Overexpression of Arabidopsis hexokinase in tomato plants inhibits growth,reduces photosynthesis, and induces rapid senescence

Sugars are key regulatory molecules that affect diverse processes in higher plants. Hexokinase is the first enzyme in hexose metabolism and may be a s ugar sensor that mediates sugar regulation. We present evidence that hexoki nase is involved in sensing endogenous levels of sugars in photosynthetic t issues and that it participates in the regulation of senescence, photosynth esis, and growth in seedlings as well as in mature plants. Transgenic tomat o plants overexpressing the Arabidopsis hexokinase-encoding gene AtHXK1 wer e produced. independent transgenic plants carrying single copies of AtHXK1 were characterized by growth inhibition, the degree of which was found to c orrelate directly to the expression and activity of AtHXK1. Reciprocal graf ting 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 p hotosynthesis rates, and reduced photochemical quantum efficiency of photos ystem II reaction centers compared with plants without increased AtHXK1 act ivity. In addition, the transgenic plants underwent rapid senescence, sugge sting that hexokinase is also involved in senescence regulation. Fruit weig ht, starch content in young fruits, and total soluble solids in mature frui ts were also reduced in the transgenic plants. The results indicate that en dogenous hexokinase activity is not rate limiting for growth; rather, they support the role of hexokinase as a regulatory enzyme in photosynthetic tis sues, in which it regulates photosynthesis, growth, and senescence.