P.N. Hills

The plant growth promoting substance, lumichrome, mimics starch and ethylene-associated symbiotic responses in lotus and tomato roots

Symbiosis involves responses that maintain the plant host and symbiotic partner’s genetic program; yet these cues are far from elucidated. Here we describe the effects of lumichrome, a flavin identified from Rhizobium spp., applied to lotus (Lotus japonicus) and tomato (Solanum lycopersicum). Combined transcriptional and metabolite analyses suggest that both species shared common pathways that were altered in response to this application under replete, sterile conditions. These included genes involved in symbiosis, as well as transcriptional and metabolic responses related to enhanced starch accumulation and altered ethylene metabolism. Lumichrome priming also resulted in altered colonization with either Mesorhizobium loti (for lotus) or Glomus intraradices/G. mossea (for tomato). It enhanced nodule number but not nodule formation in lotus; while leading to enhanced hyphae initiation and delayed arbuscule maturation in tomato.

Cell division and turgor mediate enhanced plant growth in Arabidopsis plants treated with the bacterial signalling molecule lumichrome

Main conclusionTranscriptomic analysis indicates that the bacterial signalling molecule lumichrome enhances plant growth through a combination of enhanced cell division and cell enlargement, and possibly enhances photosynthesis.Lumichrome (7,8 dimethylalloxazine), a novel multitrophic signal molecule produced by Sinorhizobium meliloti bacteria, has previously been shown to elicit growth promotion in different plant species (Phillips et al. in Proc Natl Acad Sci USA 96:12275–12280, https://doi.org/10.1073/pnas.96.22.12275, 1999). However, the molecular mechanisms that underlie this plant growth promotion remain obscure. Global transcript profiling using RNA-seq suggests that lumichrome enhances growth by inducing genes impacting on turgor driven growth and mitotic cell cycle that ensures the integration of cell division and expansion of developing leaves. The abundance of XTH9 and XPA4 transcripts was attributed to improved mediation of cell-wall loosening to allow turgor-driven cell enlargement. Mitotic CYCD3.3, CYCA1.1, SP1L3, RSW7 and PDF1 transcripts were increased in lumichrome-treated Arabidopsis thaliana plants, suggesting enhanced growth was underpinned by increased cell differentiation and expansion with a consequential increase in biomass. Synergistic ethylene–auxin cross-talk was also observed through reciprocal over-expression of ACO1 and SAUR54, in which ethylene activates the auxin signalling pathway and regulates Arabidopsis growth by both stimulating auxin biosynthesis and modulating the auxin transport machinery to the leaves. Decreased transcription of jasmonate biosynthesis and responsive-related transcripts (LOX2; LOX3; LOX6; JAL34; JR1) might contribute towards suppression of the negative effects of methyl jasmonate (MeJa) such as chlorophyll loss and decreases in RuBisCO and photosynthesis. This work contributes towards a deeper understanding of how lumichrome enhances plant growth and development.

Effects of Strigolactones on Plant Roots

Strigolactones are the most recently identified class of plant hormones, having only been classified as phytohormones in 2008. Considerable research has since been conducted on the biosynthesis of these hormones and on their actions in plants, both on the shoots and the roots. Large parts of the biosynthetic pathway have been determined, although several crucial steps which would explain how the different strigolactones arise from a common precursor are still missing. Strigolactones affect the growth and development of all types of roots. Strigolactone action is sensitive to environmental conditions, as the hormones may have virtually opposite effects in the different roots under different carbon and nutrient regimes. Generally, strigolactones enhance root growth when nutrients, particularly phosphate, are limiting but reduce root development when nutrients are abundant. This is also in line with the important role that strigolactones play in promoting the symbiotic relationship between plant roots and arbuscular mycorrhizal fungi, suggesting that strigolactones modify the plant root structure in order to balance carbon use and economy with the ability to obtain sufficient nutrients. In this review, we deal with strigolactone biosynthesis and perception and the role of strigolactones in rooting as well as how interaction of strigolactones with plant nutritional status and other phytohormones determines root architecture.