Abstract 1626: Tumors driven by oncogene amplified extrachromosomal DNA (ecDNA) demonstrate enhanced sensitivity to cell cycle checkpoint kinase 1 (CHK1) inhibition

Patients whose tumors harbor oncogene amplification on extrachromosomal DNA (ecDNA) fail to respond to targeted or immune therapy and have poor prognosis. ecDNA are cancer-specific circular fragments of genomic DNA engendered with unique properties, including open chromatin architecture associated with hyper transcription and a predilection for structural variation. In addition, ecDNA+ tumors have tremendous copy number heterogeneity mediated through acentric, non-Mendelian segregation. These properties afford ecDNA+ tumors with unparalleled genomic plasticity permitting circumvention of therapeutic pressure. However, these features also confer heightened levels of DNA replication stress (RS), and we have found that ecDNA amplified tumor cells are hyper-reliant on CHK1, a master regulator of the cellular RS response. We identified CHK1 as an ecDNA essential target in a CRISPR genetic screen using a methotrexate-induced ecDNA amplification model in HeLa cancer cells. ecDNA-dependent cell fitness assessment of CHK1 was confirmed using a flow cytometry-based CRISPR competition assay. The target was further validated using a CHK1 inhibitor (CHK1i) tool compound in MYC-amplified COLO320 isogenic cell lines that demonstrated a 10-fold enhanced cytotoxicity in the ecDNA amplified setting.Based on these results, we developed a highly potent, selective, and orally bioavailable CHK1i optimized as an ecDNA-directed therapeutic (ecDTx). An advanced lead, BBI-cmpd1, robustly induced RS biomarkers (e.g., pRPA) in ecDNA+ COLO320 tumor cells as compared to matched chromosomally-amplified (ecDNA-) COLO320 cells, consistent with the increased reliance on CHK1 to manage elevated RS in the ecDNA amplified setting. BBI-cmpd1 demonstrated potent anti-proliferative activity against a panel of ecDNA+ oncogene amplified tumor lines as compared to non-amplified lines demonstrating oncogene and indication agnostic efficacy in ecDNA-based tumors. Oral administration of BBI-cmpd1 resulted in on-target activity against CHK1 and anti-tumor activity in an ecDNA oncogene amplified tumor model in vivo. These findings support the clinical utility of potent, selective, and oral CHK1i to address the significant unmet need driven by ecDNA oncogene amplified cancers.

Citation Format: Sudhir Chowdhry, Snezana Milutinovic, Edison Tse, Salvador Garcia, Dean Perusse, Melissa Ritland, Juyeon Ko, Deepti Wilkinson, Kristen Turner, Auzon Steffy, Joshua Plum, Ben Norman, AnnMarie Pferdekamper, Todd Meyer, Debbie Liao, Rachelle Elsdon, Joshua Lange, Anthony Pinkerton, Ryan Hansen, Christian Hassig, Shailaja Kasibhatla. Tumors driven by oncogene amplified extrachromosomal DNA (ecDNA) demonstrate enhanced sensitivity to cell cycle checkpoint kinase 1 (CHK1) inhibition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1626.

Inhibition of alkaline phosphatase impairs dyslipidemia and protects mice from atherosclerosis

Calcium accumulation in atherosclerotic plaques predicts cardiovascular mortality, but the mechanisms responsible for plaque calcification and how calcification impacts plaque stability remain debated. Tissue-nonspecific alkaline phosphatase (TNAP) recently emerged as a promising therapeutic target to block cardiovascular calcification. In this study, we sought to investigate the effect of the recently developed TNAP inhibitor SBI-425 on atherosclerosis plaque calcification and progression. TNAP levels were investigated in ApoE-deficient mice fed a high-fat diet from 10 weeks of age and in plaques from the human ECLAGEN biocollection (101 calcified and 14 non-calcified carotid plaques). TNAP was inhibited in mice using SBI-425 administered from 10 to 25 weeks of age, and in human vascular smooth muscle cells (VSMCs) with MLS-0038949. Plaque calcification was imaged in vivo with ¹⁸F-NaF-PET/CT, ex vivo with osteosense, and in vitro with alizarin red. Bone architecture was determined with µCT. TNAP activation preceded and predicted calcification in human and mouse plaques, and TNAP inhibition prevented calcification in human VSMCs and in ApoE-deficient mice. More unexpectedly, TNAP inhibition reduced the blood levels of cholesterol and triglycerides, and protected mice from atherosclerosis, without impacting the skeletal architecture. Metabolomics analysis of liver extracts identified phosphocholine as a substrate of liver TNAP, who's decreased dephosphorylation upon TNAP inhibition likely reduced the release of cholesterol and triglycerides into the blood. Systemic inhibition of TNAP protects from atherosclerosis, by ameliorating dyslipidemia, and preventing plaque calcification.

P1237TISSUE-NONSPECIFIC ALKALINE PHOSPHATASE, A CULPRIT DURING ARTERIAL MEDIA CALCIFICATION BUT INDISPENSABLE DURING PHYSIOLOGICAL BONE FORMATION/-MINERALIZATION IN RATS

Background and Aims

Vascular media calcification is frequently seen in elderly and patients with chronic kidney disease (CKD), diabetes and osteoporosis. Pyrophosphate is a well-known calcification inhibitor that binds to nascent hydroxyapatite crystals and prevents further incorporation of inorganic phosphate into these crystals. However, the enzyme tissue-nonspecific alkaline phosphatase (TNAP), which is highly expressed in calcified arteries, degrades extracellular pyrophosphate into phosphate ions, by which pyrophosphate loses its ability to block vascular calcification. Here, we aimed to evaluate whether a TNAP inhibitor is able to prevent the development of arterial calcification in a rat model of warfarin-induced vascular calcification.

Method

To induce vascular calcification, rats received a diet containing 0.30% warfarin and 0.15% vitamin K1 throughout the entire study and were subjected to the following daily treatments: (i) vehicle (n=10) or (ii) 10 mg/kg/day TNAP-inhibitor (n=10) administered via an intraperitoneal catheter from start of the study until sacrifice at week 7. Calcium, phosphorus and parathyroid hormone (PTH) levels were determined in serum samples as these are important determinants of vascular calcification. As TNAP is also expressed in the liver, serum alanine aminotransferase (ALT) and aspartate (AST) levels were analyzed. At sacrifice, vascular calcification was evaluated by measurement of the total calcium content in the arteries and quantification of the area % calcification on Von Kossa stained sections of the aorta. The mRNA expression of osteo/chondrogenic marker genes (runx2, TNAP, SOX9, collagen 1 and collagen 2) was analyzed in the aorta by qPCR to verify whether vascular smooth muscle cells underwent reprogramming towards bone-like cells. Bone histomorphometry was performed on the left tibia to measure static and dynamic bone parameters as TNAP also regulates physiological bone mineralization.

Results

No differences in serum calcium, phosphorus and PTH levels was observed between both study groups. Warfarin exposure resulted in distinct calcification in the aorta and peripheral arteries. Daily dosing with the TNAP inhibitor (10 mg/kg/day) for 7 weeks significantly reduced vascular calcification as indicated by a significant decrease in calcium content in the aorta (vehicle 3.84±0.64 mg calcium/g wet tissue vs TNAP inhibitor 0.70±0.23 mg calcium/g wet tissue) and peripheral arteries and a distinct reduction in area % calcification on Von Kossa stained aortic sections as compared to vehicle condition. The inhibitory effects of SBI-425 on vascular calcification were without altering serum liver markers ALT and AST levels. Furthermore, TNAP-inhibitor SBI-425 did not modulate the mRNA expression of osteo/chondrogenic marker genes runx2, TNAP, SOX9, collagen 1 and 2. Dosing with SBI-425 resulted in decreased bone formation rate and mineral apposition rate, and increased osteoid maturation time and this without significant changes in osteoclast- and eroded perimeter.

Conclusion

Dosing with TNAP inhibitor SBI-425 significantly reduced the calcification in the aorta and peripheral arteries of a rat model of warfarin-induced vascular calcification and this without affecting liver function. However, suppression of TNAP activity should be limited in order to maintain adequate physiological bone mineralization.

A screen for inducers of bHLH activity identifies pitavastatin as a regulator of p21, Rb phosphorylation and E2F target gene expression in pancreatic cancer

The average survival for patients with Pancreatic Ductal Adenocarcinoma (PDA) is merely 6 months, underscoring the need for new therapeutic approaches. During PDA progression, pancreatic acinar cells lose activity of the ClassI/II bHLH factors that regulate quiescence. We previously found that promoting transcriptional activity of the Class I bHLH factor E47 in highly aggressive PDA cells induced stable growth arrest in vitro and in vivo. To translate these findings for clinical utility, we developed a high throughput screening platform to identify small molecule inducers of Class I/II bHLH activity. A screen of 4,375 known drugs identified 70 bHLH activators. Prominent among the hits were members of the statin class of HMG-CoA reductase inhibitors, cholesterol lowering drugs that are also being evaluated in cancer. Studies with pitavastatin in primary patient derived tumor cells and established PDA lines, revealed dose dependent growth inhibition. At the molecular level, pitavastatin induced expression of the cyclin dependent kinase (CDK) inhibitor p21 in a cholesterol independent manner, blocked repressive phosphorylation of the Retinoblastoma tumor suppressor protein at CDK targeted sites, and reduced expression of E2F target genes required for progression through the G1/S boundary. Together, the data provide new insight into mechanisms by which statins constrain proliferation in cancer and establish the effectiveness of a novel screening platform to identify small molecules of clinical relevance in pancreatic cancer.

High-throughput screening and bioinformatic analysis to ascertain compounds that prevent saturated fatty acid-induced β-cell apoptosis

Pancreatic β-cell lipotoxicity is a central feature of the pathogenesis of type 2 diabetes. To study the mechanism by which fatty acids cause β-cell death and develop novel approaches to prevent it, a high-throughput screen on the β-cell line INS1 was carried out. The cells were exposed to palmitate to induce cell death and compounds that reversed palmitate-induced cytotoxicity were ascertained. Hits from the screen were analyzed by an increasingly more stringent testing funnel, ending with studies on primary human islets treated with palmitate. MAP4K4 inhibitors, which were not part of the screening libraries but were ascertained by a bioinformatics analysis, and the endocannabinoid anandamide were effective at inhibiting palmitate-induced apoptosis in INS1 cells as well as primary rat and human islets. These targets could serve as the starting point for the development of therapeutics for type 2 diabetes.

Nimbolide targets BCL2 and induces apoptosis in preclinical models of Waldenströms macroglobulinemia

Neem leaf extract (NLE) has medicinal properties, which have been attributed to its limonoid content. We identified the NLE tetranorterpenoid, nimbolide, as being the key limonoid responsible for the cytotoxicity of NLE in various preclinical models of human B-lymphocyte cancer. Of the models tested, Waldenströms macroglobulinemia (WM) cells were most sensitive to nimbolide, undergoing significant mitochondrial mediated apoptosis. Notably, nimbolide toxicity was also observed in drug-resistant (bortezomib or ibrutinib) WM cells. To identify putative targets of nimbolide, relevant in WM, we used chemoinformatics-based approaches comprised of virtual in silico screening, molecular modeling and target–ligand reverse docking. In silico analysis revealed the antiapoptotic protein BCL2 was the preferential binding partner of nimbolide. The significance of this finding was further tested in vitro in RS4;11 (BCL2-dependent) tumor cells, in which nimbolide induced significantly more apoptosis compared with BCL2 mutated (Jurkat BCL2^Ser70-Ala) cells. Lastly, intraperitoneal administration of nimbolide in WM tumor xenografted mice, significantly reduced tumor growth and IgM secretion in vivo, while modulating the expression of several proteins as seen on immunohistochemistry. Overall, our data demonstrate that nimbolide is highly active in WM cells, as well as other B-cell cancers, and engages BCL2 to exert its cytotoxic activity.

Abstract 4520: Inhibition of melanoma growth by small molecules that attenuate PI3K/PDK1 signaling and promote ATF2 mitochondrial localization

Phosphoinositide-dependent kinase-1 (PDK-1) phosphorylates members of the AGC kinase superfamily, including AKT and PKC. We recently demonstrated the importance of PDK1 in melanoma development and progression (Oncogene 2013). Genetic ablation of PDK1 in melanocytes effectively delayed melanoma development and inhibited metastasis in a Braf/Cdkn2a model with WT or mutant Pten. In an effort to identify small molecule inhibitors that could affect the PDK1 signaling we characterized SBI-0089410, a small molecule identified in a screen for nuclear exclusion of the transcription factor ATF2, which attenuates its oncogenic activity while enabling its function at the mitochondrial membrane to promote apoptosis. SBI-0089410 reduced melanoma cell viability, inhibited colony formation and spheroid growth, decreased mitochondrial membrane permeability, decreased ATF2 phosphorylation by PKCϵ and altered ATF2 transcriptional activity. Notably, the effect of SBI-0089401 was attenuated by the overexpression of constitutively active PKCϵ or ATF2T52E, which carries a phosphomimetic mutation at the PKCϵ phosphorylation site. Reverse phase protein array (RPPA), validated by western blot analysis, revealed that SBI-408910 attenuates PI3K/AKT/PDK1 signaling pathways. In vivo, SBI-089410 attenuated the growth of Braf/Cdkn2a/Pten tumors in a syngeneic B6 mouse model, as well as prevented their resistance to BRAFi. Our studies substantiate the importance of targeting PI3K/PDK1 signaling in melanoma as a means for effective therapy and prevention of chemoresistance.

Citation Format: Tal Varsano, Yongmei Feng, Giuseppina Claps, Marzia Scortegagna, Eric Lau, Marilyn Leonard, Anthony Pinkerton, Maurizio Pellecchia, Michael Davies, Marcus Bosenberg, Ze'ev Ronai. Inhibition of melanoma growth by small molecules that attenuate PI3K/PDK1 signaling and promote ATF2 mitochondrial localization. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4520. doi:10.1158/1538-7445.AM2014-4520

Induction of β-cell replication by a synthetic HNF4α antagonist

Increasing the number of β cells is critical to a definitive therapy for diabetes. Previously, we discovered potent synthetic small molecule antagonists of the nuclear receptor transcription factor HNF4α. The natural ligands of HNF4α are thought to be fatty acids. Because obesity, in which there are high circulating levels of free fatty acids, is one of the few conditions leading to β-cell hyperplasia, we tested the hypothesis that a potent HNF4α antagonist might stimulate β-cell replication. A bioavailable HNF4α antagonist was injected into normal mice and rabbits and β-cell ablated mice and the effect on β-cell replication was measured. In normal mice and rabbits, the compound induced β-cell replication and repressed the expression of multiple cyclin-dependent kinase inhibitors, including p16 that plays a critical role in suppressing β-cell replication. Interestingly, in β-cell ablated mice, the compound induced α- and δ-cell, in addition to β-cell replication, and β-cell number was substantially increased. Overall, the data presented here are consistent with a model in which the well-known effects of obesity and high fat diet on β-cell replication occur by inhibition of HNF4α. The availability of a potent synthetic HNF4α antagonist raises the possibility that this effect might be a viable route to promote significant increases in β-cell replication in diseases with reduced β-cell mass, including type I and type II diabetes.

Data quality as a driver for ever more subtle charge density analyses

The constant progress in both X-ray source and detector technology has clearly led not only to faster data collection, but also to improvements in data quality. As we move towards the unattainable "holy grail" of error free diffraction data, it is possible to address ever more subtle questions concerning the electron density distribution and associated properties. Current applications to the study of weak interactions, core polarization, heavy element compounds, anharmonic atomic displacements, and anisotropic motion of hydrogen atoms will be presented. The reliability of these studies will be discussed based on evaluations of data quality and refinement statistics.

Identification and characterization of novel human glucose-6-phosphate dehydrogenase inhibitors

Glucose-6-phosphate dehydrogenase (G6PD) is the key enzyme of the pentose phosphate pathway, converting glucose-6-phosphate to 6-phosphoglucono-δ-lactone with parallel reduction of NADP(+). Several human diseases, including cancer, are associated with increased G6PD activity. To date, only a few G6PD inhibitors have been available. However, adverse side effects and high IC(50) values hamper their use as therapeutics and basic research probes. In this study, we developed a high-throughput screening assay to identify novel human G6PD (hG6PD) inhibitors. Screening the LOPAC (Sigma-Aldrich; 1280 compounds), Spectrum (Microsource Discovery System; 1969 compounds), and DIVERSet (ChemBridge; 49 971 compounds) small-molecule compound collections revealed 139 compounds that presented ≥50% hG6PD inhibition. Hit compounds were further included in a secondary and orthogonal assay in order to identify false-positives and to determine IC(50) values. The most potent hG6PD inhibitors presented IC(50) values of <4 µM. Compared with the known hG6PD inhibitors dehydroepiandrosterone and 6-aminonicotinamide, the inhibitors identified in this study were 100- to 1000-fold more potent and showed different mechanisms of enzyme inhibition. One of the newly identified hG6PD inhibitors reduced viability of the mammary carcinoma cell line MCF10-AT1 (IC(50) ~25 µM) more strongly than that of normal MCF10-A cells (IC(50) >50 µM).

High-throughput screening for small-molecule inhibitors of plasmodium falciparum glucose-6-phosphate dehydrogenase 6-phosphogluconolactonase

Plasmodium falciparum causes severe malaria infections in millions of people every year. The parasite is developing resistance to the most common antimalarial drugs, which creates an urgent need for new therapeutics. A promising and attractive target for antimalarial drug design is the bifunctional enzyme glucose-6-phosphate dehydrogenase-6-phosphogluconolactonase (PfGluPho) of P. falciparum, which catalyzes the key step in the parasites' pentose phosphate pathway. In this study, we describe the development of a high-throughput screening assay to identify small-molecule inhibitors of recombinant PfGluPho. The optimized assay was used to screen three small-molecule compound libraries-namely, LOPAC (Sigma-Aldrich, 1280 compounds), Spectrum (MicroSource Discovery Systems, 1969 compounds), and DIVERSet (ChemBridge, 49 971 compounds). These pilot screens identified 899 compounds that inhibited PfGluPho activity by at least 50%. Selected compounds were further studied to determine IC(50) values in an orthogonal assay, the type of inhibition and reversibility, and effects on P. falciparum growth. Screening results and follow-up studies for selected PfGluPho inhibitors are presented. Our high-throughput screening assay may provide the basis to identify novel and urgently needed antimalarial drugs.

The combination of X-ray-mediated radiosurgery and gene-mediated immunoprophylaxis for advanced intracerebral gliosarcomas in rats

Rats with advanced, imminently lethal, approximately 4 mm diameter, left-sided intracerebral 9L gliosarcoma (9LGS), a well characterized malignant tumor with some similarities to human high-grade astrocytomas, were used as a therapy model 14 days post-implantation of 10(4) cells. Such tumor-bearing rats die within two weeks (median, 6 days) thereafter if untreated. However, if these tumors are exposed on day 14 to 12-25 Gy of an electron-equilibrated 6 MV photon beam (radiosurgery), survival is extended about 5-6 fold to a median of 34 days, but long-term survival (> 1 year) is increased only to approximately 18%. Multiple subcutaneous inoculations of radiation-disabled 9LGS cells post-radiosurgery (immunoprophylaxis) extended lifespan and long-term (> 1 year) survival minimally (median, 37 days; 25%, respectively). In sharp contrast, radiosurgery followed by multiple subcutaneous inoculations of radiation-disabled 9LGS cells that had been transfected with granulocyte macrophage colony stimulating factor (GMCSF), a cytokine with demonstrated immune-enhancing properties (i.e. gene-mediated immunoprophylaxis, GMIMPR) increased long-term survival to approximately 67%. To our knowledge, these results are the first to show that the combination of photon radiosurgery and GMIMPR is effective for an advanced, imminently lethal brain tumor in a mammal. These data raise the possibility that GMIMPR following radiation therapy might prove effective for the treatment of some human malignant gliomas.

Critical sulfur concentrations in oilseed rape (Brassica napus) in relation to nitrogen supply and to plant age

Summary. Oilseed rape was grown in a sand culture experiment in a glasshouse to derive values for plant testing for the diagnosis of sulfur (S) deficiency and for the prediction of seed yield. Five rates of S, combined factorially with 4 rates of nitrogen (N), maintained constant throughout the experiment, were used to determine critical concentrations of S fractions and ratios (total S, St; sulfate-S, SO4; total N/total S, N/St; SO4/St). The most satisfactory indices of rapeseed S status for diagnosis or prediction were St and SO4. Whole shoots and youngest fully expanded leaves exhibited similar critical values in plants at the rosette stage, and critical values (St = 0.20–0.25%; SO4 = 230–460 mg/kg) changed little with time. Critical values for N/St changed with time, required 2 analyses, and gave no indication of the degree of deficiency when used to predict yield. Critical values of SO4/St depended on N supply, so 3 analyses were needed. It is argued that high critical values reported previously for prediction of seed yield have been obtained when there was a decline in soil-available S and plants relied on S taken up during early growth.

Recovery of field-grown canola from sulfur deficiency

Sulfate-sulfur was applied to sulfur (S)-deficient canola at several growth stages in a field experiment at Cargo near Orange, New South Wales. Applications of 0, 10 or 40 kg S/ha (S0, S10 and S40) as mixtures of potassium sulfate and potassium chloride were made at sowing, the 5-6 leaf rosette stage, flower buds visible, stem elongation and first flowering. The plots received either 80 or 160 kg nitrogen (N)/ha at sowing. Plants from the S0 plots showed symptoms of severe S deficiency during rapid stem elongation, and had a 52% reduction in seed yield and a 21% reduction in seed oil concentration compared with the S40 plants. Application of S10 at sowing, or topdressing S-deficient plants with this rate of S, was inadequate because, although seed oil concentrations were normal (39-42%), seed yields were 25% lower than those from plots that received S40. Topdressing S-deficient plants with S40 at either the 5-6 leaf rosette stage, flower buds visible or stem elongation resulted in the same seed yields and seed oil concentrations as obtained when S40 was applied at sowing. However, there was a 15% reduction in seed yield but no reduction in seed oil concentration when the S40 topdressing was delayed until flowering. Although S10 was inadequate to correct the S deficiency, there was no reduction in either seed yield or seed oil concentration when S10 was topdressed as late as flowering, when compared with this rate of S applied at sowing. Seed meal protein levels were increased by the S40 topdressings. Concentrations of S in seed from the S0 and S10 plants were below the critical value of 0.36% for canola. Seed N:S concentration ratios of S-deficient plants were greater than 10, but 7.5 for plants which received adequate S. Total glucosinolates in seed were increased by the application of S, but the levels were still well below the limit set for the canola standard.

Internal phosphorus requirements of six legumes and two grasses

Critical phosphorus (P) values, both total (P,) and inorganic Pi, for the diagnosis of P deficiency were derived for a number of pasture species that are increasingly used in Australia. Trifolium balansae and Medicago murex, with T. subterraneum for comparison, were grown for 2 seasons in field experiments to which 7 rates of phosphate fertiliser were applied. The legumes T. balansae, M. murex, M. polymorpha, Ornithopus compressus, and Lotus pedunculatus, and the grasses Phalaris aquatica and Danthonia richardsonii, were grown in sand culture in glasshouse experiments and provided with nutrient solutions containing 6 rates of P (0.05-1.6 mmol/L). Diagnostic indices were derived for blades of the youngest open leaves (YOL) or youngest expanded blades, and for whole shoots. Critical Pt concentration in the YOL of T. balansae did not decline until full flowering and was the most stable indicator. The range of critical concentrations was 0.45-0.50% for both diagnosis of deficiency and prediction of seasonal yield. A Pi concentration of 150 mg/kg was critical for T. balansae during vegetative growth only. Critical concentrations in M. murex declined from an early stage, but a Pt concentration in YOL of 0.40% was the most useful indicator for diagnosis until flowering. The critical values for T. subterraneum agreed well with previously published data. Critical P concentrations in O. compressus were similar at 2 sampling times. For the remaining species, critical concentrations declined with time and it was necessary to know plant age when interpreting them.

Critical phosphorus concentrations in oilseed rape (Brassica napus) and Indian mustard (Brassica juncea) as affected by nitrogen and plant age

Oilseed rape and Indian mustard were grown in sand culture experiments in a glasshouse to derive values for a tissue test for the diagnosis of phosphorus (P) deficiency. Seven rates of P, combined factorially with 3 rates of nitrogen (N), were used to determine critical P concentrations. Suitable tissues to sample for a diagnostic test were the whole shoot of both species at any stage, or the youngest fully expanded leaf of rape and leaves 4-6 of mustard at the rosette stage. Critical P concentrations depended on both plant age and N supply. The critical values reported here for rape agreed closely with critical values found previously in tissues of field-grown crops of similar phenological age. Critical P levels in whole rape shoots adequately supplied with N decreased from 0.29% at the early rosette stage to 0.21% at the late rosette or yellow bud stage, while critical values in mustard fell from 0.25% at the early rosette stage to 0.18% at stem elongation to full flower. Critical P concentrations for prediction of seed yield were slightly higher (0.05% higher at the rosette stage). A nutrient supply with high P and high N reduced the seed oil concentration of both species; a low P and high N supply reduced the oil concentration in rape seed but increased it in mustard seed.

Assessment of the phosphorus status of oilseed rape by plant analysis

Phosphorus (P) concentrations in young plants of rapeseed (Brassica napus cv. Wesway) were related to seed and oil yields to develop a tissue test for the diagnosis of P deficiency. Critical P concentrations were defined as those concentrations required to sustain 90% of maximum yield. In 2 field experiments in successive seasons on a P-deficient soil, rates of triple superphosphate from 2.5 to 120 kg/ha were banded with the seed. The lowest P concentration in young shoots (17-19 weeks from sowing) associated with a P supply that was adequate for plant growth was approximately 0.31%. The youngest fully-expanded leaf was a reliable plant part to sample, its P concentration being about 0.05% lower than the concentration of the whole shoot. Critical P concentrations in young plants for sustaining 90% of maximum seed and oil yields were higher, namely 0.33 and 0.28% for whole shoots and youngest fully-expanded leaves respectively. The critical P concentration in seeds was about 0.35%.

Responses of some crop plants to correction of subsoil acidity

The effects of subsoil acidity and lime incorporation on plant growth were examined for several crop species (wheat, rape, lucerne and phalaris) growing in reconstituted soil profiles. The pH levels of the profiles had been differentially lowered because they had been under improved subterranean clover pastures for various periods up to 55 years. Acidities in the upper layers differed by at least one pH unit and the depth of the acidified horizons reached 60 cm in the most severely affected soil. All species showed good root growth in soil from 'unimproved' native pasture. Root growth was much poorer in the more acid profiles, particularly between 10- and 40-cm depths, where CaCl2-extractable aluminium was high (> 6 �g g-1). Liming throughout the profile reduced exchangeable aluminium (to about 1 �gg-1) and increased root lengths in the most acid profile by about 60% (0-40 cm depth), so that root lengths then approached those in the 'unimproved' soil. Liming increased shoot yields of all crops except phalaris. The increase was greatest for lucerne (400%), with smaller increases for wheat (39%) and rape (26%). Liming the surface horizon (0- 10 cm) had little effect on most species, but substantially improved root length and shoot yield of lucerne (75 and 300% respectively). The agronomic implications of these findings in crop establishment and drought resistance on acidified soils are discussed.

Interactions of surface drying and subsurface nutrients affecting plant growth on acidic soil profiles from an old pasture

Previous studies on soils from old pastures in southern New South Wales have demonstrated that nutrients have accumulated at the soil surface, but that the 40-100-mm depth layer in many profiles has become strongly acidic (e.g. pH 4.7), and high in extractable aluminium. Poor growth of subterranean clover has occurred on such soils during dry periods and may be associated with poor root growth in the acidic, nutrient-poor subsurface layers. Possible nutritional causes of these observations were investigated using reconstituted soil profiles. The root and shoot growth of subterranean clover, wheat, oats and lucerne were compared in unamended profiles and in profiles amended by applying nutrients or calcium carbonate (lime) to correct the more obvious deficiencies of the subsurface layers. Subterranean clover grew well as long as the surface soil remained moist, so that plants could utilise the nutrients potentially available within it. When the surface (0-40 mm) was allowed to dry but the subsurface layers remained moist, growth was poor unless phosphate was applied to the moist layer. Subsurface application of lime alone was ineffective. Nitrogen application increased clover growth in the presence of added phosphate or surface moisture, but nitrogen alone did little to alleviate the effects of surface drought. Wheat, and to a lesser extent oats, responded to subsurface lime when the surface was moist, and both responded to subsurface phosphate when the surface was dry. Lucerne responded to subsurface phosphate similarly to subterranean clover but the response was more than doubled in the presence of additional borate and lime. Lime without borate was not effective. When the surface was maintained moist, liming both the surface (0-40 mm) and subsurface layers improved the response over liming the subsurface layer only. The results suggest that declining fertility and productivity in old pastures developed on acid soils may not be rectified by liming alone, but that cultivation, ripping or drilling of phosphate, and in some cases addition of borate, may be required to improve the penetration of nutrients, particularly phosphorus, to greater depth.

Effects of subsoil acidity on the shoot and root growth of some tropical and temperate forage legumes

The root and shoot growth of four tropical and two temperate summer-growing legumes were assessed when plants were grown in deep profiles of an acidic soil modified by additions of calcium carbonate. Species tested over three harvests were Desmodium intortum, Glycine wightii, Stylosanthes humilis (Townsville stylo), Macvoptilium atvopuvpureum (Siratro), Trifolium repens and Medicago sativa (lucerne). There were large and more immediate effects on root growth, particularly on fine root length, than on shoot growth. The species differed in their root responses to lime, the tropical species in general being more tolerant of subsoil acidity than the temperate species. There were marked differences between species in their responses when expressed as the ratio of fine root length to total shoot weight. The ratio of root weight to shoot weight showed much less variation with lime rate, and it is suggested that the ratio of fine root length to shoot weight is the better indicator of tolerance to subsoil acidity. S. humilis showed little response to lime at any time, and was notable for its length of fine root. Siratro grew well at first but later there was little increase in shoot weight or in length of fine root, although tap root weight increased greatly. Roots of D. intorturn, T. repens and lucerne were slow to penetrate beyond 55 cm depth. At later harvests the root lengths of these species and of G. wightii were highly responsive to lime. Agronomic implications of the results are discussed.

Interacting effects of subsoil acidity and water on the root behaviour and shoot growth of some genotypes of lucerne (Medicago sativa L.)

The effects of additions of calcium carbonate to a low-calcium, acidic subsoil, underlying a fertilized, limed surface soil, on the root and shoot growth of four lucerne genotypes were studied in deep (85 cm) soil columns. Separate surface and subsoil watering treatments were included after plants were established. A comparison showed that, at low rates of addition, calcium carbonate was more effective than magnesium carbonate in promoting root elongation, but more generally the increase in pH appeared to be the important factor. Surface droughting reduced shoot growth, but the response to subsoil lime continued as long as subsoil moisture was supplied, in accordance with the increased root penetration and ability for water uptake. Thus shoot growth under surface drought reflected (r = 0.973) the total length of deep roots (> 60 cm depth) over all genotypes and lime treatments. The genotypes, which included three from cv. Hunter River and one from cv. Siro Peruvian, differed in several respects. Our selections from Hunter River varied in root morphology, shoot:root ratios and type of response to subsoil lime. The Siro Peruvian genotype produced abundant deep, fine roots and good shoot growth under surface drought.

The use of stable strontium as a chemical tracer for root penetration

Stable strontium was used successfully, in the glasshouse and the field, as a tracer for root penetration and activity in some crop plants. Changes in strontium concentration in shoots were detected within a few days of roots coming into contact with soil enriched with strontium chloride, either mixed throughout a horizon as a powder or place at a particular depth as pellets. In glasshouse experiments there was a close correlation between dry weight of roots in the enriched zone and strontium accumulated in the shoots. Of various sizes of pellets used, 100 mg pellets were effective in raising the strontium content of shoots, while diffusing through only a limited volume of surrounding soil. Movement of strontium from larger pellets was excessive for accurate monitoring of root penetration and soil strontium concentrations were undesirably high, while smaller pellets, or a narrow band of enrichment, were ineffective. In the field, a technique of placing 100 mg pellets at 10 cm intervals in soil successfully monitored root penetration when wheat was grown under two systems of cultivation. Differences in shoot strontium concentration were greatest at early harvests and gradually disappeared. Hence it was essential to take frequent harvests in order to record root penetration with accuracy.

Effects of subsoil calcium on the root growth of some lucerne genotypes (Medicago sativa L.) in acidic soil profiles

The root growth of lucerne was examined in an acidic soil profile modified by varying additions of calcium carbonate to different layers of the subsoil. Root growth responded strongly to changes in the concentration of exchangeable soil calcium. Symptoms of thickening, distortion and poor lateral formation occurred under low calcium treatments. The results were in agreement with the interacting effects of calcium (at 0.5–5.0mM) and aluminium ions (at 0–20,µM) on lucerne in separate solution culture experiments. Differences in root penetration were observed between three lucerne clones selected from the cultivars Hunter River and Siro Peruvian. At the first harvest, the magnitude of these differences was increased by the addition of lime to the profiles. The three genotypes produced similar total dry weight yields, but differed in their distribution of growth between shoots and roots. This distribution was not affected by the addition of lime to the subsoil. However, the length of roots in the lower layer of the profile ( > 60 cm depth) was more responsive to subsoil treatment than was total dry weight. At the final harvest, the shoot yields of two genotypes were affected by lime treatments, but that of the deepest-rooted genotype was not. The results suggest that improved genotypes could be selected from Australian lucernes for establishment in areas with acidic subsoils, but that selection on root penetration alone would not necessarily lead to increased shoot yields.