Abstract
Both the evolution of tree taxa and whole-genome duplication (WGD) have occurred many times during angiosperm evolution. Transcription factors are preferentially retained following WGD suggesting that functional divergence of duplicates could contribute to traits distinctive to the tree growth habit. We used gain- and loss-of-function transgenics, photoperiod treatments, and circannual expression studies in adult trees to study the diversification of three Populus FLOWERING LOCUS D-LIKE (FDL) genes encoding bZIP transcription factors. Expression patterns and transgenic studies indicate that FDL2.2 promotes flowering and that FDL1 and FDL3 function in different vegetative phenophases. Study of dominant repressor FDL versions indicates that the FDL proteins are partially equivalent in their ability to alter shoot growth. Like its paralogs, FDL3 overexpression delays short day-induced growth cessation, but also induces distinct heterochronic shifts in shoot development—more rapid phytomer initiation and coordinated delay in both leaf expansion and the transition to secondary growth in long days, but not in short days. Our results indicate that both regulatory and protein coding sequence variation contributed to diversification of FDL paralogs that has led to a degree of specialization in multiple developmental processes important for trees and their local adaptation.
Highlights
Distinguishing features of trees include large crowns enabled by extensive wood development and protracted flowering-incompetent phases
We studied FLOWERING LOCUS D-LIKE (FDL) expression in different vegetative tissues and developmental stages of 4-month-old WT trees grown in a Long days (LDs) greenhouse
FDL3 was more highly expressed in shoot apices during the growing season compared to autumn–winter season and was transiently upregulated in leaves during early autumn at the timepoint when leaf collection shifted from fully expanded leaf (August) to preformed leaf within a terminal bud (September; Figures 1C,E)
Summary
Distinguishing features of trees include large crowns enabled by extensive wood development and protracted flowering-incompetent phases. The tree growth habit has been lost and gained many times throughout angiosperm evolution (Groover, 2005), which has been characterized by whole-genome duplications (WGDs) as well as segmental and tandem duplications (Hanada et al, 2008; Soltis et al, 2015). These events are likely to have had a major role in the repeated evolution of trees, empirical evidence for the role of gene duplicate diversification in processes that define the tree life style is sparse
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