Most of the symplastic water transport in plants occurs via aquaporins, but the extent to which aquaporins contribute to plant water status under favorable growth conditions and abiotic stress is not clear. To address this issue, we constitutively overexpressed the Arabidopsis plasma membrane aquaporin, PIP1b, in transgenic tobacco plants. Under favorable growth conditions, PIP1b overexpression significantly increased plant growth rate, transpiration rate, stomatal density, and photosynthetic efficiency. By contrast, PIP1b overexpression had no beneficial effect under salt stress, whereas during drought stress it had a negative effect, causing faster wilting. Our results suggest that symplastic water transport via plasma membrane aquaporins represents a limiting factor for plant growth and vigor under favorable conditions and that even fully irrigated plants face limited water transportation. By contrast, enhanced symplastic water transport via plasma membrane aquaporins may not have any beneficial effect under salt stress, and it has a deleterious effect during drought stress.

Sugars are key regulatory molecules that affect diverse processes in higher plants. Hexokinase is the first enzyme in hexose metabolism and may be a sugar sensor that mediates sugar regulation. We present evidence that hexokinase is involved in sensing endogenous levels of sugars in photosynthetic tissues and that it participates in the regulation of senescence, photosynthesis, and growth in seedlings as well as in mature plants. Transgenic tomato plants overexpressing the Arabidopsis hexokinase-encoding gene AtHXK1 were produced. Independent transgenic plants carrying single copies of AtHXK1 were characterized by growth inhibition, the degree of which was found to correlate directly to the expression and activity of AtHXK1 . Reciprocal grafting experiments suggested that the inhibitory effect occurred when AtHXK1 was expressed in photosynthetic tissues. Accordingly, plants with increased AtHXK1 activity had reduced chlorophyll content in their leaves, reduced photosynthesis rates, and reduced photochemical quantum efficiency of photosystem II reaction centers compared with plants without increased AtHXK1 activity. In addition, the transgenic plants underwent rapid senescence, suggesting that hexokinase is also involved in senescence regulation. Fruit weight, starch content in young fruits, and total soluble solids in mature fruits were also reduced in the transgenic plants. The results indicate that endogenous hexokinase activity is not rate limiting for growth; rather, they support the role of hexokinase as a regulatory enzyme in photosynthetic tissues, in which it regulates photosynthesis, growth, and senescence. INTRODUCTIONSugars are central compounds in nature that serve as essential metabolic nutrients and structural components for most organisms. They are also major regulatory molecules that control gene expression, metabolism, physiology, cell cycle, and development in prokaryotes and eukaryotes (Newgard and McGarry, 1995;Ronne, 1995;Saier et al., 1995). In plants, it has been shown that sugars regulate the expression of a broad spectrum of genes involved in many essential processes (e.g., Sheen, 1990;Thomas and Rodriguez, 1994;Stitt and Sonnewald, 1995; Graham, 1996; Koch, 1996;Smeekens, 1998). Furthermore, sugars affect developmental and metabolic processes throughout the life cycle of the plant. These processes include germination, growth, flowering, senescence, sugar metabolism, and photosynthesis (von Schaewen et al., 1990;Dickinson et al., 1991;Stitt et al., 1991; Goldschmidt and Huber, 1992; Huber and Hanson, 1992; Bernier et al., 1993; Ding et al., 1993; Yang et al., 1993;Thomas and Rodriguez, 1994; Dangl et al., 1995; Kovtun and Daie, 1995; Jang and Sheen, 1997;Perata et al., 1997;Prata et al., 1997;Smeekens and Rook, 1997; Wingler et al., 1998).Photosynthesis is regulated by sugar levels, and this regulation overrides that of light, tissue type, and developmental stage (Sheen, 1990;von Schaewen et al., 1990; Krapp et al., 1993). Increased concentrations of externally supplemented sugar o...

No abstract

No abstract

No abstract

Sugars are key regulatory molecules that affect diverse processes in higher plants. Hexokinase is the first enzyme in hexose metabolism and may be a sugar sensor that mediates sugar regulation. We present evidence that hexokinase is involved in sensing endogenous levels of sugars in photosynthetic tissues and that it participates in the regulation of senescence, photosynthesis, and growth in seedlings as well as in mature plants. Transgenic tomato plants overexpressing the Arabidopsis hexokinase-encoding gene AtHXK1 were produced. Independent transgenic plants carrying single copies of AtHXK1 were characterized by growth inhibition, the degree of which was found to correlate directly to the expression and activity of AtHXK1. Reciprocal grafting experiments suggested that the inhibitory effect occurred when AtHXK1 was expressed in photosynthetic tissues. Accordingly, plants with increased AtHXK1 activity had reduced chlorophyll content in their leaves, reduced photosynthesis rates, and reduced photochemical quantum efficiency of photosystem II reaction centers compared with plants without increased AtHXK1 activity. In addition, the transgenic plants underwent rapid senescence, suggesting that hexokinase is also involved in senescence regulation. Fruit weight, starch content in young fruits, and total soluble solids in mature fruits were also reduced in the transgenic plants. The results indicate that endogenous hexokinase activity is not rate limiting for growth; rather, they support the role of hexokinase as a regulatory enzyme in photosynthetic tissues, in which it regulates photosynthesis, growth, and senescence.

No abstract

These authors equally contributed to this work. SummaryFructokinases catalyze the key step of fructose phosphorylation in plants. LeFRK2, the major fructokinaseencoding gene in tomato plants, is abundantly expressed in roots, stems, and fruits. To analyze the role of LeFRK2 in plant development, we analyzed transgenic tomato plants with sense and antisense expression of StFRK, the potato homolog of LeFRK2. Increased fructokinase activity had no effect. However, plants in which LeFRK2 was speci®cally suppressed, either via antisense suppression or via co-suppression, exhibited growth inhibition and wilting of young leaves at daytime. Grafting experiments indicated that a stem interstock of antisense plants was suf®cient to inhibit growth and cause leaf wilting. Stem secondary xylem exhibited particular suppression of LeFRK2 and the area of active xylem, estimated by eosin uptake, was signi®cantly smaller in antisense stem compared to that of wild-type plants. These results suggest that LeFRK2 might be required for proper development of xylem that affected growth and wilting.