Vaccinium arboreum Is a Potential Source for Breeding Deeper Root Systems in Southern Highbush Blueberry

Growers use costly soil amendments to provide southern highbush blueberry (SHB, Vaccinium corymbosum hybrids) plants with an adequate soil environment for growth. SHB exhibits shallow, fibrous root systems that confine plant water and nutrient uptake to superficial soil layers. V. arboreum is a wild species native to the southeastern United States and has previously been used in SHB breeding. Field observations suggest that V. arboreum exhibits deeper root systems than those of SHB; however, this has not been documented. Plants that develop deeper root systems may exhibit increased drought and nutrient deficiency tolerances because they can access water and nutrients found in deeper soil strata. We hypothesized that a) V. arboreum seedlings exhibit deeper root systems than SHB seedlings and interspecific hybrids between SHB and V. arboreum exhibit intermediate phenotypes and b) DNA markers are associated with root system depth from V. arboreum. We tested these hypotheses using seedling families grown in bench-top rhizotrons filled with sphagnum peat moss. Root systems were cleaned and scanned prior to analyses with WinRhizo Pro 2013b. Comparisons of SHB, V. arboreum, and interspecific families with different levels of V. arboreum introgression indicated that V. arboreum exhibited greater maximum root depth and lower percentage of roots in the top 8 cm of soil than SHB. Interspecific hybrids generally exhibited intermediate root depths between SHB and V. arboreum. In a subsequent experiment, a pseudo-backcross 1 family of interspecific hybrids (SHB × (SHB × V. arboreum)) was phenotyped as above and genotyped through a genotype-by-sequence approach to search for markers associated with root system depth. A total of 3,447 single nucleotide polymorphisms were identified. No markers were significantly associated with maximum root depth; however, 50 markers were significantly associated with the percentage of roots in the top 8 cm of soil. Thirty-three of the significantly associated markers were exonic, and the neighboring regions of 16 of these exonic markers were annotated based on protein homology. The annotation led to genes that are reported to affect root system architecture in model species. Altogether, these results suggest that V. arboreum exhibits deeper root systems than SHB and breeding for greater root system depth may be more efficient using the molecular markers developed in the present study.