Poplar Transformation

Changes in wax composition but not amount enhance cuticular transpiration

This study focuses on the role of the qualitative leaf wax composition in modulating the cuticular water loss using a Populus × canescens cer6 mutant line, which accumulates C34-C46 wax ester dimers and is reduced in wax monomers >C24. The two literature-based hypotheses to be tested were the importance of the amount of wax esters and the weighted mean carbon chain length in restricting cuticular water loss. The main results were acquired by chemical analysis of cuticular wax and gravimetric cuticular transpiration measurements. Besides additional physiological measurements, the leaf surface properties were characterised by scanning electron microscopy and spectrophotometric light reflectance quantification. Mutation of the CER6 gene resulted in striking changes in qualitative wax composition but not quantitative wax amount. Based on the strong accumulation of dimeric wax esters, the relative proportion of esters increased to >90%, and the weighted mean carbon chain length increased by >6 carbon atoms. These qualitative alterations were found to increase the cuticular transpiration of leaves by twofold. Our results do not support the hypotheses that enhanced amounts of wax esters or increased weighted mean carbon chain lengths of waxes lead to reduced cuticular transpiration.

Base Editing in Poplar Through an Agrobacterium-Mediated Transformation Method

CRISPR-Cas9 systems have revolutionized genome editing in plants and facilitated gene knockout and functional genomic studies in woody plants, like poplar. However, in tree species, previous studies have only focused on targeting indel mutations through CRISPR-based nonhomologous end joining (NHEJ) pathway. Cytosine base editors (CBEs) and adenine base editors (ABEs) enable C-to-T and A-to-G base changes, respectively. These base editors can introduce premature stop codons and amino acid changes, alter RNA splicing sites, and edit cis-regulatory elements of promoters. Base editing systems have only been recently established in trees. In this chapter, we describe a detailed, robust, and thoroughly tested protocol for preparing T-DNA vectors with two highly efficient CBEs, PmCDA1-BE3 and A3A/Y130F-BE3, and the highly efficient ABE8e as well as delivering the T-DNA through an improved protocol for Agrobacterium-mediated transformation in poplar. This chapter will provide promising application potential for precise base editing in poplar and other trees.

A Constitutively Active Cytokinin Receptor Variant Increases Cambial Activity and Stem Growth in Poplar

The cambial meristem is responsible for bark and wood formation in woody plants. The activity of the cambial meristem is controlled by various factors; one of them is the plant hormone cytokinin. Here, we have explored different approaches to genetically engineering cambial activity in poplar plants by the ectopic expression of a cytokinin biosynthesis gene with enhanced activity (named ROCK4) or of a gene encoding a constitutively active cytokinin receptor variant (ROCK3). Both genes are derived from Arabidopsis thaliana and were expressed in poplar trees under the control of their own promoter or the cambium-specific pHB8 promoter. pIPT3:ROCK4- and pHB8:ROCK4-expressing plants were smaller than wild-type plants and formed more lateral branches; pHB8:ROCK4 transgenic plants additionally showed an increased stem diameter. In contrast, pAHK3:ROCK3- and pHB8:ROCK3-expressing plants grew taller than wild type without an altered branching pattern and formed more cambial cells, leading to increased radial stem growth. The effectivity of ROCK3 when expressed in either secondary phloem cells or in cambial cells is consistent with a dual, tissue-autonomous and non-autonomous activity of cytokinin in regulating cambial activity. We propose ROCK3 as a novel gene to enhance biomass formation in woody plants.

A single gene underlies the dynamic evolution of poplar sex determination

Although hundreds of plant lineages have independently evolved dioecy (that is, separation of the sexes), the underlying genetic basis remains largely elusive¹. Here we show that diverse poplar species carry partial duplicates of the ARABIDOPSIS RESPONSE REGULATOR 17 (ARR17) orthologue in the male-specific region of the Y chromosome. These duplicates give rise to small RNAs apparently causing male-specific DNA methylation and silencing of the ARR17 gene. CRISPR-Cas9-induced mutations demonstrate that ARR17 functions as a sex switch, triggering female development when on and male development when off. Despite repeated turnover events, including a transition from the XY system to a ZW system, the sex-specific regulation of ARR17 is conserved across the poplar genus and probably beyond. Our data reveal how a single-gene-based mechanism of dioecy can enable highly dynamic sex-linked regions and contribute to maintaining recombination and integrity of sex chromosomes.