Sugar Levels Modulate Differential Expression of Maize Sucrose Synthase Genes.

The two genes encoding sucrose synthase in maize (Sh1 and Sus1) show markedly different responses to changes in tissue carbohydrate status. This enzyme is widely regarded as pivotal to sucrose partitioning, import, and/or metabolism by developing plant organs. Excised maize root tips were incubated for varying periods in different sugars and a range of concentrations. The Sh1 mRNA was maximally expressed under conditions of limited carbohydrate supply (~0.2% glucose). In contrast, Sus1 transcript levels were low or nondetectable under sugar-depleted conditions and peaked at 10-fold greater glucose concentrations (2.0%). Responses to other metabolizable sugars were similar, but L-glucose and elevation of osmolarity with mannitol had little effect. Plentiful sugar supplies thus increased expression of Sus1, whereas reduced sugar availability enhanced Sh1. At the protein level, shifts in abundance of subunits encoded by Sh1 and Sus1 were much less pronounced but corresponded to changes in respective mRNA levels. Although total enzyme activity did not show net change, cellular localization of sucrose synthase protein was markedly altered. In intact roots, sucrose synthase was most prevalent in the stele and apex. In contrast, sugar depletion favored accumulation in peripheral cells, whereas high sugar levels resulted in elevated expression in all cell types. The differential response of the two sucrose synthase genes to sugars provides a potential mechanism for altering the pattern of enzyme distribution in response to changing carbohydrate status and also for adjusting the sucrose-metabolizing capacity of importing cells relative to levels of available photosynthetic products.

Sucrose synthase and invertase in isolated vascular bundles.

Vascular bundles were isolated from grapefruit (Citrus paradisi Macf.) during periods of rapid sucrose translocation into fruit. Invertase and sucrose synthase activities were assayed in these strands and compared with immediately adjacent tissues (inner most peel and segment epidermis) and phloem-free juice sacs during four growing seasons. Although sucrose synthase was present in sink cells, the significantly greater activity in vascular strands (per unit fresh weight and protein) indicated that the role of this enzyme in translocation may include a vascular function in addition to its proposed involvement in metabolism of importing cells.

Organ-specific invertase deficiency in the primary root of an inbred maize line.

An organ-specific invertase deficiency affecting only the primary root system is described in the Oh 43 inbred maize (Zea mays). Invertases (acid and neutral/soluble and insoluble) were assayed in various tissues of hybrid (NK 508) and inbred (Oh 43, W 22) maize lines to determine the basis for an early report that Oh 43 root tips were unable to grow on sucrose agar (27). Substantial acid invertase activity (7.3 to 16.1 micromoles of glucose per milligram of protein per hour) was evident in extracts of all tissues tested except the primary root system of Oh 43. This deficiency was also evident in lateral roots arising from the primary root. In contrast, morphologically identical lateral roots from the adventitious root system had normal invertase levels. These results suggest that ontogenetic origin of root tissues is an important determinant of invertase expression in maize. Adventitious roots (including the seminals) arise above the scutellar node and are, therefore, of shoot origin. The Oh 43 deficiency also demonstrated that invertase activity was not essential for maize root growth. Sucrose synthase was active in extracts from all root apices and theoretically provided the only available avenue for sucrose degradation in primary root tips of Oh 43. The deficiency described here will provide a useful avenue of investigation into the expression and significance of root invertase.

ACCUMULATION OF PHOSPHORUS, DRY MATTER AND BETAINE DURING NaCl STRESS OF SPLIT-ROOT CITRUS SEEDLINGS COLONIZED WITH VESICULAR-ARBUSCULAR MYCORRHIZAL FUNGI ON ZERO, ONE OR TWO HALVES.

Sodium chloride tolerance and phosphorus content were examined in split-root Carrizo citrange seedlings [Poncirus trifoliata (L.) Raf. ×Citrus sinensis (L.) Osbeck] colonized with a vesicular-arbuscular mycorrhizal fungus (Glomus intraradices Schenck & Smith) on zero, one or two root halves. Plants were treated with NaCl at 0, 25, 50 or 100 mM, and the degree of stress was measured as reduction of dry matter accumulation and rise in level of leaf proline-betaine (stachydrine). Shoot and root dry weight production during this period decreased with increasing levels of salt. Absolute reductions were similar for plants inoculated on one vs two half-root systems, but percentage decreases were less in the latter due to greater overall growth in all treatments. Betaine levels in leaf tissues were positively related to soil salt levels for each mycorrhizal treatment. Significant differences in betaine levels were also detected in plants with and without mycorrhizal fungi, and mean levels tended to be higher for those colonized on one vs two halves of their root system. In contrast, a half-root system and its fungal symbiont supplied enough phosphorus to allow concentrations of leaf P to equal those of fully infected root systems, yet the two groups did not show equal growth under control conditions or percentage reductions with NaCl stress.