Joanne Tilbrook

Roles of aquaporins in root responses to irrigation

Due to current environmental issues concerning the use of water for irrigation, the improvement of crop water-use efficiency and a reduction in water consumption has become a priority. New irrigation methods that reduce water use, while still maintaining production have been developed. To optimise these techniques knowledge of above- and below-ground plant physiological responses is necessary. During growth, plant roots are exposed to cycles of wetting and drying in normal rain-fed and irrigation situations. This review concentrates on the below-ground aspects, in particular the water permeability of roots. Significant research has been conducted on the root anatomy and hydraulic conductivity of desert plants subjected to wetting and drying. Major intrinsic proteins (MIPs), most of which show aquaporin (water-channel) activity are likely to be involved in balancing the water relations of the plants during water deficit. However, many MIPs seem to allow permeation of other small neutral solutes and some may allow permeation of ions under certain conditions. The ability of the plant to rapidly respond to rewetting may be important in maintaining productivity. It has been suggested that aquaporins may be involved in this rapid response. The down-regulation of the aquaporins during dry conditions can also limit water loss to the soil, and intrinsic sensitivity of aquaporins to water potential is shown here to be very strong in some cases (NOD26). However, the response of aquaporins in various plant species to water deficits has been quite varied. Another component of aquaporin regulation in response to various stresses (hypoxia/anoxia, salinity and chilling) may be related to redistribution of flow to more favourable regions of the soil. Some irrigation techniques may be triggering these responses. Diurnal fluctuations of root hydraulic conductance that is related to aquaporin expression seem to match the expected transpirational demands of the shoot, and it remains to be seen if shoot-to-root signalling may be important in regulation of root aquaporins. If so, canopy management typical of horticultural crops may impact on root hydraulic conductance. An understanding of the regulation of aquaporins may assist in the development of improved resistance to water stress and greater efficiency of water use by taking into account where and when roots best absorb water.

Cell death in grape berries: varietal differences linked to xylem pressure and berry weight loss

Some varieties of Vitis vinifera L. can undergo berry weight loss during later stages of ripening. This defines a third phase of development in addition to berry formation and berry expansion. Berry weight loss is due to net water loss, but the component water flows through different pathways have remained obscure. Because of the very negative osmotic potential of the cell sap, the maintenance of semipermeable membranes in the berry is required for the berry to counter xylem and apoplast tensions that may be transferred from the vine. The transfer of tension is determined by the hydraulic connection through the xylem from the berry to the vine, which changes during development. Here we assess the membrane integrity of three varieties of V. vinifera berries (cvv. Shiraz, Chardonnay and Thompson seedless) throughout development using the vitality stains, fluorescein diacetate and propidium iodide, on fresh longitudinal sections of whole berries. We also measured the xylem pressure using a pressure probe connected to the pedicel of detached berries. The wine grapes, Chardonnay and Shiraz, maintained fully vital cells after veraison and during berry expansion, but began to show cell death in the mesocarp and endocarp at or near the time that the berries attain maximum weight. This corresponded to a change in rate of accumulation of solutes in the berry and the beginning of weight loss in Shiraz, but not in Chardonnay. Continuous decline in mesocarp and endocarp cell vitality occurred for both varieties until normal harvest dates. Shiraz grapes classified as high quality and sourced from a different vineyard also showed the same death response at the same time after anthesis, but they displayed a more consistent pattern of pericarp cell death. The table grape, Thompson seedless, showed near to 100% vitality for all cells throughout development and well past normal harvest date, except for berries with noticeable berry collapse that were treated with giberellic acid. The high cell vitality in Thompson seedless berries corresponded to negative xylem pressures that contrasted to the slightly positive pressures for Shiraz and Chardonnay. We hypothesise that two variety dependent strategies exist for grapevine berries late in development: (1) programmed cell death in the pericarp and loss of osmotically competent membranes that requires concomitant reduction in the hydraulic conductance via the xylem to the vine; (2) continued cell vitality and osmotically competent membranes that can allow high hydraulic conductance to the vine.

Hydraulic connection of grape berries to the vine: varietal differences in water conductance into and out of berries, and potential for backflow

Weight loss in Vitis vinifera L. cv. Shiraz berries occurs in the later stages of ripening from 90-100 days after anthesis (DAA). This rarely occurs in varieties such as Chardonnay and Thompson seedless. Flow rates of water under a constant pressure into berries on detached bunches of these varieties are similar until 90-100 DAA. Shiraz berries then maintain constant flow rates until harvest maturity, and Chardonnay inflow tapers to almost zero. Thompson seedless maintains high xylem inflows. Hydraulic conductance for flow in and out of individual Shiraz and Chardonnay berries was measured using a root pressure probe. From 105 DAA, during berry weight loss in Shiraz, there were significant varietal differences in xylem hydraulic conductance. Both varieties showed flow rectification such that conductance for inflow was higher than conductance for outflow. For flow into the berry, Chardonnay had 14% of the conductance of Shiraz. For flow out of the berry Chardonnay was 4% of the conductance of Shiraz. From conductance measurements for outflow from the berry and stem water potential measurements, it was calculated that Shiraz could loose ~7% of berry volume per day, consistent with rates of berry weight loss. A functional pathway for backflow from the berries to the vine via the xylem was visualised with Lucifer Yellow CH loaded at the cut stylar end of berries on potted vines. Transport of the dye out of the berry xylem ceased before 97 DAA in Chardonnay, but was still transported into the torus and pedicel xylem of Shiraz at 118 DAA. Xylem backflow could be responsible for a portion of the post-veraison weight loss in Shiraz berries. These data provide evidence of varietal differences in hydraulic connection of berries to the vine that we relate to cell vitality in the mesocarp. The key determinates of berry water relations appear to be maintenance or otherwise of semi permeable membranes in the mesocarp cells and control of flow to the xylem to give variable hydraulic connection back to the vine.

Comparison of Leaf Sheath Transcriptome Profiles with Physiological Traits of Bread Wheat Cultivars under Salinity Stress

Salinity stress has significant negative effects on plant biomass production and crop yield. Salinity tolerance is controlled by complex systems of gene expression and ion transport. The relationship between specific features of mild salinity stress adaptation and gene expression was analyzed using four commercial varieties of bread wheat (Triticum aestivum) that have different levels of salinity tolerance. The high-throughput phenotyping system in The Plant Accelerator at the Australian Plant Phenomics Facility revealed variation in shoot relative growth rate and salinity tolerance among the four cultivars. Comparative analysis of gene expression in the leaf sheaths identified genes whose functions are potentially linked to shoot biomass development and salinity tolerance. Early responses to mild salinity stress through changes in gene expression have an influence on the acquisition of stress tolerance and improvement in biomass accumulation during the early “osmotic” phase of salinity stress. In addition, results revealed transcript profiles for the wheat cultivars that were different from those of usual stress-inducible genes, but were related to those of plant growth. These findings suggest that, in the process of breeding, selection of specific traits with various salinity stress-inducible genes in commercial bread wheat has led to adaptation to mild salinity conditions.

Variation in shoot tolerance mechanisms not related to ion toxicity in barley

Soil salinity can severely reduce crop growth and yield. Many studies have investigated salinity tolerance mechanisms in cereals using phenotypes that are relatively easy to measure. The majority of these studies measured the accumulation of shoot Na+ and the effect this has on plant growth. However, plant growth is reduced immediately after exposure to NaCl before Na+ accumulates to toxic concentrations in the shoot. In this study, nondestructive and destructive measurements are used to evaluate the responses of 24 predominately Australian barley (Hordeum vulgare L.) lines at 0, 150 and 250mM NaCl. Considerable variation for shoot tolerance mechanisms not related to ion toxicity (shoot ion-independent tolerance) was found, with some lines being able to maintain substantial growth rates under salt stress, whereas others stopped growing. Hordeum vulgare spp. spontaneum accessions and barley landraces predominantly had the best shoot ion independent tolerance, although two commercial cultivars, Fathom and Skiff, also had high tolerance. The tolerance of cv. Fathom may be caused by a recent introgression from H. vulgare L. spp. spontaneum. This study shows that the most salt-tolerant barley lines are those that contain both shoot ion-independent tolerance and the ability to exclude Na+ from the shoot (and thus maintain high K+:Na+ ratios).

Whole-Plant Phenomics

Over the last decade, biological sciences have been revolutionised by the adoption of high-throughput omics technologies, and many of the discoveries that have underpinned this revolution have now been adopted for use in plant phenotyping. The field of whole-plant phenomics resulting from this combines robotics, image capture and high-performance computational analysis and provides plant scientists with the ability to characterise dozens of phenotypes on thousands of plants daily.