Citation: Pérez-Fernández, M.A.; De-Lara-Del-Rey, I.A.; Magadlela, A. Bacterial Products and Their Effect on the Shrubby Legume Calicotome villosa (Poir.) Link. Diversity 2023, 15, 1101. https://doi.org/10.3390/ d15101101 Academic Editors: Alessandra Salvioli Di Fossalunga and Vincenza Cozzolino Received: 24 September 2023 Revised: 10 October 2023 Accepted: 18 October 2023 Published: 23 October 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). diversity Article Bacterial Products and Their Effect on the Shrubby Legume Calicotome villosa (Poir.) Link María A. Pérez-Fernández 1, * , Irene Ariadna De-Lara-Del-Rey 1 and Anathi Magadlela 2 1 Department of Physical, Chemical and Natural Systems, Ecology Area, Pablo de Olavide University, Carretera de Utrera, K1, 41013 Seville, Spain; iadedel@alu.upo.es 2 School of Life Sciences, College of Agriculture, Engineering and Science, University of KwaZulu-Natal, Westville Campus, Private Bag X54001, Durban 4000, South Africa; anathimagadlela@icloud.com * Correspondence: maperfer@upo.es Abstract: Calicotome villosa is a eurioic legume with broad distribution in the south of Europe. It can grow in almost any type of soil as well as in humid and dry and nutrient-deficient ecosystems. The broad distribution and eurioic nature of C. villosa may be attributed to its ability to establish endophytic associations with plant-growth-promoting (PGP) bacteria housed in the nodules and rootlets. This study examined the legume–microbe interaction of C. villosa growing in two contrasting locations: a hilltop with high sun irradiance and drought, and a valley bottom with a low level of radiation and sufficient soil humidity for plant growth. Calicotome villosa adult plants established endophytic interactions with bacteria in six genera: Pseudomonas, Stenotrophomonas, Bacillus, Paeni- bacillus, Brevicacterium, and Rhizobium. Plants growing on the hilltop had associated lower bacterial richness than those grown on the valley bottom. All strains were drought-tolerant and produced siderophores, IAA, HCN, and NH 3 that stimulated plant performance in C. villosa plants and the other four legumes commonly present in the understory of the shrub communities. The legumes’ capacity to selectively host symbiotic bacteria that enhance plant survival in harsh conditions partly accounts for the diverse partnerships between C. villosa plants and their symbionts, ultimately explaining the wide distribution of this plant species. Keywords: legumes; siderophores; IAA; biomass; bacterial richness 1. Introduction Shrubs play a vital ecological role on the Iberian Peninsula due to their multifaceted contributions to its ecosystems [1]. These plants are particularly well adapted to the Mediterranean climate, characterized by hot and dry summers [2]. Shrubs provide habitat and food resources for a wide range of wildlife, including insects, birds, and mammals, contributing to biodiversity conservation [3]. Their intricate root systems help stabilize soil, preventing erosion and supporting water infiltration, which is essential in the face of frequent droughts [4]. Additionally, many shrub species have developed strategies to cope in nutrient-poor soils, promoting nutrient cycling and enriching the soil by estab- lishing symbiotic interactions with soil-borne microorganisms [5,6]. Their ability to thrive in challenging environmental conditions makes shrubs resilient components of Iberian ecosystems, acting as pioneers in ecosystem restoration after disturbances such as fires [7]. The establishment of leguminous shrubs under Mediterranean type of climates faces several significant constraints that shape their distribution and abundance. One primary limitation is the nutrient-poor soils, which affect the growth and survival of leguminous species. These plants rely on a symbiotic relationship with nitrogen(N)-fixing bacteria to obtain essential nutrients, particularly nitrogen (N) and phosphorus (P) [7,8]. Moreover, the Mediterranean climate can pose a challenge to water availability for these plants, especially during critical establishment phases [1]. Competition with other vegetation, including non- native species, further adds to the challenges leguminous shrubs face in securing a foothold in Diversity 2023, 15, 1101. https://doi.org/10.3390/d15101101 https://www.mdpi.com/journal/diversity