Rice cycles between drought and well-watered-adapted phenotypes by changing lateral root formation.

Background And Aims: Natural rainfed conditions present drought episodes interspersed with periods of moderate to high soil moisture levels. This study investigates the genetic variation in root-to-shoot growth in response to a wet-drought-wet cycle and aims to identify rice (Oryza sativa) lines differing in drought recovery, focusing on detailed root trait investigations.

Methods: 100 different rice accessions were screened under fluctuating moisture across three field seasons for GWAS (genome wide association study) analysis. In a subset of 20 genotypes, crown root number and leaf length were recorded regularly to calculate a Water Recovery Index. Two lines contrasting in Water Recovery Index were grown in a greenhouse experiment to resolve detailed root phenotypes in simulated field drought and re-watering.

Key Results: GWAS co-locations indicated drought recovery-associated loci that included candidate genes previously reported for several abiotic stressors. In the subset of 20 genotypes, crown root growth was impacted most by the transition from drought to re-watering. The calculated Water Recovery Index distinguishes different responses to drought and re-watering. A greenhouse study reproduced the contrasting growth of two selected lines, with 'ADT 12' shoot and root growth being strongly impaired by drought, while 'ARC 18202' growth was not suppressed. Drought caused a significant decrease in S-type lateral root production in both lines, while a significant increase in L-type lateral root proportion was only found for 'ADT 12'. These phenotypes were reversed seven days after re-watering to values of the well-watered control plants.

Conclusions: Overall, in-depth root phenotyping confirmed the drought-resistance and recovery ability of 'ARC18202' in the field and highlighted the importance of S-type and L-type lateral root formation already under well-watered conditions prior to drought. 'ARC18202' had a higher amount of thick lateral roots before drought and, therefore, less change in formation under drought and re-watering conditions.