Heterologous over-expression of ACC SYNTHASE8 (ACS8) in Populus tremula x P. alba clone 717-1B4 results in elevated levels of ethylene and induces stem dwarfism and reduced leaf size through separate genetic pathways

Abstract

Plant height is an important agronomic and horticultural trait that impacts plant productivity, durability and esthetic appeal. A number of the plant hormones such as gibberellic acid (GA), auxin and ethylene have been linked to control of plant architecture and size. Reduction in GA synthesis and auxin transport result in dwarfism while ethylene may have a permissive or repressive effect on tissue growth depending upon the age of plant tissues or the environmental conditions considered. We describe here an activation-tagged mutant of Populus tremula x P. alba clone 717-1B4 identified from 2000 independent transgenic lines due to its significantly reduced growth rate and smaller leaf size. Named dwarfy, the phenotype is due to increased expression of PtaACC SYNTHASE8, which codes for an enzyme in the first committed step in the biosynthesis of ethylene. Stems of dwarfy contain fiber and vessel elements that are reduced in length while leaves contain fewer cells. These morphological differences are linked to PtaACS8 inducing different transcriptomic programs in the stem and leaf, with genes related to auxin diffusion and sensing being repressed in the stem and genes related to cell division found to be repressed in the leaves. Altogether, our study gives mechanistic insight into the genetics underpinning ethylene-induced dwarfism in a perennial model organism.

Adaptive evolution and segregating load contribute to the genomic landscape of divergence in two tree species connected by episodic gene flow

“…Gathering direct evidence for an involvement of NBS‐LRR's (and other groups of genes) in RI in poplars would benefit from testing the phenotypic and fitness effects of different alleles and haplotypes in common garden trials, and identifying the most relevant mutations by functional assays (Plett et al . ; Suarez‐Gonzalez et al . ).…”

“…Thus, this group of genes appears to represent an example of ecologically important loci (host-pathogen interactions) that nevertheless exert their effects on RI and plant speciation via essentially intrinsic, nonecological mechanisms. Gathering direct evidence for an involvement of NBS-LRR's (and other groups of genes) in RI in poplars would benefit from testing the phenotypic and fitness effects of different alleles and haplotypes in common garden trials, and identifying the most relevant mutations by functional assays (Plett et al 2014;Suarez-Gonzalez et al 2016). This could be complemented by proteomic and metabolomic approaches (e.g.…”

The Pivotal Role of Ethylene in Plant Growth

“…A third cell-cycle inhibitory mechanism relies on the downregulation of the CYCLIN genes. Overexpression of ACS8 in poplar leaves results in downregulation of several A- and B-type CYCLIN genes and a CDKA -like gene [26] . Notably, in roots, where ethylene also represses the cell cycle, CYCB1;1 expression is unaffected, but at the protein level CYCB1;1 was degraded in the presence of ethylene, highlighting a post-translational regulatory mechanism [27] .…”

“…However, similarly to cell division, the influence of ethylene on cell growth in leaves is almost exclusively negative. Overproduction of ethylene or overexpression of proteins of the signaling cascade results in smaller leaves because of restricted cell expansion, as illustrated by overexpression of ACS8 [26] , EIN2 [10] , BOLITA [22] , ERF6 [23] , and multiple other ERF-encoding genes, discussed in the next section. By contrast, mutants with a reduced ethylene sensitivity have an increased leaf size resulting from enhanced cell expansion, as demonstrated in ein2 [10] and in lines overexpressing negative regulators of ethylene signaling of the ARGOS family [12] .…”

SIAMESE-RELATED1 Is Regulated Posttranslationally and Participates in Repression of Leaf Growth under Moderate Drought

“…In poplar (Populus 570 spp. ), ethylene-induced dwarfism has also been proposed to result in the down-571 regulation of multiple cell cycle genes, including CYCLINs (Plett et al, 2014). 572…”