Cell cycle regulation under pattern-triggered immunity

Abstract

In a world with a constantly growing population, improving crop yields is necessary to ensure food security. However, plant diseases severely impact crop yields, and breeding efforts to enhance plant immunity can be limited by trade-offs between immune activation and growth. Yet, the molecular mechanisms behind these growth-immunity trade-offs remain largely unknown. Therefore, the aim of this work was to investigate the influence of pattern-triggered immunity (PTI) on mitotic cell cycle regulators during root growth inhibition (RGI). Previous findings indicated that the intensity of the immunity-related RGI varies depending on the plant growth medium used. In this thesis, PTI responses in Arabidopsis thaliana (A. thaliana) roots grown on two standard growth media were compared. Depending on the medium, roots exhibited a much stronger response to elicitor flg22 (the active epitope of bacterial flagellin), in terms of RGI, reactive oxygen species production as well as immunity gene induction, and this was largely independent of gelling agent or media buffering. Using fluorescent PTI reporter lines, it was also shown that depending on the growth media used, different root zones responded to PTI elicitor treatment. Together, the findings that growth media can have a strong influence on PTI levels might be helpful for researchers to employ suitable growth conditions for PTI studies. In the second part of this thesis, to gain a better understanding of the growth-immunity cross-talk, the expression of key cell cycle regulator genes was analyzed after PTI activation by exogenous application of the flg22 elicitor to A. thaliana roots. The results showed that the expression of various cell cycle regulators (e.g. CYCLIN DEPENDENT KINASE B2;1 (CDKB2;1) and CYCLIN D1;1 (CYCD1;1)) was reduced during flg22-induced PTI. Furthermore, the effect of flg22 treatment on key cell cycle regulator protein levels in Nicotiana benthamiana was investigated, and it was found that flg22-triggered PTI may disrupt protein accumulation (e.g. CYCD2;1). Finally, it was shown that supporting the cell cycle by overexpressing some cell cycle regulators (e.g. CDKB2;1) could reduce root growth inhibition caused by flg22 treatment in A. thaliana. Together, these results support that during RGI, immune activation by flg22 treatment may directly affect cell cycle regulators and that over-activation of various cell cycle regulators can uncouple immunity- and growth-related processes. This may potentially help breeding strategies to obtain disease resistant crops with unimpaired growth.