Leaf-size control beyond transcription factors: Compensatory mechanisms

Abstract

Plant leaves display abundant morphological richness yet grow to characteristic sizes and shapes. Beginning with a small number of undifferentiated founder cells, leaves evolve via a complex interplay of regulatory factors that ultimately influence cell proliferation and subsequent post-mitotic cell enlargement. During their development, a sequence of key events that shape leaves is both robustly executed spatiotemporally following a genomic molecular network and flexibly tuned by a variety of environmental stimuli. Decades of work on Arabidopsis thaliana have revisited the compensatory phenomena that might reflect a general and primary size-regulatory mechanism in leaves. This review focuses on key molecular and cellular events behind the organ-wide scale regulation of compensatory mechanisms. Lastly, emerging novel mechanisms of metabolic and hormonal regulation are discussed, based on recent advances in the field that have provided insights into, among other phenomena, leaf-size regulation.

Keywords: Arabidopsis thaliana; cell proliferation; cell-autonomous; compensation; leaf morphogenesis; non-cell-autonomous; post-mitotic cell expansion.

Figure 1

Cellular spatial relationships during induction of CCE. (A) Mesophyll cells exploit cell-to-cell communication to stimulate CCE in response to deficient cell proliferation in an3, which has class I compensation (Kawade et al., 2010). By contrast, epidermal cells trigger CCE in a cell-autonomous manner, preventing cell-to-cell communication across tissue layers (Nozaki et al., 2020). (B) Cell-to-cell communication between epidermal and mesophyll cells is absent in fugu5, which has class II compensation (Gunji et al., 2022). Given that excess PPi inhibits metabolic reactions, fugu5-mediated induction of CCE is likely controlled in a cell-autonomous manner. (C) Direct inhibition of cell cycle progression by KRP overexpression, a representative of class III compensation, induces CCE in a cell- and tissue-autonomous manner. This was determined using chimeric KRP2-overexpressing leaves generated using the Cre/lox-P system (Kawade et al., 2010) and KRP1-overexpressing leaves with a tissue-specific expression system (Bemis and Torii, 2007). Epidermal cell-to-cell communication remains to be explored. Circles and crosses indicate, respectively, the presence and absence of cell-to-cell communication that stimulates CCE. Question marks are added when cell-to-cell communication is still untested.

Figure 2

Molecular machinery of the induction and response phases in class II CCE. The decreased cell number (induction phase) in Arabidopsis fugu5-mutant cotyledons has been shown to be exclusively due to a decreased level of TAG-derived Suc, and IBA-derived IAA (Katano et al., 2016; Takahashi et al., 2017) has been suggested to mediate CCE (response phase) as follows: First, upon seed imbibition, excess cytosolic PPi in fugu5 leads to the inhibition of de novo Suc synthesis from TAG, a major seed-storage lipid and substrate for the conversion of fatty acids to acetyl-CoA for glyoxylate bypass that takes place in the glyoxysome (Ferjani et al., 2011). This is owing to inhibition of gluconeogenesis (Ferjani et al., 2018). Second, during seedling establishment, metabolic disorder associated with the reduced Suc content (4-6 DAS; left panel) is converted into an ‘output instructive signal’ (6-10 DAS; right panel) that promotes the conversion of IBA, an auxin precursor, to IAA, the natural phytohormone auxin, leading to an increase in endogenous IAA concentration, which is crucial for CCE (Tabeta et al., 2021). Third, increased endogenous IAA (IAA concentration peaks at 8-10 DAS) triggers the TIR/AFB-dependent auxin signaling pathway through the AUXIN RESPONSE FACTORs ARF7 and ARF19, transcriptional activators of early auxin-response genes. This subsequently activates the vacuolar type V-ATPase, leading to an increase in turgor pressure, which ultimately drives an increase in cell size and CCE (Tabeta et al., 2021; Nakayama et al., 2022). β-ox, β-oxidation; Succ, succinate. Glyoxysome, the single membrane-bound organelles that house most of the biochemical machinery required to convert fatty acids derived from TAG to 4-carbon compounds. TCA, the tricarboxylic acid cycle. PEN3, PENETRATION (PEN) 3 is a membrane-localized ATP-binding cassette (ABC) transporter. PDR9, is a member of the pleiotropic drug resistance (PDR) family of ATP-binding cassette transporters. ABCG transporter, G-type ATP-binding cassette (ABCG) transporter. DAS, days after seed sowing.