Overexpressing PURPLE ACID PHOSPHATASE 2 inhibits primary root elongation in response to heat stress

Roots play a critical role in plant adaptation to heat stress, yet the underlying mechanisms of heat tolerance remain poorly understood. This study uncovers that overexpression of PURPLE ACID PHOSPHATASE 2 (PAP2) in Arabidopsis leads to shortened primary root length and reduced sensitivity to 1-naphthaleneacetic acid (NAA) treatment under moderate heat stress conditions. Phytohormone analysis revealed that under moderate-high temperature conditions, PAP2 overexpression leads to an overaccumulation of indole-3-acetic acid (IAA) while concomitantly reducing levels of its precursor, indole-3-pyruvate acid (IPyA), and the IAA storage conjugates, indole-3-acetyl-aspartate (IAA-Asp) and indole-3-acetyl-glutamate (IAA-Glu). Additionally, IAA is distributed to the elongation zone of the root tips, as demonstrated by pDR5-GUS staining. RNA sequencing (RNA-seq) and qRT-PCR analysis indicated that PAP2 overexpression upregulates the expression of several YUCs, key genes involved in IAA biosynthesis. Furthermore, the expression levels of GH3.9, ABCB16 and ABCB17 related to auxin metabolism and distribution in the PAP2 overexpression lines are significantly upregulated compared with those in the wild type under moderate-high temperature. Taken together, these results suggest that the reduced root elongation under heat stress in PAP2 overexpression lines is linked to changes in auxin metabolism and distribution, providing new insights into the mechanisms of plant root responses to heat stress.