Sucrose is required for auxin-induction of the tomato expansin LeExp2

.Plant cell wall consists of a cellulose microfibrils network associated with matrix components. Expansins are thought to mediate cell wall loosening required for cell expansion in a pH-dependent manner (acid growth theory). We studied the regulation of the tomato (Lycopersicon esculentum cv St. Pierre) expansin gene LeExp2 in isolated segments from etiolated hypocotyl, since LeExp2 is the major expansin gene expressed in growing hypocotyl tissues (Catalá et al., 2000). We tested the ability of hormones with a known role in cell elongation (auxin, brassinolides and gibberellins), alone or combined with sucrose, to induce LeExp2 mRNA, and their effects on hypocotyl segments growth. Both auxin and brassinolide trigger hypocotyl elongation but the induction of LeExp2 by auxin/brassinolide stimulus is strongly dependent on sucrose. Auxin and brassinolide-induced elongation is instead sugar-independent. This work highlights the existence of an auxin/BL-sugar cross-talk, and it provides new insights into LeExp2 role in tomato hypocotyl growth.