Dear Auxin: An ode to the master hormone

A simple Google search shows that the word “auxin” derives from the Greek “auxein” which means, “to grow”. Auxin steals the spotlight as being both the first plant hormone discovered by Darwin and being immensely important for plant growth. Plant processes like flowering, fruit production and growing towards light, water, and gravity are elegantly orchestrated by molecules like auxin. Auxin is beautiful in its complexity. The phytohormone exerts opposite effects on root and shoot growth, with it inhibiting growth at the roots, yet stimulating growth at the shoot. Even within a single or- gan, like the primary root, high concentrations inhibit growth and lower concentrations stimulate growth. As a hormone, auxin is the antithesis of death and destruction; however, too much of a ‘good thing’ makes auxin toxic as an herbicide. As a compound with morphogenic properties, auxin does not impart a singular effect on a cell or process. Rather, auxin is simply a messenger, and the message depends on the tissue type, its concentration, and time (Leyser 2018). This makes auxin one of the most fascinating things I had encountered in plant biology.

My first hands-on research experiences (and naivete) studying auxin were in the context of calcium signaling during my PhD. I found that the calcium channel CNGC2 and auxin work together during plant development. In simple terms, auxin prompts CNGC2 to release calcium, while auxin itself is under the control of the calcium signals from CNGC2 it helped produce, as a part of a feedback loop (Chakraborty 2021). But the story is likely more complex. Technologies like the DR5 type transcriptional reporters and DII-VENUS type signaling sensors have informed scientists about aux- in regulation and distribution. Although they have been im- mensely useful, they also limit our understanding of auxin as they are merely proxies of the actual hormone. It’s almost like looking at the shadow of auxin, but not auxin itself.

So dear auxin, I hope this letter finds you well, wherever you are (perhaps at the edges of meristems where you like to accumulate). The very mention of your name makes my ears perk up at any conference talk or poster session. I delight in talking about you and what you might be doing at any given moment. Things have not always been easy between us. You have at times been the bane of my graduate school existence and I might have even professed that I was never going to work with auxin ever again. But that’s not your fault, you were simply misunderstood. Thankfully, your future is bright (and I mean that quite literally). Emerging technologies like rapid and reversible hormone biosensors with high binding affinity and sensitivity can directly visualize you at the subcellular resolution (Balcerowicz 2021). With such systems at the forefront of hormone research, we can finally figure out what you do and maybe even topple some long-held misconceptions about you. I look forward to exchanging glances with you under the microscope some day. Until we meet again,

Your not-so-secret admirer Sonhita

Contributed by Dr. Sonhita Chakraborty

sonhita.chakraborty@mail.utoronto.ca

Dr. Sonhita Chakraborty (she/her) just finished her PhD under the supervision of Dr. Keiko Yoshioka at the Cell & Systems Biology department, University of Toronto. Her research focused on the cal- cium channel CNGC2 and its role in auxin signaling. Dr. Chakraborty is taking a short breather after completing her PhD and is actively looking for post-doctoral positions and fellowships; if you are interested in collaborating, please reach out!