Arabidopsis sucrose synthase 2 and 3 modulate metabolic homeostasis and direct carbon towards starch synthesis in developing seeds

Abstract

Two genes encoding sucrose synthase (SUS), namely SUS2 (At5g49190) and SUS3 (At4g02280), are strongly and differentially expressed in Arabidopsis seed. Detailed biochemical analysis was carried out in developing seeds 9-21 days after flowering (DAF) of wild type and two knockouts. SUS2 and SUS3 are not redundant genes since single knockouts show a phenotype in developing seeds. The mutants had 30-50% less SUS activity and therefore accumulated 40% more sucrose and 50% less fructose at 15 DAF. This did not affect the hexose-P pool, but led to 30-70% less starch in embryo and seed coat. Lipids were 55% higher in both mutants at 9-15 DAF. It seems that sucrolysis via SUS is not required for oil or protein synthesis but rather for channeling carbon toward ADP-glucose and starch in seeds. Metabolite profiling with GC-TOF revealed specific downstream changes in primary metabolism as a consequence of signaling or regulatory fine-tuning. While sucrose increased, hexoses and specific amino acids decreased reciprocally. There was a developmental shift regarding an earlier timing of dry weight accumulation, germinative maturity, oil deposition, sugar levels, transient starch buildup, and protein storage. Nevertheless, final seed size and composition were unaltered due to an earlier cessation of growth, thus giving rise to an apparent silent phenotype of mature mutant seeds. We conclude that SUS is important for metabolite homeostasis and timing of seed development, and propose that an altered sucrose/hexose ratio can modify carbon partitioning and the pattern of storage compounds in Arabidopsis.

Aislamiento, caracterización molecular y evaluación de cepas fijadoras de nitrógeno en la promoción del crecimiento de frijol

“…These gibberellins activate genes that synthesize mRNA which promote the synthesis of hydrolytic enzymes such as α-amylase that splits starch into sugars, providing a significant source of carbon atoms to organs and plant tissue, promoting its growth, development and grain filling. This is consistent with the increase in carbohydrate content observed in growing organs and tissues (Angeles-Núnez and Fait et al, 2006;Tiessen, 2010). In other crops like barley and corn, the starch that is accumulated in the stem during the development of the plant is used as source of carbon for grain filling (Blum, 1998;Mi et al, 2009;Madani et al, 2010).…”

“…Esto es coherente con el incremento del contenido de carbohidratos que se observan en órganos y tejidos en desarrollo (Angeles-Núnez y Fait et al, 2006;Tiessen, The changes in root dry weight and soluble sugar content in stems from treatment with Ramlibacter sp. weren't observed on seedlings treated with other isolates.…”

Starch Turnover and Metabolism during Flower and Early Embryo Development

“…Within the cytosol, the expression of two isoforms derived from each of the SPS and SPP genes suggests high Suc biosynthetic activities paralleling starch biosynthesis. SUS gene expression levels remained unaltered during all stages of pollen development and in vitro pollen tube growth, pointing toward SUS-independent starch biosynthesis, a finding that contrasts with the SUSbased starch biosynthesis suggested for other tissues (Déjardin et al, 1997;Angeles-Núñez and Tiessen, 2010). Low SUS and high invertase expression levels imply a higher abundance of Glc and Fru compared with UDP-Glc.…”

Oil and Protein Accumulation in Developing Seeds Is Influenced by the Expression of a Cytosolic Pyrophosphatase in Arabidopsis

“…Thus, the use of PPi appears to be part of an ATP-saving, SUS-initiated pathway of cytosolic Suc catabolism characteristic of the maturation phase of seed development with highest rates of seed storage compound accumulation. However, direct, genetic evidence for the essential nature of the pathway in Arabidopsis seed filling is lacking, as SUS quadruple mutants show wild-type levels of oil and protein in dry seed and only a transient obstruction of seed fill is observed in developing seed of SUS mutants (Barratt et al, 2009;Angeles-Núñez and Tiessen, 2010). Moreover, to our knowledge, no other mutants with altered seed composition have been described that carry lossof-function alleles of genes encoding components of this pathway.…”