Transcripts switched off at the stop of phloem unloading highlight the energy efficiency of sugar import in the ripening V. vinifera fruit

Abstract

Transcriptomic changes at the cessation of sugar accumulation in the pericarp of Vitis vinifera were addressed on single berries re-synchronised according to their individual growth patterns. The net rates of water, sugars and K+ accumulation inferred from individual growth and solute concentration confirmed that these inflows stopped simultaneously in the ripe berry, while the small amount of malic acid remaining at this stage was still being oxidised at low rate. Re-synchronised individual berries displayed negligible variations in gene expression among triplicates. RNA-seq studies revealed sharp reprogramming of cell-wall enzymes and structural proteins at the stop of phloem unloading, associated with an 80% repression of multiple sugar transporters and aquaporins on the plasma or tonoplast membranes, with the noticeable exception of H+/sugar symporters, which were rather weakly and constitutively expressed. This was verified in three genotypes placed in contrasted thermo-hydric conditions. The prevalence of SWEET suggests that electrogenic transporters would play a minor role on the plasma membranes of SE/CC complex and the one of the flesh, while sucrose/H+ exchangers dominate on its tonoplast. Cis-regulatory elements present in their promoters allowed to sort these transporters in different groups, also including specific TIPs and PIPs paralogs, and cohorts of cell wall-related genes. Together with simple thermodynamic considerations, these results lead to propose that H+/sugar exchangers at the tonoplast, associated with a considerably acidic vacuolar pH, may exhaust cytosolic sugars in the flesh and alleviate the need for supplementary energisation of sugar transport at the plasma membrane.

The sugarless grape trait characterized by single berry phenotyping

“…In most comparative studies, authors predetermine an average harvest date, ripening duration, thermal time or even [Hex] threshold to compare accessions at an hopefully similar ripe stage, in contradiction with the recognized impact of GxE on these variables (Liu et al, 2007;Dai et al, 2011;Duchêne et al, 2012). To circumvent this inconsistency and lack of consensus, the moment at which berry phloem unloading stops was recently proposed as a relevant definition of ripe stage both on the physiological and transcriptomic point of views (Bigard et al, 2018 3 and 2020; Shahood et al, 2015 andSavoi et al, 2021). This key transition which marks the arrest of the most intensive fruit biochemical pathways is associated with the transcriptional extinction of many genes encoding, amongst other, sugar transporters, aquaporins and cell wallrelated enzymes (Savoi et al, 2021).…”

“…To circumvent this inconsistency and lack of consensus, the moment at which berry phloem unloading stops was recently proposed as a relevant definition of ripe stage both on the physiological and transcriptomic point of views (Bigard et al, 2018 3 and 2020; Shahood et al, 2015 andSavoi et al, 2021). This key transition which marks the arrest of the most intensive fruit biochemical pathways is associated with the transcriptional extinction of many genes encoding, amongst other, sugar transporters, aquaporins and cell wallrelated enzymes (Savoi et al, 2021). Unfortunately, this developmental stage can't be directly inferred from [Hex] kinetics, which continue to evolve after the completion of sugar storage (or accumulation) due to subsequent berry shriveling.…”

“…Thus, additional information on berry growth is required to address the net rates of sugar accumulation and malic acid breakdown in berries, together with their respective timings. Very recently, single berry phenotyping approaches improved our understanding of berry growth and metabolism during ripening, avoiding the biases due to berry asynchronicity (Shahood et al, 2020;Savoi et al, 2021). According to this new paradigm, this study aims to decipher the genetic differences existing for [Hex] and fruit acidity in a set of genotypes encompassing traditional varieties and new hybrids exhibiting low…”

Plasmodesmata and their role in the regulation of phloem unloading during fruit development

“…Increase in apoplastic sugars and acid invertase expression at the onset of ripening. [ 55 , 66 ] Strawberry ( Fragaria x ananassa ) Apoplastic pathway. Sucrose Sorbitol The abundance of PD between PCs, rare presence between SE-CC complexes.…”

“…In kiwi fruits, high numbers of PD were observed in the SE-CC interface but not in the post-phloem unloading domains (CC-PC or SE-PC) concomitant with restricted symplasmic unloading of CF [ 54 ]. RNA-seq analysis in grapes collected after phloem unloading stops (at the end of ripening), reveals downregulation in the expression of the SWEET transporters, of cell wall genes and aquaporins, marking the cessation of growth [ 66 ].…”