Moving on up - controlling internode growth.

Sarah M. McKim

doi:10.1111/nph.16439

Abstract

Plant reproductive success depends on making fertileflowers but also upon developing appropriate shoot internodes that optimally arrange and support the flowering shoot. Compared to floral morphogenesis, we understand little about the networks directing internode growth during flowering. However, new studies reveal that long-range signals, local factors, and age-dependent micoRNA-networks are all important toharmonize internode morphogenesis with shoot development. Some of the same players modulate symplastic transport to seasonally regulate internode growth in perennial species. Exploring possible hierarchical control amongst symplastic continuity, age, systemic signals and local regulators during internode morphogenesis will help elucidate the mechanisms coordinating axial growth with the wider plant body.

Highlights

PENNYWISE/ REPLUMLESS/ BELLRINGER/ VAAMANA (PNY) likely works by interacting with specific KNOTTED-Like homeoboxes (KNOXs) to limit lignification (Khan et al, 2012) and promote meristematic function and cytokinin accumulation (Byrne, 2003; Yanai et al, 2005), the latter module being necessary for axial elongation in both flowering plants and moss (Coudert et al, 2019)
This research reveals some central themes: (1) localized control of GA signalling is sensitive to the reproductive phase and responds to long-distance signals; (2) cell division orientation, boundary gene expression patterns and vascularization are important regulatory targets during reproductive internode morphogenesis; (3) control of axial growth can be uncoupled from the floral transition; and (4) phase-specific regulators may exploit JA to modulate developmental growth switches in the apex and internode
It will be satisfying to learn whether BELL-like homeodomain (BLH) interact with meristem regulators, seasonal cues and phytohormone signalling to control cell division orientation and vascularization rate, and whether ATH1 uses similar mechanisms to suppress internode elongation in vegetative rosettes

Summary

Compress and release

Signals orchestrating increased axial growth with inflorescence maturation likely modify gibberellin (GA) hormone concentrations and/or responsiveness (Fig. 2). Rib and intercalary meristems lack a clear histological boundary from their daughter cells, it is possible that localized micro-domains of elevated GA are established distal to the cytokinin-accumulating stem cells to promote internode growth while preserving subapical indeterminacy. PNY helps reduce and confine the transcripts of ATH1, LIGHT DEPENDENT SHORT HYPOCOTYLS4 and BLADE-ON-PETIOLE1/2 (all boundary genes) to the base of floral buds and the internode periphery, excluding them from the central rib meristem in order to promote both photoperiodic flowering and axial elongation (Ragni et al, 2008; Andres et al, 2015; Khan et al, 2015; Bencivenga et al, 2016). PNY likely works by interacting with specific KNOXs to limit lignification (Khan et al, 2012) and promote meristematic function and cytokinin accumulation (Byrne, 2003; Yanai et al, 2005), the latter module being necessary for axial elongation in both flowering plants and moss (Coudert et al, 2019)

Independence

Not so fast

Conclusion