Response of Calcium-Dependent Protein Kinase Genes’ Expression in ‘Feizixiao’ Litchi Pulp to Foliar Nutrient Treatment of Calcium–Magnesium Mixed Solution and Their Regulation of Sugar Transformation

Previous studies have shown that foliar spraying with a 0.3% CaCl₂ + 0.3% MgCl₂ solution can mitigate the "sugar receding" phenomenon in fruit pulp, partly by regulating sugar conversion in the pulp of 'Feizixiao' litchi (<i>Litchi chinensis</i> Sonn.). Given that calcium-dependent protein kinases (CDPKs) in plants regulate sugar metabolism by modulating the activity of key sugar conversion enzymes, this study investigated the expression response of <i>CDPK</i> genes in 'Feizixiao' litchi pulp to foliar calcium-magnesium nutrient treatment and their regulatory characteristics on sugar conversion. After the fruit set, 'Feizixiao' litchi trees were subjected to three consecutive foliar spray applications of 0.3% CaCl₂ + 0.3% MgCl₂, with water spraying as the control. The dynamic changes in peel <i>h</i> values and soluble sugar and monosaccharides, water-soluble calcium (Ca²⁺) and magnesium (Mg²⁺), plant hormones, and the concentration of CDPKs in the pulp were compared throughout fruit development. Key differentially expressed members of the <i>CDPK</i> gene family were screened through real-time quantitative PCR analysis. The results showed that the peel color transition occurred earlier in the control (CK) than in the treatment (T), but the coloration process accelerated in the treated fruit, leading to no significant difference in peel <i>h</i> values between the groups at 76 days after anthesis (DAA), when both reached the lowest levels. The total of soluble sugar in the pulp peaked at 70 DAA in both groups, but while the CK exhibited a significant decline thereafter, T maintained stable sugar levels, thereby mitigating the "sugar receding" phenomenon. Water-soluble calcium and water magnesium levels were significantly higher in the T at 42 and 63 DAA, with water calcium remaining significantly higher at 70 DAA. Furthermore, sucrose, glucose, fructose, abscisic acid (ABA) contents, and CDPK concentration were significantly higher in the T at 70 and 76 DAA. The <i>CDPK</i> gene family members <i>LcCDPK1</i>, <i>LcCDPK2</i>, <i>LcCDPK3</i>, <i>LcCDPK4</i>, <i>LcCDPK5</i>, <i>LcCDPK9</i>, <i>LcCDPK15</i>, and <i>LcCDPK17</i> were upregulated in response to T. Among them, <i>LcCDPK1</i>, <i>LcCDPK4</i>, <i>LcCDPK5</i>, <i>LcCDPK9</i>, and <i>LcCDPK17</i> were identified as key structural genes due to their significant correlation with soluble sugar content and CDPK concentration, as well as their differential expression between T and CK. In conclusion, foliar calcium-magnesium nutrient treatment upregulates the expression of these five CDPK gene family members by increasing the ABA levels in the pulp, leading to more CDPK accumulation. This accumulation inhibits sugar conversion and promotes sucrose and fructose accumulation, thereby mitigating the "sugar receding" phenomenon in 'Feizixiao' litchi pulp.