This simplified model was based on the models proposed by Wang et al. [24]. Cold stress induces an increase in NO through the accumulation of NOS-like activity followed by GC stimulation resulting in cGMP enhancing. As a consequence, the proline (Pro) is stimulated. Together, these pathways lead to tip growth contraction. Black arrows indicate the links established in the induction of pollen tube development; broken arrows represent indirect or still undescribed pathways in pollen tube tip growth. NOS, nitric oxide synthase; GC, guanylyl cyclase; cGMP, cyclic guanosine monophosphate; PDE, phosphodiesterase; P5CS, delta 1-pyrroline-5-carboxylate synthase; P5CR, Pyrroline-5-carboxylate reductase; ProDH, Pro dehydrogenase; ⊥, inhibition.