A new practical tool for deriving a functional signature for herbaceous vegetation

Hypothesis: For any one time and place a ‘functional signature’ can be derived for a sample of herbaceous vegetation in a way that concisely represents the balance between the different clusters of functional attributes that are present among component species. Methods: We developed a spreadsheet‐based tool for calculating functional signatures within the context of the C‐S‐R system of plant functional types. We used the tool to calculate and compare signatures for specimen British vegetation samples which differed in management regime and location in time. Conclusion: The integrative power of the ‘C‐S‐R signature’ is useful in comparative studies involving widely differing samples. Movements in the signature can be used to indicate degree of resistance, resilience, eutrophication and dereliction. Systems of plant functional types other than C‐S‐R might also be approached in this way. Availability: The tool can be downloaded free of charge from the first author's web pages or from the journal's electronic archive.     

Plant DELLAs restrain growth and promote survival of adversity by reducing the levels of reactive oxygen species

Plant growth is adaptively modulated in response to environmental change. The phytohormone gibberellin (GA) promotes growth by stimulating destruction of the nuclear growth-repressing DELLA proteins [1-7], thus providing a mechanism for environmentally responsive growth regulation [8, 9]. Furthermore, DELLAs promote survival of adverse environments [8]. However, the relationship between these survival and growth-regulatory mechanisms was previously unknown. Here, we show that both mechanisms are dependent upon control of the accumulation of reactive oxygen species (ROS). ROS are small molecules generated during development and in response to stress that play diverse roles as eukaryotic intracellular second messengers [10]. We show that Arabidopsis DELLAs cause ROS levels to remain low after either biotic or abiotic stress, thus delaying cell death and promoting tolerance. In essence, stress-induced DELLA accumulation elevates the expression of genes encoding ROS-detoxification enzymes, thus reducing ROS levels. In accord with recent demonstrations that ROS control root cell expansion [11, 12], we also show that DELLAs regulate root-hair growth via a ROS-dependent mechanism. We therefore propose that environmental variability regulates DELLA activity [8] and that DELLAs in turn couple the downstream regulation of plant growth and stress tolerance through modulation of ROS levels.     

DELLAs control plant immune responses by modulating the balance of jasmonic acid and salicylic acid signaling

In Arabidopsis, the flagellin-derived peptide flg22 elevates antibacterial resistance [1] and inhibits growth [2] upon perception via the leucine-rich repeat receptor-like kinase Flagellin-Sensitive 2 (FLS2) [3]. DELLA proteins are plant growth repressors whose degradation is promoted by the phytohormone gibberellin [4]. Here, we show that DELLA stabilization contributes to flg22-induced growth inhibition. In addition, we show that DELLAs promote susceptibility to virulent biotrophs and resistance to necrotrophs, partly by altering the relative strength of salicylic acid and jasmonic acid (JA) signaling. A quadruple-DELLA mutant (which lacks four out of the five Arabidopsis DELLA proteins [5]) was partially insensitive to gene induction by Methyl-Jasmonate (MeJA), whereas the constitutively active dominant DELLA mutant gai[6] was sensitized for JA-responsive gene induction, implicating DELLAs in JA-signaling and/or perception. Accordingly, the elevated resistance of gai to the necrotrophic fungus Alternaria brassicicola and susceptibility to the hemibiotroph Pseudomonas syringae pv. tomato strain DC3000 (Pto DC3000) was attenuated in the JA-insensitive coi1-16 mutant [7]. These findings suggest an explanation for why the necrotrophic fungus Gibberella fujikuroi, causal agent of the foolish-seedling disease of rice, makes gibberellin.     

The Arabidopsis mutant sleepy1gar2-1 protein promotes plant growth by increasing the affinity of the SCFSLY1 E3 ubiquitin ligase for DELLA protein substrates

DELLA proteins restrain the cell proliferation and enlargement that characterizes the growth of plant organs. Gibberellin stimulates growth via 26S proteasome-dependent destruction of DELLAs, thus relieving DELLA-mediated growth restraint. Here, we show that the Arabidopsis thaliana sleepy1gar2-1 (sly1gar2-1) mutant allele encodes a mutant subunit (sly1gar2-1) of an SCF(SLY1) E3 ubiquitin ligase complex. SLY1 (the wild-type form) and sly1gar2-1 both confer substrate specificity on this complex via specific binding to the DELLA proteins. However, sly1gar2-1 interacts more strongly with the DELLA target than does SLY1. In addition, the strength of the SCFSLY1-DELLA interaction is increased by target phosphorylation. Growth-promoting DELLA destruction is dependent on SLY1 availability, on the strength of the interaction between SLY1 and the DELLA target, and on promotion of the SCFSLY1-DELLA interaction by DELLA phosphorylation.     

Studies of Physcomitrella patens reveal that ethylene‐mediated submergence responses arose relatively early in land‐plant evolution

Colonization of the land by multicellular green plants was a fundamental step in the evolution of life on earth. Land plants evolved from fresh‐water aquatic algae, and the transition to a terrestrial environment required the acquisition of developmental plasticity appropriate to the conditions of water availability, ranging from drought to flood. Here we show that extant bryophytes exhibit submergence‐induced developmental plasticity, suggesting that submergence responses evolved relatively early in the evolution of land plants. We also show that a major component of the bryophyte submergence response is controlled by the phytohormone ethylene, using a perception mechanism that has subsequently been conserved throughout the evolution of land plants. Thus a plant environmental response mechanism with major ecological and agricultural importance probably had its origins in the very earliest stages of the colonization of the land.     

The Angiosperm Gibberellin-GID1-DELLA Growth Regulatory Mechanism: How an “Inhibitor of an Inhibitor” Enables Flexible Response to Fluctuating Environments

The phytohormone gibberellin (GA) has long been known to regulate the growth, development, and life cycle progression of flowering plants. However, the molecular GA-GID1-DELLA mechanism that enables plants to respond to GA has only recently been discovered. In addition, studies published in the last few years have highlighted previously unsuspected roles for the GA-GID1-DELLA mechanism in regulating growth response to environmental variables. Here, we review these advances within a general plant biology context and speculate on the answers to some remaining questions. We also discuss the hypothesis that the GA-GID1-DELLA mechanism enables flowering plants to maintain transient growth arrest, giving them the flexibility to survive periods of adversity.