NOX enzyme, by producing ROS, is needed to sustain the normal rate of pollen tube growth and this is likely to be a general mechanism in the control of tip growth of polarized plant cells [
26,
27]. NOXs generate tip-localized, pulsating amounts of H
2O
2 that functions, possibly through Ca
2+ channel activation, to maintain a steady, tip-focused Ca
2+ gradient in pollen tube tip during growth [
28]. On the other hand, pollen NOX can be activated by Ca
2+ and Ca
2+ can increase its activity in vivo [
26,
29]. This process would happen especially when the pollen grains are hydrated under mild conditions, so the activity of pollen NOX could be concentrated in those insoluble fractions, which could facilitate the exposure of tissues to ROS produced by this enzyme. It is worth mentioning that the extent of this exposure could differ among the plant families according to where the NOX resides in pollen grains and during hydration in the mucosa [
30]. In addition, pollen NOX can also be activated by low abundant signaling phospholipids, such as phosphatidic acid (PA) and phosphatidylinositol 4, 5-bisphosphate in vitro and in vivo, so there is a possible synergism between Ca
2+ and phospholipid-mediated NOX activation in pollen [
29]. In plants, ROP/RAC GTPases (the subfamily of Rho-type GTPases, which belong to the Rat sarcoma superfamily of small GTP-binding proteins) are also necessary for normal pollen tube growth by regulating ROS production [
29]. Besides these items mentioned above, O
3 was also indicated to increase ragweed pollen allergenicity through stimulation of ROS-generating NADPH oxidase [
31]. Recently, two NOXs/RBOHs, AtRbohH and AtRbohJ, have been shown to localize at the plasma membrane of pollen tube tip and the ROS production by the NOXs/RBOHs presumably plays a critical role in the positive feedback regulation that maintains the pollen tube tip growth [
32,
33]. Furthermore, apoplastic ROS derived from AtRbohH and AtRbohJ are involved in pollen tube elongation by affecting the cell wall metabolism [
33]. More intriguing is that ectopic expressing of AtRbohC/hair root2 (hrd2) in pollen tubes could restore
atrbohH and
atrbohJ defects in tip growth of pollen tubes [
34], which implies that AtRbohC/hrd2 also plays roles in regulating the development of pollen tubes. Moreover, AtRbohE was also suggested to be critical for programmed cell death (PCD) and pollen development in
Arabidopsis thaliana L. [
35]. Meanwhile, OeRbohH, possessing a high degree of identity with AtRbohH/J, plays an important role in pollen germination and pollen tube growth in olive [
36]. Genetic interference with the temporal ROS pattern by manipulating
NOX/RBOH genes, affected the timing of tapetal PCD (programmed cell death) and resulted in aborted male gametophytes [
35]. All in all, we still cannot figure out how many factors are related to the regulation of pollen NOX. Nevertheless, what we already know is that pollen NOXs play a significant part in the regulation of pollen germination.