Warm temperature and mild water stress cooperatively promote root elongation.

Warm temperatures have a dramatic effect on plant development. In shoots, stems elongate, and leaves are raised in a developmental program called thermomorphogenesis. This results in enhanced leaf cooling capacity. Roots also undergo thermomorphogenesis, but the mechanism by which this occurs is less well understood. Main root elongation is enhanced at warm temperatures, in an autonomous response that likely requires other signaling pathways than those involved in light and temperature signaling of the shoot. It was recently speculated that due to the close correlation between warm temperature and soil moisture content, root temperature signaling could feasibly be related to water availability signals. In this study, we tested the interaction between warm temperature and water stress signaling in plant roots. We found that these environmental factors cooperatively enhance main root elongation. This interaction effect was dependent on sucrose non-fermenting-related kinase 2.2 (SnRK2.2) and SnRK2.3 and the E3 ubiquitin ligase constitutively photomorphogenic 1 (COP1). We found that SnRK2.2/2.3 and COP1 have opposite effects on the stability of the transcription factor elongated hypocotyl 5 (HY5) in elongation zone trichoblasts. The stability of HY5 in these cell types generally corresponded to the degree of root elongation seen in each mutant background. Our study reveals several molecular components of root thermomorphogenesis and highlights the importance of an integrative approach to plant environmental signaling. Our results may have direct implications for agricultural land management, especially as global climates become more unpredictable.