Phosphorus deficiency suppresses the <i>trans-</i>zeatin riboside accumulation by up-regulating <i>CeIPT5</i>, promoting root elongation and phosphorus absorption efficiency of <i>Casuarina equisetifolia</i>

Phosphorus (P) deficiency is critical to the renewal barrier of she-oak (Casuarina equisetifolia), an important tree species used for coastal protection. However, the response of she-oak to P deficiency remains unclear. In this study, we compared the phenotypes of two she-oak cultivars, the P deficiency-sensitive 'Chihu219' and the insensitive 'Chihu397', and found that P deficiency significantly increased root growth, P concentration and phosphorus absorption efficiency (PAE) in Chihu219, but not in Chihu397. We also analyzed the transcriptome and metabolome of these cultivars under different P conditions and showed that trans-zeatin riboside (tZR) levels were highly suppressed by P deficiency in Chihu219, but not in Chihu397. Furthermore, exogenous tZR suppressed both root P concentration and PAE while promoting P-use efficiency. We also identified CeIPT5 (isopentenyltransferase 5) as a key regulatory gene of tZR biosynthesis and found that its expression was more highly induced by P deficiency in Chihu219 than in Chihu397. We also showed that overexpression of CeIPT5 in insensitive she-oak lines reduced tZR concentration and increased root P concentration compared with the vector control. Taken together, P deficiency can greatly reduce tZR accumulation in P deficiency-insensitive she-oak at least by activating the tZR accumulation regulatory gene, CeIPT5, thereby promoting root elongation and P concentration. This study not only provides a genetic basis for enhancing PAE in woody plants but also establishes a theoretical basis for optimizing root structure and improving nutrient utilization efficiency, thereby promoting sustainable forestry development.