Tomato Root Architecture and the Influence of Atbzr1-d & Atbes1-d in the Phosphate Deprivation Response

Response to phosphate nutrient deprivation in plant roots is critical to a plants survival however many of the pathways regulating this response are unknown. Many plants, including Arabidopsis, respond to phosphate stress by a reduction in the length of the primary root and an increase in the number and length of lateral roots. Yet the developmental root system architecture (RSA) of tomato under phosphate stress has not yet been described. We identified natural variation in RSA between two Solanum lycopersicum cultivars (M82 & New Yorker) and S. pennellii in response to phosphate deprivation, 9 days after germination. New Yorker showed a greater response to phosphate deprivation with a larger increase in the number of lateral roots and significantly longer lateral roots compared to M82. S. pennellii showed insensitivity for phosphate deprivation for primary root length, the number of lateral roots and lateral root length. Additionally, we investigated the transcriptomic response of phosphate starvation. S. pennellii does not respond to phosphate deprivation, this is reflected by a lack of transcriptional response. Comparatively few genes are differentially expressed in S. pennellii, 37, compared to 1,190 for S. lycopersicum strongly suggesting that transcriptional regulation plays a major role in the Solanum phosphate deprivation response. In Arabidopsis, a dominant mutation of homologous brassinosteroid transcription factors BRASSINAZOLE-RESISANT1-d (Atbzr1-d) & BRASSINAZOLE-RESISANT2-d (Atbes1-d) prevents the remodulation of the RSA under phosphate stress. New Yorker plants expressing the dominant negative mutant alleles - Atbes1-d and Atbzr1-d display an inhibition of lateral root elongation in response to phosphate starvation. Lateral root initiation and primary root length is not influenced. However, brassinosteroid signaling via bzr1-d and bes1-d confers insensitivity to phosphate deprivation in terms of lateral root elongation. Thus, in response to phosphate starvation BR signaling is exploited to regulate different aspects of root development in Arabidopsis and S. lycopersicum.