Effect of salicylic acid on morphological traits and the expression of key rubber biosynthesis genes in Parthenium argentatum A. Gray

Guayule ( Parthenium argentatum A. Gray) serves as a critical, drought-tolerant alternative source of natural rubber for medical and industrial applications. While salicylic acid is a fundamental signaling molecule in plant development, stress responses, and secondary metabolism, its specific influence on the gene expression of rubber biosynthesis in guayule remains poorly understood. This study evaluated the effects of exogenous salicylic acid on the morphological traits and regulation of cis-prenyltransferase 3 ( CPT3 ), small rubber particle protein ( SRPP ), farnesyl pyrophosphate synthase ( FPPS ), and allene oxide synthase ( AOS ) in guayule seedlings grown in vitro condition. Seedlings at the four-leaf stage were treated with varying concentrations of salicylic acid (0, 2.5, 5, and 7.5 µM) for four weeks. Treatment with exogenous salicylic acid induced a biphasic morphological response in P . argentatum seedlings. Morphological analysis revealed a dose-dependent response: although moderate concentrations (2.5 and 5 µM) showed a trend toward increased shoot and root elongation, these differences were not statistically significant. However, 7.5 µM inhibited elongation but significantly stimulated leaf and shoot proliferation. This dose-dependent shift highlighted the hormetic potential of salicylic acid in modulating plant morphogenesis without affecting seedling viability. In addition, quantitative gene expression analysis showed significant, concentration-dependent upregulation of CPT3 and SRPP , alongside gradual increases in AOS expression. However, FPPS was significantly induced only at 7.5 µM salicylic acid. These results demonstrate that salicylic acid is a potent modulator of both vegetative growth and the genes expression of rubber biosynthesis pathway. Consequently, these findings provide a molecular foundation for employing hormonal elicitors to upregulate key biosynthetic genes in guayule biotechnology.