Arabidopsis sucrose synthase 2 and 3 modulate metabolic homeostasis and direct carbon towards starch synthesis in developing seeds |
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Authors: | Juan Gabriel Angeles-Núñez Axel Tiessen |
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Institution: | 1. Departamento de Ingeniería Genética, CINVESTAV Unidad Irapuato, Km 9.8 Libramiento Norte, CP 36821, Irapuato, Guanajuato, México
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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. |
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