Carbonic anhydrases are upstream regulators of CO₂ signaling in guard cells
Honghong Hu^1*_, Julian I. Schroeder²

¹College of Life Sciences and Technology, Huazhong Agricultural University, Wuhan, 430070
² Cell and Developmental Biology Section, Division of Biological Sciences, University of California, San Diego, La Jolla, CA 92093-0116, USA.
*Corresponding Author：Tel: (+8627) 87385931; E-mail: huhh@mail.hzau.edu.cn

Abstract
We know ambient CO₂ is continuing rising and this rising CO₂ causes plants stomatal pores closing and thus regulates plant gas exchange---CO₂ influx into plants and water loss by transpiration. However, little is known about the early CO₂ sensing mechanisms that control plant gas exchange. Recently, we found that CO₂-binding proteins, βCA1 and βCA4, are required for [CO₂] regulation of stomatal movements and also stomatal development, functioning as the earliest components in guard cell signaling pathway. Interestingly, CA-mediated stomatal CO₂ responses are not directly linked to photosynthesis and originate in guard cells. Guard cell over-expression of either CA1 or CA4 in wild-type plants greatly enhances water use efficiency. Further physiological research showed that CA-mediated catalysis is a major mechanism for βCA-mediated CO₂-induced stomatal closing and elevated HCO₃^-, more than CO₂ and protons, functions as a small-molecule transducer to activate anion channels in guard cells. In this pathway, Ca²⁺ is required for HCO₃^- activation of S-type anion channel and the activation is mediated by SLAC1. Furthermore, several proteins were identified as interactors of CA1 and CA4, indicating there is another mechanism for CA1 and CA4 in CO₂ regulation of stomatal movement. The detailed data will be presented at the meeting.

Key Words: βCA1 and βCA4; CO₂; HCO₃^-; guard cell; stomatal movement; signaling