Ovary signals for directional pollen tube growth

In angiosperms, the female gametophyte has a secluded life; it is protected by several concentric layers that envelop each other. The embryo sac is su rrounded by the nucellus, which in turn is wrapped by the integuments formi ng the ovule, which is nested in the ovary. These wrappings are not hermeti c, but contain little "gates" the pollen tube must traverse on its way towa rds the embryo sac. Information is emerging that shows that the ovary and o vule provide signals orienting and directing the pollen tube on the right c ourse. There are three main bodies of evidence supporting this hypothesis. One relates to developmental changes in the female tissues and how they aff ect pollen tube growth. The second refers to defective ovule mutants, which induce defective pollen tube guidance. And the third relates to the possib le molecules involved in this signalling. Here, information gathered along these three main lines of evidence is reviewed. All converge to the conclus ion that different checkpoints exist all along the pollen tube pathway. The se checkpoints provide active signalling that guides the pollen tube to its destination, the embryo sac.