Vegetative phase change in Populus tremula × alba

Abstract

Plants transition through juvenile and adult phases of vegetative development in a process known as vegetative phase change (VPC). In poplars (genus Populus) the differences between these stages are subtle, making it difficult to determine when this transition occurs. Previous studies of VPC in poplars have relied on plants propagated in vitro, leaving the natural progression of this process unknown.We examined developmental morphology of seed-grown and in vitro derived Populus tremula 9 alba (clone 717-1B4), and compared the phenotype of these to transgenics with manipulated miR156 expression, the master regulator of VPC.In seed-grown plants, most traits changed from node-to-node during the first 3 months of development but remained constant after node 25. Many traits remained unchanged in clones over-expressing miR156, or were enhanced when miR156 was lowered, demonstrating their natural progression is regulated by the miR156/SPL pathway. The characteristic leaf fluttering of Populus is one of these miR156-regulated traits.Vegetative development in plants grown from culture mirrored that of seed-grown plants, allowing direct comparison between plants often used in research and those found in nature. These results provide a foundation for further research on the role of VPC in the ecology and evolution of this economically important genus.

MicroRNA156‐mediated changes in leaf composition lead to altered photosynthetic traits during vegetative phase change

“…Populus tremula × alba line 717‐1B4 and two independent miR156 overexpressor lines, 40 and 78, described in Lawrence et al . (2020), were obtained by in vitro propagation and hardened on propagation media as described in Meilan & Ma (2006). Plants were then transplanted to Fafard‐2 growing mix (Sun Gro Horticulture, Agawam, MA, USA) in 0.3 l pots in a glasshouse at the University of Pennsylvania (39.9493°N, 75.1995°W, 22.38 m above sea level (asl)) and kept in plastic bags for increased humidity for 2 wk.…”

“…Leaves 1–52 were measured in poplar plants germinated from seed. The juvenile‐to‐adult transition in these plants occurred between leaves 20 and 30, as determined by the change in petiole shape and trichome density (Lawrence et al ., 2020). In Z. mays , leaves 2–11 were measured with leaves 1–5 juvenile in wild‐type plants and all leaves juvenile in Cg1 mutants.…”

“…In P. tremula × alba 717-1B4 and miR156 overexpressor lines, leaves 10, 15, 20, and 25 were measured. Leaves 10 and 15 in the wild-type 717-1B4 line were juvenile and leaves 20 and 25 were adult, as determined by petiole shape and abaxial trichome density as described in Lawrence et al (2020). All leaves measured in the miR156 overexpressor lines were juvenile.…”

Florigen governs shoot regeneration

“…or pear 21 by cycling expression year-round. Florigen action in the meristem to induce flowering 27 , 32 functions to transform the leaf into a mature organ changing its shape 33 and reducing its capacity to regenerate. While the phase change from vegetative to reproductive growth in a plant is accompanied by changing leaf shape 33 , we found that inducing early flowering and reducing regeneration ability is not related to this shape change.…”

“…Florigen action in the meristem to induce flowering 27 , 32 functions to transform the leaf into a mature organ changing its shape 33 and reducing its capacity to regenerate. While the phase change from vegetative to reproductive growth in a plant is accompanied by changing leaf shape 33 , we found that inducing early flowering and reducing regeneration ability is not related to this shape change. While the shape changes are correlated with reduced mir156 and increased SPL genes 33 , this study shows that independently from flowering and other meristematic effects, FT genes function in the leaf tissue as a determinant of juvenility or maturity depending if the rejuvenator or the florigen is expressed without changing the leaf shape that is typical of plant maturity.…”