The role of leaf hydraulic conductance dynamics on the timing of leaf senescence

Cell-level anatomy explains leaf age-dependent declines in mesophyll conductance and photosynthetic capacity in the evergreen Mediterranean oak Quercus ilex subsp. rotundifolia

Leaves of Mediterranean evergreen tree species experience a reduction in net CO2 assimilation (AN) and mesophyll conductance to CO2 (gm) during aging and senescence, which would be influenced by changes in leaf anatomical traits at cell level. Anatomical modifications can be accompanied by the dismantling of photosynthetic apparatus associated to leaf senescence, manifested through changes at the biochemical level (i.e., lower nitrogen investment in photosynthetic machinery). However, the role of changes in leaf anatomy at cell level and nitrogen content in gm and AN decline experienced by old non-senescent leaves of evergreen trees with long leaf lifespan is far from being elucidated. We evaluated age-dependent changes in morphological, anatomical, chemical and photosynthetic traits in Quercus ilex subsp. rotundifolia Lam., an evergreen oak with high leaf longevity. All photosynthetic traits decreased with increasing leaf age. The relative change in cell wall thickness (Tcw) was less than in chloroplast surface area exposed to intercellular air space (Sc/S), and Sc/S was a key anatomical trait explaining variations in gm and AN among different age classes. The reduction of Sc/S was related to ultrastructural changes in chloroplasts associated to leaf aging, with a concomitant reduction in cytoplasmic nitrogen. Changes in leaf anatomy and biochemistry were responsible for the age-dependent modifications in gm and AN. These findings revealed a gradual physiological deterioration related to the dismantling of the photosynthetic apparatus in older leaves of Q. ilex subsp. rotundifolia.

Standoff Optical Glucose Sensing in Photosynthetic Organisms by a Quantum Dot Fluorescent Probe

Glucose is a major product of photosynthesis and a key energy source for cellular respiration in organisms. Herein, we enable in vivo optical glucose sensing in wild-type plants using a quantum dot (QD) ratiometric approach. The optical probe is formed by a pair of QDs: thioglycolic acid-capped QDs which remain invariable to glucose (acting as an internal fluorescent reference control) and boronic acid (BA)-conjugated QDs (BA-QD) that quench their fluorescence in response to glucose. The fluorescence response of the QD probe is within the visible light window where photosynthetic tissues have a relatively low background. It is highly selective against other common sugars found in plants and can be used to quantify glucose levels above 500 μM in planta within the physiological range. We demonstrate that the QD fluorescent probe reports glucose from single chloroplast to algae cells ( Chara zeylanica) and plant leaf tissues ( Arabidopsis thaliana) in vivo via confocal microscopy and to a standoff Raspberry Pi camera setup. QD-based probes exhibit bright fluorescence, no photobleaching, tunable emission peak, and a size under plant cell wall porosity offering great potential for selective in vivo monitoring of glucose in photosynthetic organisms in situ.

Nitroaromatic detection and infrared communication from wild-type plants using plant nanobionics

Plant nanobionics aims to embed non-native functions to plants by interfacing them with specifically designed nanoparticles. Here, we demonstrate that living spinach plants (Spinacia oleracea) can be engineered to serve as self-powered pre-concentrators and autosamplers of analytes in ambient groundwater and as infrared communication platforms that can send information to a smartphone. The plants employ a pair of near-infrared fluorescent nanosensors-single-walled carbon nanotubes (SWCNTs) conjugated to the peptide Bombolitin II to recognize nitroaromatics via infrared fluorescent emission, and polyvinyl-alcohol functionalized SWCNTs that act as an invariant reference signal-embedded within the plant leaf mesophyll. As contaminant nitroaromatics are transported up the roots and stem into leaf tissues, they accumulate in the mesophyll, resulting in relative changes in emission intensity. The real-time monitoring of embedded SWCNT sensors also allows residence times in the roots, stems and leaves to be estimated, calculated to be 8.3 min (combined residence times of root and stem) and 1.9 min mm^-1 leaf, respectively. These results demonstrate the ability of living, wild-type plants to function as chemical monitors of groundwater and communication devices to external electronics at standoff distances.

A mutation that eliminates bundle sheath extensions reduces leaf hydraulic conductance, stomatal conductance and assimilation rates in tomato (Solanum lycopersicum)

Bundle sheath extensions (BSEs) are key features of leaf structure whose distribution differs among species and ecosystems. The genetic control of BSE development is unknown, so BSE physiological function has not yet been studied through mutant analysis. We screened a population of ethyl methanesulfonate (EMS)-induced mutants in the genetic background of the tomato (Solanum lycopersicum) model Micro-Tom and found a mutant lacking BSEs. The leaf phenotype of the mutant strongly resembled the tomato mutant obscuravenosa (obv). We confirmed that obv lacks BSEs and that it is not allelic to our induced mutant, which we named obv-2. Leaves lacking BSEs had lower leaf hydraulic conductance and operated with lower stomatal conductance and correspondingly lower assimilation rates than wild-type leaves. This lower level of function occurred despite similarities in vein density, midvein vessel diameter and number, stomatal density, and leaf area between wild-type and mutant leaves, the implication being that the lack of BSEs hindered water dispersal within mutant leaves. Our results comparing near-isogenic lines within a single species confirm the hypothesised role of BSEs in leaf hydraulic function. They further pave the way for a genetic model-based analysis of a common leaf structure with deep ecological consequences.