Sugar Starvation Disrupts Lipid Breakdown by Inducing Autophagy in Embryonic Axes of Lupin (Lupinus spp.) Germinating Seeds

Abstract

Under nutrient deficiency or starvation conditions, the mobilization of storage compounds during seed germination is enhanced to primarily supply respiratory substrates and hence increase the potential of cell survival. Nevertheless, we found that, under sugar starvation conditions in isolated embryonic axes of white lupin (Lupinus albus L.) and Andean lupin (Lupinus mutabilis Sweet) cultured in vitro for 96 h, the disruption of lipid breakdown occurs, as was reflected in the higher lipid content in the sugar-starved (-S) than in the sucrose-fed (+S) axes. We postulate that pexophagy (autophagic degradation of the peroxisome—a key organelle in lipid catabolism) is one of the reasons for the disruption in lipid breakdown under starvation conditions. Evidence of pexophagy can be: (i) the higher transcript level of genes encoding proteins of pexophagy machinery, and (ii) the lower content of the peroxisome marker Pex14p and its increase caused by an autophagy inhibitor (concanamycin A) in -S axes in comparison to the +S axes. Additionally, based on ultrastructure observation, we documented that, under sugar starvation conditions lipophagy (autophagic degradation of whole lipid droplets) may also occur but this type of selective autophagy seems to be restricted under starvation conditions. Our results also show that autophagy occurs at the very early stages of plant growth and development, including the cells of embryonic seed organs, and allows cell survival under starvation conditions.

Identification and Potential Participation of Lipases in Autophagic Body Degradation in Embryonic Axes of Lupin (Lupinus spp.) Germinating Seeds

“…The action of exogenous asparagine, which slowed down the degradation of autophagic bodies and decreased lipolytic activity in the −S+Asn axes, convinced us to use this amino acid in our research concerning the role of vacuolar lipases in autophagic body degradation. The comparison of our previously obtained results [ 15 , 16 , 18 ] with the currently performed transcriptome analysis (NGS and SRA databases, BioProject accession number for white lupin: PRJNA953600, and Andean lupin: PRJNA953433, deposited in 2023) suggested that vacuolar lipases have a high potential to be involved in the degradation of autophagic bodies in plants. This preliminary study provides new insights into one of the final, poorly understood stages of autophagy in plants, which is the degradation of autophagic bodies.…”

“…Research was performed on the embryonic axes isolated from the imbibed seeds of the white lupin ( Lupinus albus L.) and Andean lupin ( Lupinus mutabilis Sweet) and cultured in vitro for 96 h on a liquid mineral Heller medium [ 22 ] with or without 60 mM of sucrose (+S and −S, respectively) and with 35 mM of asparagine (−S+Asn and +S+Asn, respectively). A detailed description of the morpho-physiological parameters of the 96 h lupin embryonic axes was published previously [ 16 , 18 ]. Based on the results of the mass sequencing of the transcriptomes of the white and Andean lupin isolated embryonic axes cultured in vitro for 96 h, we identified 90 and 111 transcripts of the genes encoding lipases for the embryonic axes of the white lupin and Andean lupin, respectively ( Table S1 ).…”

“…Carbon starvation is a well-known stressor enhancing autophagy in plant cells [ 15 , 16 , 17 , 18 ]. The intensification of autophagic degradation provides cells with respiratory substrates, which simultaneously enhances their survival potential [ 19 ].…”

“…seeds cultured in vitro on a mineral medium for 96 h under carbon (sugar) starvation conditions (−S). The presence of autophagy was reflected in the huge cell vacuolization [ 18 ], decreased content of phosphatidylcholine [ 18 ], and higher level of atg8 gene transcripts [ 18 ]. We also induced a slowdown in the degradation of autophagic bodies by simultaneously nourishing the lupin embryonic axes with asparagine (−S+Asn), a central amino acid in the metabolism of lupin seeds [ 15 , 20 ].…”

“…This suggested that asparagine in plants has an inhibitory effect on this late step of autophagy. Apart from the accumulation of autophagic bodies, it was also noted that in the −S+Asn axes, the lipolytic activity was significantly decreased in comparison to the −S axes [ 16 , 18 ]. The action of exogenous asparagine, which slowed down the degradation of autophagic bodies and decreased lipolytic activity in the −S+Asn axes, convinced us to use this amino acid in our research concerning the role of vacuolar lipases in autophagic body degradation.…”