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

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 <i>Camellia sinensis</i> 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 <i>C. sinensis</i> under Al stress and could aggravate the damage of Al³⁺ to <i>C. sinensis</i> pollen tubes by promoting the uptake of Al³⁺. In addition, application of the NO-specific scavenger significantly alleviated stress damage in <i>C. sinensis</i> pollen tube under Al stress. Moreover, 18 <i>CsALMT</i> 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 <i>CsALMT8</i> showed significant positive correlation with the Al³⁺ concentration gradient and NO levels, but a significant negative correlation with pollen germination rate and pollen tube length. The expression level of <i>CsALMT5</i> was negatively correlated with the Al³⁺ concentration gradient and NO level, and positively correlated with pollen germination rate and pollen tube length. The expression level of <i>CsALMT17</i> showed a significant negative correlation with Al³⁺ 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 <i>CsALMTs</i> revealed that <i>CsALMT8</i> was regulated by environmental Al³⁺ and NO to assist Al³⁺ entry into pollen tubes; <i>CsALMT5</i> might be influenced by the Al³⁺ signal, stimulate malate efflux in vacuoles and chelate with Al³⁺ to detoxify Al in <i>C. sinensis</i> pollen tube.