and miR390; leaf morphology is regulating by miR164 and miR319; phase initiation is determined by miR156 and miR172; leaf senescence is determined by miR164 and miR319 (the arrows indicate positive and T-shaped indicate negative regulation).