Phospholipases in the Response of Fortune Mandarins to Heat and Cold Stress and in the Heat-Induced Cross-Adaption to Chilling

Phospholipases (PLs) are key enzymes involved in membrane remodeling, structure, and signaling during plant stress responses. This study analyzed early and long-term transcriptional regulation of PL isoforms—PLD (CsPLDα,β,δ,γ,ζ), PLA (CsPLA2α,2β, CsPAT1), and PLC (CsNPC3, CsPI-PLC)—in Fortune mandarins exposed to thermal stress. Three conditions were evaluated: heat stress (37 °C, 90–95% relative humidity), cold stress (2 °C), and cold stress in fruit exposed to a prior heat conditioning treatment (3 days at 37 °C) that reduces chilling injury (CI). All genes—except CsNPC3—were rapidly upregulated by heat (4–12 h), suggesting a role as early heat sensors and their putative participation in the heat-induced chilling tolerance. Moreover, after transferring heat-conditioned fruit to cold, CsPLDζ, CsPI-PLC, and CsNPC3 showed marked expression increases, highlighting their relevance in such cross-adaptation. In non-conditioned fruit, CsPLDγ was strongly and consistently induced by cold and associated with CI development, as further confirmed by tissue-specific analysis comparing necrotic and healthy flavedo. Conversely, cold-induced CsPLDα, CsPLDζ, CsPLA2α, and CsNPC3 may contribute to protective responses via early signaling cascades; although CsPLA2α might contribute to CI only by 60 days. These findings highlight the isoform-specific and time-dependent regulation of PL genes under thermal stress and their potential role in citrus stress tolerance.