SUMMARY
Biosynthesis of plant cell walls requires UDP‐glucose as the substrate for cellulose biosynthesis, and as an intermediate for the synthesis of other matrix polysaccharides. The sucrose ...cleaving enzyme sucrose synthase (SUS) is thought to have a central role in UDP‐glucose biosynthesis, and a long‐held and much debated hypothesis postulates that SUS is required to supply UDP‐glucose to cellulose biosynthesis. To investigate the role of SUS in cellulose biosynthesis of Arabidopsis thaliana we characterized mutants in which four or all six Arabidopsis SUS genes were disrupted. These sus mutants showed no growth phenotypes, vascular tissue cell wall defects, or changes in cellulose content. Moreover, the UDP‐glucose content of rosette leaves of the sextuple sus mutants was increased by approximately 20% compared with wild type. It can thus be concluded that cellulose biosynthesis is able to employ alternative UDP‐glucose biosynthesis pathway(s), and thereby the model of SUS requirements for cellulose biosynthesis in Arabidopsis can be refuted.
Significance Statement
Many models of cellulose biosynthesis in plants imply that sucrose synthase channels the substrate UDP‐glucose to the plasma membrane localized cellulose synthase complex. Here we used sextuple Arabidopsis mutants devoid of sucrose synthase activity to test this hypothesis directly. The results show that sucrose synthase is not required for cellulose biosynthesis in Arabidopsis.
All photosynthetic organisms balance CO2 assimilation with growth and carbon storage. Stored carbon is used for growth at night and when demand exceeds assimilation. Gaining a mechanistic ...understanding of carbon partitioning between storage and growth in trees is important for biological studies and for estimating the potential of terrestrial photosynthesis to sequester anthropogenic CO2 emissions.1,2 Starch represents the main carbon storage in plants.3,4 To examine the carbon storage mechanism and role of starch during tree growth, we generated and characterized low-starch hybrid aspen (Populus tremula × tremuloides) trees using CRISPR-Cas9-mediated gene editing of two PHOSPHOGLUCOMUTASE (PGM) genes coding for plastidial PGM isoforms essential for starch biosynthesis. We demonstrate that starch deficiency does not reduce tree growth even in short days, showing that starch is not a critical carbon reserve during diel growth of aspen. The low-starch trees assimilated up to ∼30% less CO2 compared to the wild type under a range of irradiance levels, but this did not reduce growth or wood density. This implies that aspen growth is not limited by carbon assimilation under benign growth conditions. Moreover, the timing of bud set and bud flush in the low-starch trees was not altered, implying that starch reserves are not critical for the seasonal growth-dormancy cycle. The findings are consistent with a passive starch storage mechanism that contrasts with the annual Arabidopsis and indicate that the capacity of the aspen to absorb CO2 is limited by the rate of sink tissue growth.
•Aspen trees employ a passive starch-storage mechanism during growth•Carbon assimilation is not limiting growth of aspen trees under benign conditions•Starch is not required for bud set and bud flush or its timing in aspen trees
Wang et al. create low-starch aspen mutants and discover that aspen trees employ a passive investing strategy to save carbon for future needs and that tree growth is not carbon limited under benign conditions.
A cryopreservation method was developed for a Norway spruce (
Picea abies
L. Karst.) cell line characterised by highly vacuolated cells and ability to produce natural-like extracellular lignin in a ...cell suspension culture. Spruce callus cultured in a photoperiod of 16 h light, 8 h dark contained two types of callus morphologies. Soft callus was composed of loosely bound cells that dispersed into single cells and small cell aggregates when transferred into liquid medium. The callus with hard morphology had also cells that were more tightly attached to each other; this callus formed bigger cell aggregates in liquid medium in addition to single cells and small cell aggregates. The hard callus contained higher concentration of intracellular phenolic compounds as compared to soft callus. For cryopreservation, a vitrification method with plant vitrification solution 2 (PVS2) was used. To reduce cellular water content, spruce calli were pre-cultured on a culture medium with increasing sucrose concentration (0.2 and 0.4 M; one day on each). The cryopreservation survival rate of callus with hard morphology was significantly higher than that with soft morphology (45 ± 8% and 5 ± 5%, respectively). Pre-culturing in continuous light for several weeks led exclusively to formation of a hard-type callus, which had a survival rate of 48 ± 16% in cryopreservation. Expression of candidate genes of the monolignol biosynthesis pathway, Fourier transform infrared spectra and pyrolysis breakdown products of extracellular lignin were similar in control cultures and those originating from cryopreserved cells suggesting that cryopreservation is a feasible method for long-term storage of the lignin-forming cell line.
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