Lysine metabolism is concurrently regulated by synthesis and catabolism in both reproductive and vegetative tissues

The functional role of Lys catabolism in balancing Lys levels in plants has only been directly demonstrated in developing seeds. Seed-specific expression of a bacterial feedback-insensitive dihydrodipicolinate synthase (DHPS) in an Arabidopsis knockout mutant of the AtLKR/SDH gene that regulates Lys catabolism synergistically boosted Lys accumulation in mature seeds, but it also severely reduced the growth of seedlings derived from them. Here we further tested whether the inhibition of seedling growth was due to a negative physiological effect of excess Lys on seed maturation or to defective postgermination catabolism of Lys, which accumulated in the mature seeds. To address these questions, we coexpressed a bacterial DHPS gene with an RNAi construct of AtLKR/SDH, both under control of the same seed-specific promoter, to restrict Lys synthesis and catabolism to the developing seeds. Coexpression of these genes boosted seed Lys content and caused a significant, metabolically unanticipated increase in Met content, similarly to our previous report using plants expressing the bacterial DHPS on an AtLKR/SDH knockout background. However, postgermination seedling growth was significantly improved when the reduction of Lys catabolism was restricted to seed development, suggesting that defective postgermination Lys catabolism was responsible for inhibition of seedling growth in the AtLKR/SDH knockout plants expressing the bacterial DHPS gene in a seed-specific manner. Constitutive expression of the bacterial DHPS in the AtLKR/SDH knockout mutant boosted Lys levels in vegetative tissues in a similar manner to that observed in seeds, further demonstrating that Lys catabolism plays an important regulatory role in balancing Lys levels.