Informed by vegetation maps across high-latitude landscapes, terrestrial biosphere models are a tool that can be used to predict changes in the composition and function of vegetation, above- and belowground, across the land surface in response to changing environmental conditions. However, terrestrial biosphere models represent vegetation characteristics at a finer grain than mapped vegetation communities. These models group plant species that colonize high-latitude biomes by their functional trait variation into plant functional types (PFTs) that characterize the impacts of plant species on, and their response to changes in, their surrounding abiotic and biotic environment. Blume-Werry et al. (2023) found that vegetation mapping units that broadly incorporate multiple plant species and functional types are too coarse, or encompass too much biological variation, to fully capture belowground plant trait variation. However, they did find that they could successfully cluster rooting depth observations into ‘Root Profile Types’, suggesting that modeling PFTs may be a useful tool to characterize above- and belowground linkages across high-latitude environments.
In many arctic and boreal ecosystems, plant roots are constrained by permafrost to a shallow ‘active layer’ of soil that thaws progressively over the course of each growing season. Blume-Werry et al. (2023) identified active layer thickness and the closely related minimum temperature of the coldest month as two of three main abiotic drivers constraining rooting depth distribution in their analysis (a third, cation exchange capacity, is more indicative of nutrient availability than a physical impediment). Furthermore, waterlogging can limit root distribution to surface, oxic soils, and can lead to a thick layer of poorly decomposed, organic peat at the soil surface with different characteristics from mineral soils (Fig. 1; Walker et al., 2003). Indeed, Blume-Werry et al. (2023) found that despite similarities in species composition between wetland and graminoid tundra in CAVM mapping units, rooting depth in wetland tundra was shallower than graminoid tundra. This may indicate that waterlogged conditions can constrain rooting depth distribution, even in vegetation communities dominated by species with aerenchymatous roots. Ranging from rootless mosses and plant-like lichens to vascular graminoids and shrubs, and deciduous and evergreen trees, PFTs inhabiting the arctic tundra and boreal forest vary in their rooting depth distributions, their interactions with soil microbiota, and their ratio of belowground to aboveground tissues (e.g. root : shoot ratio; Chapin et al., 1996). However, terrestrial biosphere models have often neglected the unique characteristics of the species that colonize high-latitude biomes, especially belowground (Iversen et al., 2015, 2018
The regulatory framework of leaf senescence is gradually becoming clearer; however, the fine regulation of this process remains largely unknown.
Here, genetic analysis revealed that U2 small nuclear ribonucleoprotein B (U2B″), a component of the spliceosome, is a negative regulator of leaf senescence. Mutation of U2B″ led to precocious leaf senescence, whereas overexpression of U2B″ extended leaf longevity. Transcriptome analysis revealed that the jasmonic acid (JA) signaling pathway was activated in the u2b″ mutant. U2B″ enhances the generation of splicing variant JASMONATE ZIM-DOMAIN 9β (JAZ9β) with an intron retention in the Jas motif, which compromises its interaction with CORONATINE INSENSITIVE1 and thus enhances the stability of JAZ9β protein. Moreover, JAZ9β could interact with MYC2 and obstruct its activity, thereby attenuating JA signaling. Correspondingly, overexpression of JAZ9β rescued the early senescence phenotype of the u2b″ mutant.
Furthermore, JA treatment promoted expression of U2B″ that was found to be a direct target of MYC2. Overexpression of MYC2 in the u2b″ mutant resulted in a more pronounced premature senescence than that in wild-type plants.
Collectively, our findings reveal that the spliceosomal protein U2B″ fine-tunes leaf senescence by enhancing the expression of JAZ9β and thereby attenuating JA signaling.
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