An auxin signaling network translates low-sugar-state input into compensated cell enlargement in the fugu5 cotyledon

Abstract

In plants, the effective mobilization of seed nutrient reserves is crucial during germination and for seedling establishment. The Arabidopsis H+-PPase-loss-of-function fugu5 mutants exhibit a reduced number of cells in the cotyledons. This leads to enhanced post-mitotic cell expansion, also known as compensated cell enlargement (CCE). While decreased cell numbers have been ascribed to reduced gluconeogenesis from triacylglycerol, the molecular mechanisms underlying CCE remain ill-known. Given the role of indole 3-butyric acid (IBA) in cotyledon development, and because CCE in fugu5 is specifically and completely cancelled by ech2, which shows defective IBA-to-indoleacetic acid (IAA) conversion, IBA has emerged as a potential regulator of CCE. Here, to further illuminate the regulatory role of IBA in CCE, we used a series of high-order mutants that harbored a specific defect in IBA-to-IAA conversion, IBA efflux, IAA signaling, or vacuolar type H+-ATPase (V-ATPase) activity and analyzed the genetic interaction with fugu5–1. We found that while CCE in fugu5 was promoted by IBA, defects in IBA-to-IAA conversion, IAA response, or the V-ATPase activity alone cancelled CCE. Consistently, endogenous IAA in fugu5 reached a level 2.2-fold higher than the WT in 1-week-old seedlings. Finally, the above findings were validated in icl–2, mls–2, pck1–2 and ibr10 mutants, in which CCE was triggered by low sugar contents. This provides a scenario in which following seed germination, the low-sugar-state triggers IAA synthesis, leading to CCE through the activation of the V-ATPase. These findings illustrate how fine-tuning cell and organ size regulation depend on interplays between metabolism and IAA levels in plants.

Tissue-targeted inorganic pyrophosphate hydrolysis in a fugu5 mutant reveals that excess inorganic pyrophosphate triggers developmental defects in a cell-autonomous manner

“…Finally, to evaluate the impact of tissue-specific expression of IPP1 on gluconeogenesis, we quantified Suc content by GC-MS/MS ( Tabeta et al, 2021 ). The Suc level in PDF1 pro ::IPP1 tended to increase ( Figure 3D ), whereas that in CLV1 pro ::IPP1 was indistinguishable from fugu5-1 ( Figure 3E ).…”

“…Taken together, these findings demonstrate that palisade tissue requires FUGU5 for a few days after seed imbibition, but prolonged H + -PPase activity is required for PCs to adopt their typical jigsaw puzzle shape ( Higaki et al, 2016 ). In other words, the decision to trigger CCE in cotyledons occurs early during postembryonic development and is followed by large-scale metabolic reprogramming involving IBA-to-IAA conversion as a key step in CCE induction ( Tabeta et al, 2021 ). By contrast, PCs require more time than the palisade to mature and gradually adopt complex shapes ( Fu et al, 2002 , 2005 ; Armour et al, 2015 ; Higaki et al, 2016 ).…”

“…For TMS derivatization, the mixture was incubated for 30 min at 1,200 rpm at 37°C as above. Finally, 50 μl of the derivatized samples were dispensed into vials for GC-QqQ-MS analysis (AOC-5000 Plus with GCMS-TQ8040; Shimadzu) as described elsewhere ( Tabeta et al, 2021 ). The raw data were collected, and GC-MS peak areas were calculated with GCMS software (Shimadzu).…”

“…We reported that excess PPi in the H + -PPase loss-of-function fugu5 mutant severely compromises gluconeogenesis from seed storage lipids and inhibits a hypocotyl elongation in etiolated seedlings ( Ferjani et al, 2011 , 2014a , 2018 ), arrests cell division in cotyledonary palisade tissue, and triggers a compensated cell enlargement (CCE; Tsukaya, 2002 , 2008 ; Horiguchi et al, 2005 , 2006a , b ; Ferjani et al, 2007 , 2008 , 2010 , 2011 , 2013a , b ; Amano et al, 2015 ; Katano et al, 2016 ; Takahashi et al, 2017 ; Tabeta et al, 2021 ; Nakayama et al, 2022 ). It is important to note that these phenotypes were recovered by exogenous sucrose (Suc) supply or specific removal of PPi from the fugu5 background by the yeast cytosolic PPase IPP1 under the control of the vacuolar H + -pyrophosphatase AVP1 / FUGU5 promoter in fugu5-1 AVP1 pro ::IPP1 transgenic lines ( Ferjani et al, 2011 , 2014a , b ; Asaoka et al, 2019 ; Gunji et al, 2020 ).…”

CINCINNATA-Like TCP Transcription Factors in Cell Growth – An Expanding Portfolio

“…Another homeodomain transcription factor HAT2 is also known to promote cell elongation in hypocotyl in response to auxin signaling ( Sawa et al, 2002 ). A recent report suggests that increased endogenous auxin, triggered by a low-sugar state in the cotyledons following seed germination, promotes compensation-mediated cell enlargement ( Tabeta et al, 2021 ). Another recent study demonstrates that constitutively activated salicylic acid (SA) signaling suppresses compensation-induced cell expansion in leaves, thus assigning a role for SA in organ size regulation ( Fujikura et al, 2020 ).…”