Effects of pod removal on the transport and accumulation of abscisic Acid and indole-3-acetic Acid in soybean leaves

Dynamic root exudate chemistry and microbial substrate preferences drive patterns in rhizosphere microbial community assembly

Like all higher organisms, plants have evolved in the context of a microbial world, shaping both their evolution and their contemporary ecology. Interactions between plant roots and soil microorganisms are critical for plant fitness in natural environments. Given this co-evolution and the pivotal importance of plant-microbial interactions, it has been hypothesized, and a growing body of literature suggests, that plants may regulate the composition of their rhizosphere to promote the growth of microorganisms that improve plant fitness in a given ecosystem. Here, using a combination of comparative genomics and exometabolomics, we show that pre-programmed developmental processes in plants (Avena barbata) result in consistent patterns in the chemical composition of root exudates. This chemical succession in the rhizosphere interacts with microbial metabolite substrate preferences that are predictable from genome sequences. Specifically, we observed a preference by rhizosphere bacteria for consumption of aromatic organic acids exuded by plants (nicotinic, shikimic, salicylic, cinnamic and indole-3-acetic). The combination of these plant exudation traits and microbial substrate uptake traits interact to yield the patterns of microbial community assembly observed in the rhizosphere of an annual grass. This discovery provides a mechanistic underpinning for the process of rhizosphere microbial community assembly and provides an attractive direction for the manipulation of the rhizosphere microbiome for beneficial outcomes.

Significant contribution from foliage-derived ABA in regulating gas exchange in Pinus radiata

The complex regulatory system controlling stomata involves physical and chemical signals that affect guard cell turgor to bring about changes in stomatal conductance (gs). Abscisic acid (ABA) closes stomata, yet the mechanisms controlling foliar ABA status in tree species remain unclear. The importance of foliage-derived ABA in regulating gas exchange was evaluated under treatments that affected phloem export through girdling and reduced water availability in the tree species, Pinus radiata (D. Don). Branch- and whole-plant girdling increased foliar ABA levels leading to declines in gs, despite no change in plant water status. Changes in gs were largely independent of the more transient increases in foliar non-structural carbohydrates (NSC), suggesting that gradual accumulation of foliar ABA was the primary mechanism for reductions in gs and assimilation. Whole-plant girdling eventually reduced root NSC, hindering root water uptake and decreasing foliar water potential, causing a dramatic increase in ABA level in leaves and concentrations in the xylem sap of shoots (4032 ng ml-1), while root xylem sap concentrations remained low (43 ng ml-1). Contrastingly, the drought treatment caused similar increases in xylem sap ABA in both roots and shoots, suggesting that declines in water potential result in relatively consistent changes in ABA along the hydraulic pathway. ABA levels in plant canopies can be regulated independently of changes in root water status triggered by changes by both phloem export and foliar water status.

Long-distance phloem transport of glucosinolates in Arabidopsis

Glucosinolates are a large group of plant secondary metabolites found mainly in the order Capparales, which includes a large number of economically important Brassica crops and the model plant Arabidopsis. In the present study, several lines of evidence are provided for phloem transport of glucosinolates in Arabidopsis. When radiolabeled p-hydroxybenzylglucosinolate (p-OHBG) and sucrose were co-applied to the tip of detached leaves, both tracers were collected in the phloem exudates at the petioles. Long-distance transport of [(14)C]p-OHBG was investigated in wild-type and transgenic 35S::CYP79A1 plants, synthesizing high amounts of p-OHBG, which is not a natural constituent of wild-type Arabidopsis. In both wild-type and 35S::CYP79A1 plants, radiolabeled p-OHBG was rapidly transported from the application site into the whole plant and intact p-OHBG was recovered from different tissues. The pattern of distribution of the radioactivity corresponded to that expected for transport of photoassimilates such as sucrose, and was consistent with translocation in phloem following the source-sink relationship. Radiolabeled p-OHBG was shown to accumulate in the seeds of wild-type and 35S::CYP79A1 plants, where p-OHBG had been either exogenously applied or endogenously synthesized from Tyr in the leaves. p-OHBG was found in phloem exudates collected from cut petioles of leaves from both wild-type and 35S::CYP79A1 plants. Phloem exudates were shown to contain intact glucosinolates, and not desulphoglucosinolates, as the transport form. It is concluded that intact glucosinolates are readily loaded into and transported by the phloem.

Abscisic Acid Alters the Metabolism of Indole-3-Acetic Acid in Senescing Flowers of Cucumis melo L

Experiments were conducted to investigate indole-3-acetic acid (IAA) and abscisic acid (ABA) metabolism associated with postanthesis senescence of ovaries from nonpollinated muskmelon (Cucumis melo L.) flowers. Flowers attached to the vine were allowed to senesce for 4 days after anthesis or were harvested at full anthesis and aged for the same time interval with or without access to water. The IAA ester, amide-linked forms of IAA, free ABA, and ABA esters increased in senescent ovaries from flowers left attached to the vine. Detaching flowers from the vine resulted in an accumulation of free and amide-linked IAA in the senescing ovary but suppressed accumulation of ester IAA. Free ABA failed to increase in ovaries detached from the vine. Subjecting detached flowers to water stress had no effect on the endogenous level of free ABA but resulted in the accumulation of ABA ester and suppression of any increase in free IAA. However, detached flowers treated with 0.1 millimolar ABA accumulated 75% less free IAA and initiated the synthesis of ester IAA. Detached flowers treated with ABA also accumulated high levels of ester ABA. These results suggest that the metabolism of free IAA in muskmelon ovary tissue is regulated in situ and not the consequence of external synthesis and importation. ABA appears to be transported into the senescing ovary from an external source and alters the IAA metabolism in such a manner as to suppress the level of free IAA while stimulating accumulation of the ester IAA.

Accumulation of C-radiolabel in leaves and fruits after injection of [C]tryptophan into seeds of soybean

Injection of [(14)C]tryptophan into one seed in a soybean fruit resulted in recovery of radiolabel in a fraction that cochromatographed with indoleacetic acid (IAA) through three successive high performance liquid chromatography separations. Label was found in the putative IAA fraction in all of the fruit tissues sampled and in the blade of the leaf subtending the pod into which the radiolabeled tryptophan had been injected. This suggested that IAA or an IAA precursor was transported from seeds to other parts of the fruit and to subtending leaves.