Heat stress reveals a specialized variant of the pachytene checkpoint in meiosis of Arabidopsis thaliana

Abstract

Plant growth and fertility strongly depend on environmental conditions such as temperature. Remarkably, temperature also influences meiotic recombination and thus, the current climate change will affect the genetic make-up of plants. To better understand the effects of temperature on meiosis, we followed male meiocytes in Arabidopsis thaliana by live cell imaging under three temperature regimes: at 21°C; at heat shock conditions of 30°C and 34°C; after an acclimatization phase of 1 week at 30°C. This work led to a cytological framework of meiotic progression at elevated temperature. We determined that an increase from 21°C to 30°C speeds up meiosis with specific phases being more amenable to heat than others. An acclimatization phase often moderated this effect. A sudden increase to 34°C promoted a faster progression of early prophase compared to 21°C. However, the phase in which cross-overs mature was prolonged at 34°C. Since mutants involved in the recombination pathway largely did not show the extension of this phase at 34°C, we conclude that the delay is recombination-dependent. Further analysis also revealed the involvement of the ATAXIA TELANGIECTASIA MUTATED kinase in this prolongation, indicating the existence of a pachytene checkpoint in plants, yet in a specialized form.

Caught in the Act: Live-Cell Imaging of Plant Meiosis

“…This method also allows the addition of drugs such as oryzalin to the imaging medium; in the case of oryzalin, the effects induced by microtubule depolymerization can then be monitored in living meiocytes (Sofroni et al, 2020). Moreover, this imaging setup was also used to follow meiotic progression under heat stress, which was induced by using a heated incubation chamber surrounding the microscopic stage (De Jaeger-Braet et al, 2021).…”

“…Live-cell imaging revealed then that an increase of the ambient temperature to 30 • C resulted in an acceleration of most but not all meiotic phases in Arabidopsis (De Jaeger-Braet et al, 2021). For instance, the duration of metaphase I to anaphase I was shortened from nearly 1 h to approximately 0.5 h at 30 • C. An additional temperature raise to 34 • C sped up specific phases of meiosis even further, e.g., late leptotene to early pachytene lasted approximately 14 h at 21 • C, 9 h at 30 • C and only 7 h at 34 • C. At 34 • C, however, pachytene to diakinesis was considerably delayed from 6 h at 21 • C and 30 • C to almost 9 h at 34 • C. This delay, together with genetic and cell biological data, indicated the presence of a recombination checkpoint in plants (see below) (De Jaeger-Braet et al, 2021).…”

“…Being able to follow recombination in real time will open the door to a new level of understanding of this central aspect of meiosis. A first example comes from live-cell imaging of meiosis in Arabidopsis under heat stress (34 • C) that uncovered the presence of a long-doubted recombination checkpoint, also known as pachytene checkpoint (De Jaeger-Braet et al, 2021).…”

“…However, imaging dmc1 and other mutants in which recombination is abolished such as spo11-1 at 34 • C revealed that the duration of pachytene reverted to almost the level seen in the wildtype at 21 • C indicating that the pachytene delay is recombination dependent. Importantly, the pachytene delay in Arabidopsis is also absent in atm mutants exposed to 34 • C (De Jaeger-Braet et al, 2021). Thus, it appears that there is a pachytene checkpoint in plants.…”

“…Furthermore, live-cell imaging has also the great potential to contribute to an understanding of species-specific difference in meiosis. For instance, the duration of leptotene and early pachytene was extended in spo11-1 mutants in Arabidopsis while in yeast, the corresponding mutants progress faster through meiosis than the wildtype (Klapholz et al, 1985;Jiao et al, 1999;Cha et al, 2000;De Jaeger-Braet et al, 2021). Thus, the era of live-cell imaging for meiosis has just begun, with many exciting discoveries ahead of us.…”