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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 (<i>CPT3</i>), small rubber particle protein (<i>SRPP</i>), farnesyl pyrophosphate synthase (<i>FPPS</i>), and allene oxide synthase (<i>AOS</i>) in guayule seedlings grown <i>in vitro</i> 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 <i>P</i>. <i>argentatum</i> 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.