Study on pseudomonas producing ACC deaminase in increasing growth and survival of soybean against disease

Abstract

1-aminocyclopropane-1-carboxylate (ACC) deaminase-producing bacteria have been known to help plant growth by preventing the inhibition-effect of increase concentration of ethylene in higher plants, which is commonly triggered by high concentration of indole-3-acetic acid (IAA) and/or by the presence of plant pathogens in the vicinity of the root. The enzyme degrades ACC (precursor of ethylene) into ammonia and α-ketobutirate as bacterial sole source of nitrogen. This study examined the potential use of Pseudomonas isolates producing ACC deaminase in increasing soybean growth and survival against pathogenic fungi Fusarium oxysporum, Sclerotium rolfsii and Rhizoctonia solani. Eighty one isolates of IAA-producing Pseudomonas (1.13 to 20.49 μg IAA ml-1) were screened in planta for their ability to promote soybean growth in growth chamber conditions. The selected isolates were assessed in vitro for their ACC deaminase activity and compatibility with root-nodule bacteria Bradyrhizobium japonicum BJ11 and Sinorhizobium fredii Rif5, and tested to promote soybean growth and reduce disease incidence caused by F. oxysporum, S. rolfsii and R. solani under greenhouse conditions. The results showed that 13 out of 81 isolates significantly increased soybean root length and weight, up to 50% from untreated plants. Of 13 isolates, 11 isolates were able to use ACC as their sole source of nitrogen as an indication of their ACC deaminase activities. Two isolates that did not show ACC deaminase activities had lower capacity to produce IAA (5.45 to 6,31 μg IAA ml-1). Three out of 11 isolates inhibited at least one strain of root-nodule bacteria, thereby limiting their use for soybean. Eight selected isolates increased soybean height and fresh weight although not all were significantly different from untreated control. On the other hand, most isolates significantly increased the survival rates of soybean in soil containing pathogenic fungi. Disease suppression of the isolates caused by F. oxysporum, S. rolfsii and R. solani was 17.1, 36.4 and 64.5%, respectively. The higher the destructive effect of the pathogens as shown by S. rolfsii and R. solani treatments, the better was the ability of the isolates to reduce the disease. Further work is worth to evaluate whether or not the fertility status of the soil or a particular pathogen influence the beneficial effects of these bacteria on soybean.

Enzim 1-aminocyclopropane-1-carboxylate deaminase (ACC deaminase) (E.C.4.1.99.4) merupakan enzim yang diproduksi oleh beberapa rizobakteri untuk mendegradasi substrat ACC (prekursor hormon etilen) sebagai sumber nitrogen. Degradasi ACC secara aktif oleh rizobakteri akan mengurangi biosintesis hormon etilen. Dalam banyak kasus, biosintesis etilen yang berlebihan dalam jaringan akar akan menghambat perkembangan akar dan melemahkan ketahanan tanaman terhadap serangan penyakit. Beberapa faktor pemicu biosintesis etilen pada tanaman banyak terkait dengan cekaman (stres) akibat kondisi lingkungan yang tidak mendukung, serangan hama penyakit, dan peningkatan konsentrasi hormon asam indol asetat (indole acetic acid, IAA). Dari berbagai hasil penelitian, inokulasi bakteri penghasil ACC deaminase terbukti mampu meningkatkan pertumbuhan tanaman dengan mengoptimalkan peran IAA dalam memacu pertumbuhan tanaman dan meningkatkan daya tahan tanaman terhadap serangan patogen. Di Indonesia, studi penggunaan rizobakteri penghasil ACC deaminase sebelum penelitian ini belum pernah dilaporkan. Pemanfaatannya pada tanaman kedelai merupakan salah satu alternatif teknologi yang menjanjikan untuk meningkatkan kualitas pertumbuhan tanaman dan mengatasi banyaknya serangan penyakit pada pertanaman kedelai. Penggunaan bakteri ini dapat mengurangi pemakaian senyawa agrokimia sintetik yang berlebihan (pupuk dan pestisida) yang berpotensi mencemari lingkungan. Tantangannya adalah terletak pada kemampuan menapis dan memilih bakteri penghasil ACC deaminase yang unggul yang dapat bekerja secara sinergis dengan bakteri bintil akar kedelai (rhizobia).