The locoweed endophyte Alternaria oxytropis affects root development in Arabidopsis in vitro through auxin signaling and polar transport.

Locoweeds are leguminous forbs known for their toxicity to livestock caused by the endophytic fungi Alternaria sect. Undifilum. Unlike the defensive mutualism reported in many toxin producing endophytes and their plant hosts, the benefits that A. sect. Undifilum can confer to it host plants has been unclear. Here we conducted physiological and genetic analyses to show that A. (sect. Undifilum) oxytropis influences the growth, especially root development in its locoweed host and Arabidopsis. A. oxytropis significantly decreased primary root length while increasing the number of lateral roots and root hairs, plant leaf area and fresh weight. It enhanced the levels of plant endogenous auxin, and the expression of key genes in auxin biosynthesis, signaling and transport. Such influence on root development was abolished in auxin signaling and polar transport mutants. A. oxytropis down-regulated PIN1 but increased PIN2, PIN7, and AUX1, which may reflect altered spatial auxin accumulation responsible for the change in root architecture. Plant growth was also insensitive to A. oxytropis when NPA (naphthylphthalamic acid) was applied. Our findings indicate a function of A. oxytropis in promoting the growth and development of Arabidopsis via the regulation of auxin. This suggests the possible role of A. oxytropis to benefit its locoweed hosts, via a process irrelevant to its toxin production.