Table_1_Exploring Rain as Source of Biological Control Agents for Fire Blight on Apple.XLSX

Poor survival on plants can limit the efficacy of Biological Control Agents (BCAs) in the field. Yet bacteria survive in the atmosphere, despite their exposure to high solar radiation and extreme temperatures. If conditions in the atmosphere are similar to, or more extreme than, the environmental conditions on the plant surface, then precipitation may serve as a reservoir of robust BCAs. To test this hypothesis, two hundred and fifty-four rain-borne isolates were screened for in vitro inhibition of Erwinia amylovora, the causal agent of fire blight, as well as of other plant pathogenic bacteria, fungi and oomycetes. Two isolates showed strong activity against E. amylovora and other plant pathogenic bacteria, while other isolates showed activity against fungal and oomycete pathogens. Survival assays suggested that the two isolates that inhibited E. amylovora were able to survive on apple blossoms and branches similarly to E. amylovora. Pathogen population size and associated fire blight symptoms were significantly reduced when detached apple blossoms were treated with the two isolates before pathogen inoculation, however, disease reduction on attached blossoms within an orchard was inconsistent. Using whole genome sequencing, the isolates were identified as Pantoea agglomerans and P. ananatis, respectively. A UV-mutagenesis screen pointed to a phenazine antibiotic D-alanylgriseoluteic acid synthesis gene cluster as being at the base of the antimicrobial activity of the P. agglomerans isolate. Our work reveals the potential of precipitation as an under-explored source of BCAs, whole genome sequencing as an effective approach to precisely identify BCAs, and UV-mutagenesis as a technically simple screen to investigate the genetic basis of BCAs. More field trials are needed to determine the efficacy of the identified BCAs in fire blight control.

生物防治剂（Biological Control Agents, BCAs）在植株表面的定殖存活率低下，会限制其田间应用效果。然而，细菌尽管暴露于强太阳辐射与极端温度环境中，仍可在大气中存活。若大气环境与植物表面环境相似甚至更为严苛，则降水或可成为高效生防菌剂的储存库。为验证该假说，研究人员对254株雨水来源的分离菌株开展了体外抑菌试验，检测其对火疫病致病菌解淀粉欧文氏菌（Erwinia amylovora），以及其他植物病原细菌、真菌和卵菌的抑制活性。结果显示，两株菌株对解淀粉欧文氏菌及其他植物病原细菌具有强烈抑制活性，其余菌株则可抑制真菌与卵菌病原物。定殖存活试验表明，这两株具有解淀粉欧文氏菌抑制活性的菌株，可在苹果花与枝条上存活，其存活能力与解淀粉欧文氏菌相当。在离体苹果花上预先接种这两株菌株后再接种病原物，可显著降低病原物种群数量及伴随的火疫病症状；但在果园内的活体苹果花上，病害防控效果并不稳定。通过全基因组测序，这两株菌株分别被鉴定为成团泛菌（Pantoea agglomerans）与菠萝泛菌（Pantoea ananatis）。紫外诱变（UV-mutagenesis）筛选结果显示，成团泛菌菌株的抗菌活性源于吩嗪类抗生素D-丙氨酰灰黄菌素酸（D-alanylgriseoluteic acid）合成基因簇。本研究揭示了降水作为未被充分发掘的生防菌剂来源的潜力，同时证明全基因组测序可精准鉴定生防菌剂，而紫外诱变筛选则是一种技术简便的遗传基础研究手段。未来仍需开展更多田间试验，以明确所筛选得到的生防菌剂在火疫病防控中的应用效果。