Abstract
GIGANTEA (GI) genes have a central role in plant development and influence several processes. Hybrid aspen T89 (Populus tremula x tremuloides) trees with low GI expression engineered through RNAi show severely compromised growth. To study the effect of reduced GI expression on leaf traits with special emphasis on leaf senescence, we grafted GI-RNAi scions onto wild-type rootstocks and successfully restored growth of the scions. The RNAi line had a distorted leaf shape and reduced photosynthesis, probably caused by modulation of phloem or stomatal function, increased starch accumulation, a higher carbon-to-nitrogen ratio, and reduced capacity to withstand moderate light stress. GI-RNAi also induced senescence under long day (LD) and moderate light conditions. Furthermore, the GI-RNAi lines were affected in their capacity to respond to “autumn environmental cues” inducing senescence, a type of leaf senescence that has physiological and biochemical characteristics that differ from those of senescence induced directly by stress under LD conditions. Overexpression of GI delayed senescence under simulated autumn conditions. The two different effects on leaf senescence under LD or simulated autumn conditions were not affected by the expression of FLOWERING LOCUS T. GI expression regulated leaf senescence locally—the phenotype followed the genotype of the branch, independent of its position on the tree—and trees with modified gene expression were affected in a similar way when grown in the field as under controlled conditions. Taken together, GI plays a central role in sensing environmental changes during autumn and determining the appropriate timing for leaf senescence in Populus.
Highlights
Every year deciduous trees go through a seasonal cycle of bud flush, growth, vegetative growth cessation, leaf senescence, dormancy, and development of cold hardiness
Given that GI regulates phenology through FT2, which is a mobile signal in Populus (Miskolczi et al, 2019), we tested whether grafting GI-RNAi scions onto a WT rootstock (Figure 1B, “simple grafting”; GI-RNAi scion/WT rootstock), which allows mobile signals such as FLOWERING LOCUS T (FT) to move from WT leaves in the rootstock to the apex, could rescue the effects on growth and bud set
In Arabidopsis, lowered GI expression leads to malfunctioning phloem loading (Edwards et al, 2010), and our observations are consistent with compromised phloem export in Populus too
Summary
Every year deciduous trees go through a seasonal cycle of bud flush, growth, vegetative growth cessation, leaf senescence, dormancy, and development of cold hardiness.
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