BON1 gives plants a boost in the cold.

Hua/CSH Press

An increase in the rate of exocytosis has been put forth as one hypothesis to explain how plants maintain their growth rate in cold weather, in work from Jian Hua (now at Cornell University, New York, NY), Gerald Fink (Whitehead Institute, Cambridge, MA) and colleagues. They base their suspicion on a plant mutant that is a midget only in the cold.The mutant, bonzai1 (bon1), was isolated by Hua. “In the beginning I was just looking for something that would affect cell division and expansion,” she says. But she ended up with bon1, which at 2°C is almost normal but at 22°C is a real shrimp, with epidermal cells and stems that are seven and eight times shorter than normal, respectively. The mutant also has fewer cells when grown in the cold, although this may be secondary to the reduction in cell volume.

BON1 belongs to a poorly characterized family of proteins called the copines. Like other members of this family it has a calcium-stimulated phospholipid binding activity, and can enhance vesicle aggregation in vitro. Little else is known about copines, but the binding of one to secretory vesicles, and the localization of BON1 to the plasma membrane have led to suggestions that copines participate in exocytosis.

The expression of BON1 in growing tissues, and its increased expression at low temperatures, suggest that BON1 could compensate for otherwise inefficient membrane fusion at low temperature. “It makes sense having this hypothesis,” says Hua, “but there is no direct evidence.” ▪


Hua, J., et al.
Genes Dev.