Liming can decrease legume crop yield and leaf gas exchange by enhancing root to shoot ABA signalling

Bi-directional, long-distance hormonal signalling between roots and shoots of soil water availability

While the importance of plant water relations in determining crop response to soil water availability is difficult to over-emphasise, under many circumstances, plants maintain their leaf water status as the soil dries yet shoot gas exchange and growth is restricted. Such observations lead to development of a paradigm that root-to-shoot signals regulate shoot physiology, and a conceptual framework to test the importance of different signals such as plant hormones in these physiological processes. Nevertheless, shoot-to-root (hormonal) signalling also plays an important role in regulating root growth and function and may dominate when larger quantities of a hormone are produced in the shoots than the roots. Here, we review the evidence for acropetal and basipetal transport of three different plant hormones (abscisic acid, jasmonates, strigolactones) that have antitranspirant effects, to indicate the origin and action of these signalling systems. The physiological importance of each transport pathway likely depends on the specific environmental conditions the plant is exposed to, specifically whether the roots or shoots are the first to lose turgor when exposed to drying soil or elevated atmospheric demand, respectively. All three hormones can interact to influence each other's synthesis, degradation and intracellular signalling to augment or attenuate their physiological impacts, highlighting the complexity of unravelling these signalling systems. Nevertheless, such complexity suggests crop improvement opportunities to select for allelic variation in the genes affecting hormonal regulation, and (in selected crops) to augment root-shoot communication by judicious selection of rootstock-scion combinations to ameliorate abiotic stresses.

Estimating the importance of maize root hairs in low phosphorus conditions and under drought

BACKGROUND AND AIMS

Root hairs are single-cell extensions of the epidermis that face into the soil and increase the root-soil contact surface. Root hairs enlarge the rhizosphere radially and are very important for taking up water and sparingly soluble nutrients, such as the poorly soil-mobile phosphate. In order to quantify the importance of root hairs for maize, a mutant and the corresponding wild type were compared.

METHODS

The rth2 maize mutant with very short root hairs was assayed for growth and phosphorus (P) acquisition in a slightly alkaline soil with low P and limited water supply in the absence of mycorrhization and with ample P supply.

KEY RESULTS

Root and shoot growth was additively impaired under P deficiency and drought. Internal P concentrations declined with reduced water and P supply, whereas micronutrients (iron, zinc) were little affected. The very short root hairs in rth2 did not affect internal P concentrations, but the P content of juvenile plants was halved under combined stress. The rth2 plants had more fine roots and increased specific root length, but P mobilization traits (root organic carbon and phosphatase exudation) differed little.

CONCLUSIONS

The results confirm the importance of root hairs for maize P uptake and content, but not for internal P concentrations. Furthermore, the performance of root hair mutants may be biased by secondary effects, such as altered root growth.

Long-distance ABA transport can mediate distal tissue responses by affecting local ABA concentrations

Environmental stresses that perturb plant water relations influence abscisic acid (ABA) concentrations, but it is unclear whether long-distance ABA transport contributes to changes in local ABA levels. To determine the physiological relevance of ABA transport, we made reciprocal- and self-grafts of ABA-deficient flacca mutant and wild-type (WT) tomato plants, in which low phosphorus (P) conditions decreased ABA concentrations while salinity increased ABA concentrations. Whereas foliar ABA concentrations in the WT scions were rootstock independent under conditions, salinity resulted in long-distance transport of ABA: flacca scions had approximately twice as much ABA when grafted on WT rootstocks compared to flacca rootstocks. Root ABA concentrations were scion dependent: both WT and flacca rootstocks had less ABA with the flacca mutant scion than with the WT scion under conditions. In WT scions, whereas rootstock genotype had limited effects on stomatal conductance under conditions, a flacca rootstock decreased leaf area of stressed plants, presumably due to attenuated root-to-shoot ABA transport. In flacca scions, a WT rootstock decreased stomatal conductance but increased leaf area of stressed plants, likely due to enhanced root-to-shoot ABA transport. Thus, long-distance ABA transport can affect responses in distal tissues by changing local ABA concentrations.

ABA flow modelling in Ricinus communis exposed to salt stress and variable nutrition

In a series of experiments with Ricinus communis, abscisic acid (ABA) concentrations in tissues and transport saps, its de novo biosynthesis, long-distance transport, and metabolism (degradation) were affected by nutritional conditions, nitrogen (N) source, and nutrient limitation, or salt stress. In the present study these data were statistically re-evaluated, and new correlations presented that underpin the importance of this universal phytohormone. The biggest differences in ABA concentration were observed in xylem sap. N source had the strongest effect; however, nutrient limitation (particularly phosphorus limitation) and salt also had significant effects. ABA was found in greater concentration in phloem sap compared with xylem sap; however, the effect of treatment on ABA concentration in phloem was lower. In the leaves, ABA concentration was most variable compared with the other tissues. This variation was only affected by the N source. In roots, ABA was significantly decreased by nutrient limitation. Of the compartments in which ABA was quantified, xylem sap ABA concentration was most significantly correlated with leaf stomatal conductance and leaf growth. Additionally, ABA concentration in xylem was significantly correlated to that in phloem, indicating a 6-fold concentration increase from xylem to phloem. The ABA flow model showed that biosynthesis of ABA in roots affected the xylem flow of ABA. Moreover, ABA concentration in xylem affected the degradation of the phytohormone in shoots and also its export from shoots via phloem. The role of phloem transport is discussed since it stimulates ABA metabolism in roots.

Rhizosphere bacteria containing 1-aminocyclopropane-1- carboxylate deaminase increase growth and photosynthesis of pea plants under salt stress by limiting Na+ accumulation

Although plant salt tolerance has been improved by soil inoculation with rhizobacteria containing the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase (which metabolises ACC, the immediate precursor of the phytohormone ethylene), it is not always clear whether ion homeostasis and plant water relations are affected. When pea (Pisum sativum L. cv. Alderman) was grown with 70 and 130mM NaCl, the ACC-deaminase containing rhizobacterium Variovorax paradoxus 5C-2 increased total biomass by 25 and 54% respectively. Nutrient flow modelling showed that V. paradoxus 5C-2 increased K uptake and root to shoot K flow, but decreased Na flow and increased Na deposition in roots. Thus, shoot K+:Na+ ratio increased following V. paradoxus 5C-2 inoculation. At 70 and 130mM NaCl, rhizobacterial inoculation decreased stomatal resistance by 14 and 31% and decreased xylem balancing pressure by 7 and 21% respectively. Furthermore, rhizobacterial inoculation improved photosynthetic efficiency (Fv/Fm) by 12 and 19% and increased maximal electron transport rate (ETR) by 18 and 22% at 70 and 130mM NaCl respectively. Thus V. paradoxus 5C-2 mitigates salt stress by improving water relations, ion homeostasis and photosynthesis of pea plants, and may provide an economic means of promoting growth of plants exposed to salt stress.