Scientists at the Sainsbury Laboratory Cambridge University (SLCU) have discovered a new molecular signalling pathway, triggered when leaves are exposed to low humidity, that ensures plant roots keep growing towards water. A new study has found that when the leaves of a plant are exposed to dry air (low humidity), they send a shoot-to-root signal, using abscisic acid (ABA), to tell the roots to keep growing. This is a surprising finding as ABA is usually thought to be a growth inhibitor, not a growth promoter.