MYB93 regulates responses to environmental sulphur in Arabidopsis and tomato.

Sulphur (S) is an important nutrient that has wide-ranging effects on plant health and metabolism. Several classes of transcription factor respond to S deprivation, including R2R3-MYBs. In Arabidopsis, the AtMYB93 transcription factor-encoding gene is upregulated by S deprivation. AtMYB93 has a non-redundant function in lateral root development and redundant functions in suberin biosynthesis alongside related MYB transcription factors, but AtMYB93's role in S signalling, and how it relates to lateral root development, is unknown. We show that the transcriptome of Atmyb93 mutant roots implicates AtMYB93 in responses to S, including changes in S transport and metabolism, and flavonoid- and carbohydrate metabolism. Elemental analysis demonstrates that the Atmyb93 mutant has elevated shoot S levels while tomato SlMYB93-overexpressing plants have reduced shoot S. We uncover a stimulatory effect of S deprivation on adventitious root development. However, Atmyb93 mutants do not show significant changes in sensitivity to S with respect to lateral- or adventitious root development, most likely due to some functional redundancy. Moreover, AtMYB93 promoter activity is not spatially regulated by S deprivation. Taken together, our data suggest that AtMYB93 has a role in mediating root responses to S in alongside other root transcription factors.