Plant hormones (Literature sources on phytohormones and plant signalling)

Journal of Experimental Botany (2023)

Abstract: "Twenty-five years ago, a seminal paper demonstrated that warm temperatures increase auxin levels to promote hypocotyl growth in Arabidopsis thaliana. Here we highlight recent advances in auxin-mediated thermomorphogenesis and identify unanswered questions. In the warmth, PHYTOCHROME INTERACTING FACTOR 4 (PIF4) and PIF7 bind the YUCCA 8 gene promoter and, in concert with histone modifications, enhance its expression to increase auxin synthesis in the cotyledons. Once transported to the hypocotyl, auxin promotes cell elongation. The meta-analysis of expression of auxin-related genes in seedlings exposed to temperatures ranging from cold to hot shows complex patterns of response. Changes in auxin only partially account for these responses. The expression of many SMALL AUXIN UP RNA (SAUR) genes reaches a maximum in the warmth, decreasing towards both temperature extremes in correlation with the rate of hypocotyl growth. Warm temperatures enhance primary root growth, the response requires auxin, and the hormone levels increase in the root tip but the impacts on cell division and cell expansion are not clear. A deeper understanding of auxin-mediated temperature control of plant architecture is necessary to face the challenge of global warming."

The EMBO Journal (2023)

Synopsis: Plant root growth is influenced by temperature variation. This article shows that elevated temperatures directly promote primary root growth through an auxin-mediated response that increases cell division rates in the root apical meristem. Plant roots respond to warmer temperatures by promoting primary root growth. Roots are able to sense and respond to elevated temperature independently of shoot-derived signals through an as yet unknown root thermosensor. Growth promotion in roots is primarily achieved by increasing cell division rates in the root apical meristem. Auxin mediates a temperature-dependent increase in cell division rate depending on de novo local auxin biosynthesis and temperature-sensitive organization of the polar auxin transport system.

Abstract: "Roots are highly plastic organs enabling plants to adapt to a changing below-ground environment. In addition to abiotic factors like nutrients or mechanical resistance, plant roots also respond to temperature variation. Below the heat stress threshold, Arabidopsis thaliana seedlings react to elevated temperature by promoting primary root growth, possibly to reach deeper soil regions with potentially better water saturation. While above-ground thermomorphogenesis is enabled by thermo-sensitive cell elongation, it was unknown how temperature modulates root growth. We here show that roots are able to sense and respond to elevated temperature independently of shoot-derived signals. This response is mediated by a yet unknown root thermosensor that employs auxin as a messenger to relay temperature signals to the cell cycle. Growth promotion is achieved primarily by increasing cell division rates in the root apical meristem, depending on de novo local auxin biosynthesis and temperature-sensitive organization of the polar auxin transport system. Hence, the primary cellular target of elevated ambient temperature differs fundamentally between root and shoot tissues, while the messenger auxin remains the same."

Excerpts: " Through extensive experiments, a team led by Martin Luther University Halle-Wittenberg (MLU), was able to demonstrate that roots have their own temperature sensing and response system. In a new study in The EMBO Journal, the scientists also provide a new explanation for how roots themselves detect and react to higher temperatures. The results could help develop new approaches for plant breeding."

"The researchers found in all of their experiments that root cells increased the production of the growth hormone auxin, which was then transported to the root tips. There, it stimulated cell division and enabled the roots to reach further down into the soil. "As heat and drought usually occur in tandem, it makes sense for the plants to tap into deeper and cooler soil layers that contain water," Quint explains."