2015_05 Actin - Life or Death?
What is the Topic?
Immunity needs energy – this is familiar to anybody who ever had been down with fever. There is not so much difference when we look at plants. The activation of plant defence impairs growth substantially. Therefore, defence is activated only in case of urgent need. When humans domesticated plants, they had, unconsciously, selected for fast growth and high yield. This selection was possibly too biased, because natural immunity was sacrificed and our technical agriculture demands now considerable efforts for chemical or biological plant protection.
Thus, a plant needs to find a compromise balancing reduced growth with the activation of defence. This is particularly important when an extreme form of defence, programmed cell death is employed as most efficient strategy against so called biotrophic pathogens. These are microbes that, through sophisticated chemical manipulation turn plant cells in obedient Zombies that eagerly cater the intruder with nutrients. In the current work we address the role of actin (the cellular "muscles") and auxin (the central growth hormone in plants) for the defence of grapevine cells. We discovered that auxin can quell the activation of immunity. When we investigated this phenomenon in more detail, we found, to our surprise, that an important signal for the activation of immunity – superoxide, which is generated by a signalling enzyme in the cell membrane – is also needed for auxin signalling and therefore produced to a certain extent during normal growth and development. A boost of superoxide activates the contraction of the cellular actin “muscles”, which is an important signal for the activation of programmed cell death. This means that superoxide is the key, by which plants balance growth and defence using a very simple and smart manner: both responses rely on superoxide. Since the abundance of superoxide is limited and under strict control, this competition will lead to a situation, where the activation of immunity will automatically slow down growth, such that resources otherwise consumed for growth will become available for defence. On the other hand, excessive defence can be silenced very easily by adding more auxin.
114. Chang X, Riemann M, Nick P (2015) Actin as deathly switch? How auxin can suppress cell-death related defence. doi: 10.1371/journal.pone.0125498 - pdf