The balance between plant growth and stress response is a key issue in the field of biology. In this process, mitogen-activated protein kinase 3 (MPK3) and MPK6 contribute to the construction of plants’ defense system during stress tolerance, while auxin, a growth-promoting hormone, is the key to maintaining plant growth. Nevertheless, the antagonistic or cooperative relationship between MPK3/6-mediated stress response and auxin-mediated plant growth remains unclear. Here, we demonstrate that stress-activated MPK3/6 interact with the auxin signaling repressors indoleacetic acid-induced protein 8 (IAA8) and IAA9, two key targets for regulating the auxin signaling output during stress responses. Protein phosphorylation mass spectrometry followed by a co-analysis with in vitro phosphorylation experiments revealed that MPK3/6 phosphorylated the S91, T94, and S152 residues of IAA8 and the S88 residue of IAA9. Phosphorylation significantly enhanced the protein stability of IAA8/9, thereby maintaining basal auxin signaling during plant stress adaptation. Collectively, MPK3/6-IAA8/9 are key modules that are turned on during plant stress adaptation to precisely reduce auxin signaling output, thereby preventing plants from improper vigorous growth under stress conditions.

In molecular biology studies, suitable vectors are fundamental tools; however, most vectors can only express one target gene, which limits the ability to study multiple genes simultaneously within the same plant tissue. The traditional method for achieving multi-gene co-expression involves co-transferring multiple plasmids into plant tissues, but this approach is often inefficient due to the difficulty of successfully transforming multiple plasmids at once. To overcome this limitation, we have developed a series of vectors, called pMAGs (Multigene Assembly Genetic vectors), capable of simultaneously expressing or silencing two or three different genes in plants. These vectors not only provide an optimal solution for a wide range of biological experiments but also work effectively across numerous plant species.