ABA signalling mediates biomass trade-off and allocation in poplar
New Phytol. 2019 May 3 [Epub ahead of print]
Yu D, Wildhagen H, Tylewicz S, Miskolczi PC, Bhalerao RP, Polle A

Abstract
Abscisic acid (ABA) is a well-known stress hormone regulating drought adaptation of plants. Here, we hypothesized that genetic engineering of genes involved in ABA stress signalling and photoperiodic regulation affected drought resistance by trade-off with biomass production in perennial poplar trees. We grew Populus tremula × tremuloides wildtype (T89) and various transgenic lines (two transformation events of 35S::abi1-1, 35S::RCAR, RCAR:RNAi, 35S::ABI3, 35S::AREB3, 35S::FDL1, FDL1:RNAi, 35S::FDL2, and FDL2:RNAi) outdoors and exposed them to drought in the second growth period. After the winter, the surviving lines showed a huge variation in stomatal conductance, leaf size, whole-plant leaf area, tree height, stem diameter, and biomass. Whole-plant leaf area was a strong predictor for woody biomass production. The 35S::AREB3 lines were compromised in biomass production under well-irrigated conditions compared with wildtype poplars but were resilient to drought. ABA signalling regulated FDL1 and FDL2 expression under stress. Poplar lines overexpressing FDL1 or FDL2 were drought-sensitive; they shed leaves and lost root biomass, whereas the FDL RNAi lines showed higher biomass allocation to roots under drought. These results assign a new function in drought acclimation to FDL genes aside from photoperiodic regulation. Our results imply a critical role for ABA-mediated processes in balancing biomass production and climate adaptation.

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