Involvement of Arabidopsis BIG protein in cell death mediated by Myo-inositol homeostasis

Abstract

Programmed cell death (PCD) is essential for several aspects of plant life. We previously identified the mips1 mutant of Arabidopsis thaliana, which is deficient for the enzyme catalysing myo-inositol synthesis, and that displays light-dependent formation of lesions on leaves due to Salicylic Acid (SA) over-accumulation. Rationale of this work was to identify novel regulators of plant pcD using a genetic approach. A screen for secondary mutations that abolish the mips1 PCD phenotype identified a mutation in the BIG gene, encoding a factor of unknown molecular function that was previously shown to play pleiotropic roles in plant development and defence. physiological analyses showed that BiG is required for lesion formation in mips1 via SA-dependant signalling. big mutations partly rescued transcriptomic and metabolomics perturbations as stress-related phytohormones homeostasis. in addition, since loss of function of the ceramide synthase LOH2 was not able to abolish cell death induction in mips1, we show that pcD induction is not fully dependent of sphingolipid accumulation as previously suggested. our results provide further insights into the role of the BiG protein in the control of MIPS1-dependent cell death and also into the impact of sphingolipid homeostasis in this pathway. Myo-inositol (MI) is a ubiquitous molecule and the precursor for biosynthesis of many inositol-derivatives such as inositol phosphates, phosphatidylinositol (PtdIns) phosphate, or specific classes of sphingolipids that play critical and diverse roles in vesicle trafficking, hormone signalling, and biotic and abiotic stress responses 1, 2. The rate-limiting step of MI synthesis is catalysed by l-myo-inositol 1-phosphate synthase (MIPS), using Glucose-6-P as a substrate. This reaction is followed by dephosphorylation of l-myo-inositol 1-phosphate into MI. These two reactions together, called the Loewus pathway, are the only known route for MI biosynthesis 3. Three genes encoding MIPS isoforms have been identified in the Arabidopsis genome 4, 5 , among them MIPS1 is responsible for most of MI biosynthesis in leaves. Pleiotropic developmental defects such as reduced root growth or altered venation in cotyledons have been described for the mips1 loss of function mutant 6-8. One of its most striking features is the light intensity-dependent formation of leaf lesions due to Salicylic acid (SA)dependent Programmed Cell Death (PCD), revealing the importance of MI or inositol derivatives in the regulation of these processes. However, how light affects MIPS transcript level and inositol biosynthesis was an open question for a long time until the demonstration that the two light signalling proteins, FAR-RED ELONGATED HYPOCOTYL 3 (FHY3) and its homolog, FAR-RED IMPAIRED RESPONSE 1 (FAR1), were able to directly bind MIPS1 promoter and to activate its expression, thereby promoting inositol biosynthesis to prevent lightinduced SA-dependent cell death 9. Because MI is used to synthetize numerous compounds in the cell, including PtdIns, a link between mip...

BIG participates in the Arg/N-degron pathways and the hypoxia response inArabidopsis thaliana

“…Suberisation is strongly influenced by hormones, including auxin which is associated with the growth phenotype of big alleles [48,49,50,52,54] and which has complex effects on suberin synthesis and degradation in the endodermis in Arabidopsis [79,80]. It is tempting to speculate that dysregulation of auxin synthesis and transport underpin the reduced suberisation in big-2, however, there was no significant enrichment in transcripts related to auxin signalling pathways in big-2 roots, in contrast to a previous transcriptome analysis employing leaves of a different big allele [63]. Whilst it is possible that there are tissue-specific differences in auxin-related gene expression that are not detected in the bulk root transcriptome, other mechanisms regulating suberin in big-2 roots cannot yet be ruled out.…”

“…BIG was later shown to influence multiple hormone signalling pathways and different aspects of plant development [50][51][52][53][54][55][56][57][58]. Recent studies indicate further, apparently disparate functions for BIG in the circadian clock, guard cell signalling, calcium homeostasis, regulation of C/N balance, response to pathogens, cell death, and woundinduced rooting [49,56,[59][60][61][62][63][64]. Although many big mutant phenotypes can be ascribed to dysregulation of auxin transport [46,47,48,52,53,54,57,58,65,66,67,68], this is not the case for all processes influenced by BIG and to date its precise biochemical functions have remained unclear.…”

The N-degron pathway governs autophagy to promote thermotolerance inArabidopsis

“…Arabidopsis thaliana (Ecotype Columbia, Col-0) plants were grown at 23°C under long-day conditions in a 16-h light/8-h dark cycle and short-day conditions in an 8-h light/16-h dark cycle. The mutant lines used in this study are prt6-1 (SALK_004079), big-3 (SALK_107817) (Bruggeman et al, 2020), ubr7 (SALK_034619), and atg8a (SALK_045344) from the Arabidopsis Biological Resource Center (ABRC). T-DNA insertion sites were verified by sequence analysis using gene-specific primers (Table S7).…”