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1. Convergent evolution of root system architecture in two independently evolved lineages of weedy rice

   https://doi.org/10.1111/nph.15791  

   Wedger, Marshall J. ; Topp, Christopher N. ; Olsen, Kenneth M. ( April 2019 , New Phytologist)

   Root system architecture (RSA) is a critical aspect of plant growth and competitive ability. Here we used two independently evolved strains of weedy rice, a de‐domesticated form of rice, to study the evolution of weed‐associatedRSAtraits and the extent to which they evolve through shared or different genetic mechanisms.

   We characterised 98 two‐dimensional and three‐dimensionalRSAtraits in 671 plants representing parents and descendants of two recombinant inbred line populations derived from two weed × crop crosses. A random forest machine learning model was used to assess the degree to which root traits can predict genotype and the most diagnostic traits for doing so. We used quantitative trait locus (QTL)mapping to compare genetic architecture between the weed strains.

   The two weeds were distinguishable from the crop in similar and predictable ways, suggesting independent evolution of a ‘weedy’RSAphenotype. Notably, comparativeQTLmapping revealed little evidence for shared underlying genetic mechanisms.

   Our findings suggest that despite the double bottlenecks of domestication and de‐domestication, weedy rice nonetheless shows genetic flexibility in the repeated evolution of weedyRSAtraits. Whereas the root growth of cultivated rice may facilitate interactions among neighbouring plants, the weedy rice phenotype may minimise below‐ground contact as a competitive strategy.

    

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2. Photomorphogenesis of the root system in developing sunflower seedlings: a role for sucrose

   https://doi.org/10.1111/plb.12981  

   Kutschera, U. ; Briggs, W. R. ; Hause, ed., B. ( April 2019 , Plant Biology)

   The domestic sunflower (Helianthus annuusL. cv. ‘Giganteus’) has been used since the 19th century as a model plant for the study of seedling development in darkness and white light (WL) (scoto‐versusphotomorphogenesis). However, most pertinent studies have focused on the developmental patterns of the hypocotyl and cotyledons, whereas the root system has been largely ignored.

   In this study, we analysed entire sunflower seedlings (root and shoot) and quantified organ development in the above‐ and belowground parts of the organism under natural (non‐sterile) conditions.

   We document that seedlings, raised in moist vermiculite, are covered with methylobacteria, microbes that are known to promote root development inArabidopsis. Quantitative data revealed that during photomorphogenesis inWL, the root system expands by 90%, whereas stem elongation is inhibited, and hook opening/cotyledon expansion occurs. Root morphogenesis may be mediatedviaimported sucrose provided by the green, photosynthetically active cotyledons. This hypothesis is supported by the documented effect of sucrose on the induction of lateral root initials in sunflower cuttings. Under these experimental conditions, phytohormones (auxin, cytokinin, brassinolide) exerted little effect on root and cotyledon expansion, and no hormone‐induced initiation of lateral roots was observed.

   It is concluded that sucrose not only acts as an energy source to fuel cell metabolism but is also a shoot‐derived signalling molecule that triggers root morphogenesis.

    

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3. Increased adaxial stomatal density is associated with greater mesophyll surface area exposed to intercellular air spaces and mesophyll conductance in diverse C 4 grasses

   https://doi.org/10.1111/nph.16106  

   Pathare, Varsha S. ; Koteyeva, Nuria ; Cousins, Asaph B. ( September 2019 , New Phytologist)

   Mesophyll conductance (g_m) is the diffusion ofCO₂from intercellular air spaces (IAS) to the first site of carboxylation in the mesophyll cells. In C₃species,g_mis influenced by diverse leaf structural and anatomical traits; however, little is known about traits affectingg_min C₄species.

   To address this knowledge gap, we used online oxygen isotope discrimination measurements to estimateg_mand microscopy techniques to measure leaf structural and anatomical traits potentially related tog_min 18 C₄grasses.

   In this study,g_mscaled positively with photosynthesis and intrinsic water‐use efficiency (TE_i), but not with stomatal conductance. Also,g_mwas not determined by a single trait but was positively correlated with adaxial stomatal densities (SD_ada), stomatal ratio (SR), mesophyll surface area exposed toIAS(S_mes) and leaf thickness. However,g_mwas not related to abaxial stomatal densities (SD_aba) and mesophyll cell wall thickness (T_CW).

   Our study suggests that greaterSD_adaandSRincreasedg_mby increasingS_mesand creating additional parallel pathways forCO₂diffusion inside mesophyll cells. Thus,SD_ada,SRandS_mesare important determinants of C₄‐g_mand could be the target traits selected or modified for achieving greaterg_mandTE_iin C₄species.

    

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4. Legacy effects of tree mortality mediated by ectomycorrhizal fungal communities

   https://doi.org/10.1111/nph.15993  

   Mueller, Rebecca C. ; Scudder, Crescent M. ; Whitham, Thomas G. ; Gehring, Catherine A. ( July 2019 , New Phytologist)

   Successive droughts have resulted in extensive tree mortality in the southwestern United States. Recovery of these areas is dependent on the survival and recruitment of young trees. For trees that rely on ectomycorrhizal fungi (EMF) for survival and growth, changes in soil fungal communities following tree mortality could negatively affect seedling establishment.

   We used tree‐focused and stand‐scale measurements to examine the impact of pinyon pine mortality on the performance of surviving juvenile trees and the potential for mutualism limitation of seedling establishment via alteredEMFcommunities.

   Mature pinyon mortality did not affect the survival of juvenile pinyons, but increased their growth. At both tree and stand scales, high pinyon mortality had no effect on the abundance ofEMFinocula, but led to alteredEMFcommunity composition including increased abundance ofGeoporaand reduced abundance ofTuber. Seedling biomass was strongly positively associated withTuberabundance, suggesting that reductions in this genus with pinyon mortality could have negative consequences for establishing seedlings.

   These findings suggest that whereas mature pinyon mortality led to competitive release for established juvenile pinyons, changes inEMFcommunity composition with mortality could limit successful seedling establishment and growth in high‐mortality sites.

    

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5. DRO1 influences root system architecture in Arabidopsis and Prunus species

   https://doi.org/10.1111/tpj.13470  

   Guseman, Jessica M. ; Webb, Kevin ; Srinivasan, Chinnathambi ; Dardick, Chris ( March 2017 , The Plant Journal)

   Roots provide essential uptake of water and nutrients from the soil, as well as anchorage and stability for the whole plant. Root orientation, or angle, is an important component of the overall architecture and depth of the root system; however, little is known about the genetic control of this trait. Recent reports inOryza sativa(rice) identified a role forDEEPER ROOTING 1(DRO1) in influencing the orientation of the root system, leading to positive changes in grain yields under water‐limited conditions. Here we found thatDRO1andDRO1‐related genes are present across diverse plant phyla, and fall within theIGTgene family. TheIGTfamily also includesTAC1andLAZY1, which are known to affect the orientation of lateral shoots. Consistent with a potential role in root development,DRO1homologs in Arabidopsis and peach showed root‐specific expression. Promoter–reporter constructs revealed thatAtDRO1is predominantly expressed in both the root vasculature and root tips, in a distinct developmental pattern. Mutation ofAtDRO1led to more horizontal lateral root angles. Overexpression ofAtDRO1under a constitutive promoter resulted in steeper lateral root angles, as well as shoot phenotypes including upward leaf curling, shortened siliques and narrow lateral branch angles. A conserved C‐terminalEAR‐like motif found inIGTgenes was required for these ectopic phenotypes. Overexpression ofPpeDRO1inPrunus domestica(plum) led to deeper‐rooting phenotypes. Collectively, these data indicate a potential application forDRO1‐related genes to alter root architecture for drought avoidance and improved resource use.

    

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