Nitric Oxide Participates in Aluminum-Stress-Induced Pollen Tube Growth Inhibition in Tea (Camellia sinensis) by Regulating CsALMTs

Nitric oxide (NO), as a signal molecule, is involved in the mediation of heavy-metal-stress-induced physiological responses in plants. In this study, we investigated the effect of NO on Camellia sinensis pollen tubes exposed to aluminum (Al) stress. Exogenous application of the NO donor decreased the pollen germination rate and pollen tube length and increased the malondialdehyde (MDA) content and antioxidant enzyme activities under Al stress. Simultaneously, the NO donor effectively increased NO content in pollen tube of C. sinensis under Al stress and could aggravate the damage of Al(3+) to C. sinensis pollen tubes by promoting the uptake of Al(3+). In addition, application of the NO-specific scavenger significantly alleviated stress damage in C. sinensis pollen tube under Al stress. Moreover, 18 CsALMT members from a key Al-transporting gene family were identified, which could be divided into four subclasses. Pearson correlation analysis showed the expression level of CsALMT8 showed significant positive correlation with the Al(3+) concentration gradient and NO levels, but a significant negative correlation with pollen germination rate and pollen tube length. The expression level of CsALMT5 was negatively correlated with the Al(3+) concentration gradient and NO level, and positively correlated with pollen germination rate and pollen tube length. The expression level of CsALMT17 showed a significant negative correlation with Al(3+) concentration and NO content in pollen tubes, but significant positive correlation with pollen germination rate and pollen tube length. In conclusion, a complex signal network regulated by NO-mediated CsALMTs revealed that CsALMT8 was regulated by environmental Al(3+) and NO to assist Al(3+) entry into pollen tubes; CsALMT5 might be influenced by the Al(3+) signal, stimulate malate efflux in vacuoles and chelate with Al(3+) to detoxify Al in C. sinensis pollen tube.